Bus Ele An 10507 HSF

Application Guide 10507 Effective June 2016
Protecting
semiconductors with
high speed fuses
Contents
A: Introduction 3 J: Fuses protecting regenerative drives 20

About this guide 3 Conclusion on the rectifier mode 20
Background 3 Conclusion on the regenerative mode 21
Typical fuse construction 3 Summary of voltage selection for regenerative drives 21
Fuse operation 4 K: Fuses protecting inverters 22
Power semiconductors 4 Voltage selection 22
B: High speed fuse characteristics 5 Current selection 22
How high speed fuses are different 5 I²t selection 22
Application factors 5 IGBT as switching device 22
Influencing factors 6 Protection of drive circuits 23
C: Fuse performance data 7 Bipolar power transistors and Darlington pair transistors 23
The time-current curve 7 L: Worked examples 23
The A-A curve 8 Example 1: DC Thyristor drive 23
Clearing integral information (factors K and X) 8 Example 2: High power/high current
The I²t curve 8 DC supply with redundant diodes 24
Peak let-through curve 8 Example 3: regenerative drive application 25
The arc voltage curve 9 Appendix 1: International standards 26
Watt loss correction curve 9 In the United States 26
Temperature conditions 9 In Europe 26
D: Determining fuse voltage ratings 9 Bussmann series product range 26
Voltage ratings 9 US style North American blade and flush-end 26
IEC voltage rating 9 European standard 27
North American voltage rating 9 Blade type fuses 27
Simple rated voltage determination 9 Flush-end contact type 27
Frequency dependency 9 British Standard (BS88) 27
Possible AC/ DC combinations 10 Cylindrical/ferrule fuses 27
AC fuses in DC circuits 10 Appendix 2: Fuse reference systems 28
Fuses under oscillating DC 10 European high speed fuses 28
Fuses in series 10 BS88 high speed fuses 29
E: Determining fuse amp ratings 11 US high speed fuses 30
Part 1 — Basic selection 11 Special fuses - Types SF and XL 31
Part 2 — Influence of overloads 12 Appendix 3: Installation, service, maintenance,
Part 3 — Cyclic loading and safety margins 13 environmental and storage 32
F: High speed fuse applications 14 Tightening torque and contact pressure 32
RMS currents in common bridge arrangements 14 Flush end contact fuses 32
Typical rectifier circuits 15 Special flush-end types 32
G: Fuse protection for rectifiers 16 Fuses with contact knives 32
Internal and external faults 16 DIN 43653 bolted tag fuses on busbars 32
Protection from internal faults 16 DIN 43653 bolted tag fuses in blocks 32
Protection from external faults 16 DIN 43620 bladed fuses in blocks 32
Service interruption upon device failure 17 Press Pack fuses 33
Continued service upon device failure 17 Mounting alignment 33
H: Fuses protection in DC systems 17 Surface material 33
DC fed systems 17 Tin-plated contacts 33
Battery as a load 17 Vibration and shock resistance 33
Battery as only source 18 Service and maintenance 33
Environmental issues 33
I: AC fuses in DC circuit applications 18 Storage 33
Calculation example 19
Glossary 34-35
Contact information Back cover
Bussmann® series high

speed fuse portfolio
These high speed fuse styles are
available in the voltages and ampacities
indicated. For details, see Bussmann
series high speed fuse catalog no.
10506 or full line catalog no. 1007. Compact high speed fuses IGBT fuses North American fuses
• 500 Vac/dc, 50 to 400 A • Up to 1000 Vdc, 25 to 630 A • Up to 1000 Vac/ 800 Vdc,
1 to 4000 A
British Standard BS 88 DFJ UL Class J drive fuse Square body fuses Ferrule fuses
• Up to 700 Vac/500 Vdc, • Full range, 600 Vac/450 Vdc, • Up to 1300 Vac/700 Vdc, • Up to 2000 Vac/1000 Vdc,
6 to 710 A 1 to 600 A 10 to 5500 A 5 to 100 A
2 Eaton.com/bussmannseries
A: Introduction
About this guide

This guide’s objective is to provide engineers easy access to Bussmann
series high speed fuse data. It also provides detailed information on the
Bussmann series high speed fuse reference system. The various physical
standards are covered with examples of applications along with the
considerations for selecting rated voltage, rated current and similar data
for protecting power semiconductors. Guidelines for fuse mounting is
covered, with explanations on how to read and understand product data
sheets and drawings. End plate
Screw
This document is not a complete guide for protecting all power
semiconductor applications. The market is simply too complex to make
such a document, and, in some cases, the actual fuse selection will
require detailed technical discussions between the design engineers
specifying the equipment and Application Engineering personnel. Ceramic body
Regardless, the data presented here will be of help in daily work and
Element reduced
provide the reader with the basic knowledge of our products and their
sections or
application.
“necks”/“weak spots”
Background Element
The fuse has been around since the earliest days of the telegraph and End fitting
later for protecting power distribution and other circuits.
The fuse has undergone considerable evolution since those early days.
The modern High Breaking Capacity (HBC)/high interrupting rating fuse
provides economical and reliable protection against overload and fault
currents in modern electrical systems.
Basic fuse operation is simple: excess current passes through specially
designed fuse elements causing them to melt and open, thus isolating
the overloaded or faulted circuit. Fuses now exist for many applications
with current ratings of only a few milliamps to many thousands of amps, Engineered plastic and
and for use in circuits of a few volts to 72 kV utility distribution systems. glass fiber body
The most common use for fuses is in electrical distribution systems

where they are placed throughout the system to protect cables,
transformers, switches, control gear and equipment. Along with different End connector
current and voltage ratings, fuse operating characteristics are varied to
meet specific application areas and unique protection requirements. Element
The definitions on how fuses are designed for a certain purpose (fuse
class) are included in the glossary.
Typical fuse construction

Modern high speed fuses are made in many shapes and sizes (Figure
A1), but all have the same key features. Although all fuse components
influence the total fuse operation and performance characteristics, the
key part is the fuse element. This is made from a high conductivity
material and is designed with a number of reduced sections commonly
referred to as “necks” or “weak spots.” It is these reduced sections that Outer end cap
will mainly control the fuse’s operating characteristics.
The element is surrounded with an arc-quenching material, usually End connector
Element
graded quartz, that “quenches” the arc that forms when the reduced
sections melt and “burn back” to open the circuit. It is this function that
gives the fuse its current-limiting ability.
To contain the quartz arc-quenching material, an insulated container Ceramic body
(commonly called the fuse body) is made of ceramic or engineered
plastic. Finally, to connect the fuse element to the circuit it protects
there are end connectors, usually made of copper. The other fuse Inner end cap
components vary depending on the type of fuse and the manufacturing Gasket
methods employed.
Figure A1. Typical square body and round body high speed fuse
constructions.
Eaton.com/bussmannseries 3
A: Introduction
Fuse operation
Peak fault current reached
Fuse operation depends primarily on the balance between the rate of at start of arcing Possible, unrestricted
heat generated within the element and the rate of heat dissipated to fault current
external connections and surrounding atmosphere. For current values
up to the fuse’s continuous current rating, its design ensures that all the Actual current
heat generated is dissipated without exceeding the pre-set maximum
temperatures of the element or other components.
Under conditions of sustained overloads, the rate of heat generated Start of fault
is greater than that dissipated, causing the fuse element temperature
to rise. The temperature rise at the reduced sections of the elements
(“necks” or “weak spots”) will be higher than elsewhere, and once
the temperature reaches the element material melting point it will start
arcing and “burn back” until the circuit is opened. The time it takes for
T
Pre-arcing time Arcing
the element to melt and open decreases with increasing current levels. time
The current level that causes the fuse to operate in a time of four hours Total clearing time
is called the continuous current rating, and the ratio of minimum fusing
current to the rated current is called the fusing factor of that fuse. Under
higher overloading, or short-circuit conditions, there is little time for heat Figure A2. Pre-arcing time plus arcing time equals total clearing
dissipation from the element, and the temperature at the element’s time.
reduced sections (necks) reach the melting point almost instantaneously. Transistors [IGBTs]) have found an increasing number of applications
Under these conditions, the element will commence melting well before in power and control circuit rectification, inversion and regulation.
the prospective fault current (AC) has reached its first major peak. Their advantage is the ability to handle considerable power in a very
The time taken from the initiation of the short-circuit to the element small physical size. Due to their relatively small mass, their capacity to
melting is called the pre-arcing time. This interruption of a higher current withstand overloads and overvoltages is limited and thus require special
results in an arc being formed at each reduced section with the arc protection means.
offering a higher resistance. The heat of the arcs vaporize the element In industrial applications, fault currents of many thousands of amps
material; the vapor combines with the quartz filler material to form a non- occur if a short-circuit develops somewhere in the circuit. Semiconductor
conductive, rock-like substance called fulgurite. The arcs also burn the devices can withstand these high currents for only an extremely
element away from the reduced sections to increase the arc length and short period of time. High current levels cause two harmful effects on
further increase the arc resistance. semiconductor devices.
The cumulative effect is that the arcs are extinguished in a very short First, non-uniform current distribution at the p-n junction(s) of the silicon
time along with the complete isolation of the circuit. Under such heavy creates abnormal current densities and causes damage.
overload and short-circuit conditions the total time taken from initiation
of fault to the final isolation of the circuit is very short, typically in a few Second, a thermal effect is created that’s proportional to the RMS
milliseconds. Therefore, current through the fuse has been limited. Such current squared (I²) multiplied by the amount of time (t) that the current
current limitation is obtained at current levels as low as four (4) times the flows expressed as either I2t or A2s (amps squared second).
normal continuous current rating of the fuse. As a result, the overcurrent protective device must:
The time taken from the initiation of the arcs to their being extinguished • Safely interrupt very high prospective fault currents in extremely short
is called the arcing time. The sum of the pre-arcing and arcing time is times
the total clearing time (see Figure A2). During the pre-arcing and the
arcing times a certain amount of energy will be released depending on • Limit the current allowed to pass through to the protected device
the magnitude of the current. The terms pre-arcing energy and arcing • Limit the thermal energy (I²t) let-through to the device during fault
energy are similarly used to correspond to the times. Such energy will interruption
be proportional to the integral of the square of the current multiplied by
the time the current flows, and often abbreviated as I2t, where “I” is the
RMS value of the prospective current and “t” is the time in seconds for Unfortunately, ultra-fast interruption of large currents also creates high
which the current flows. overvoltages. If a silicon rectifier is subjected to these high voltages,
it will fail due to breakdown phenomena. The overcurrent protective
For high current values, the pre-arcing time is too short for heat to be device selected must, therefore, also limit the overvoltage during fault
lost from the reduced section (is adiabatic) and pre-arcing I2t is therefore interruption.
a constant. The arcing I2t, however, also depends on circuit conditions.
The published data is based on the worst possible conditions and is So far, consideration has mainly been given to protection from high fault
measured from actual tests. These will be covered in detail later. currents. In order to obtain maximum utilization of the protected device,
coupled with complete reliability, the selected overcurrent protective
The arcing causes a voltage across the fuse element reduced sections device must also:
(necks) and is termed the arc voltage. Although this depends on the
element design, it is also governed by circuit conditions. This arc voltage • Not require maintenance
will exceed the system voltage. The design of the element allows the • Not operate at normal rated current or during normal transient overload
magnitude of the arc voltage to be controlled to known limits. The use of conditions
a number of reduced sections (necks) in the element, in series, assists in
controlling the arcing process and also the resulting arc voltage. • Operate in a predetermined manner when abnormal conditions occur
Thus, a well-designed fuse not only limits the peak fault current level,
but also ensures the fault is cleared in an extremely short time and the The only overcurrent protective device with all these qualities (and
energy reaching the protected equipment is considerably smaller than available at an economical cost) is the modern high speed fuse (also
what’s available. commonly referred to as a “semiconductor fuse” or “I2t fuse”).
While branch circuit and supplemental fuses posses all the qualities
Power semiconductors mentioned above (with the exception of special UL Class J high speed
Silicon-based power semiconductor devices (diodes, thyristors, Gate fuses), they do not operate fast enough to protect semiconductor
Turn-Off thyristors [GTOs], transistors and Insulated Gate Bipolar devices.
B: High speed fuse characteristics
How high speed fuses are different Application factors

High speed fuses are specially designed to minimize the I²t, peak Protecting semiconductors requires considering a number of device
current let-through and arc voltage. Ensuring fast opening and clearing and fuse parameters. And there are a number of influencing factors
of a fault requires rapid element melting. To achieve this, the high associated with each parameter (see Table B1). The manner in which
speed fuse element has reduced sections (necks) of a different design these are presented and interpreted will be covered in the following
than a similarly rated industrial fuse and typically have higher operating pages. These parameters and influencing factors need to be applied
temperatures. and considered with due reference to the specific requirements of the
circuit and application. These are covered in the sections on selecting the
As a result of their higher element temperatures and smaller packages,
voltage rating, current rating and applications.
high speed fuses typically have higher heat dissipation requirements
than other fuse types. To help dissipate heat, the body (or barrel) material
used is often a higher grade with a higher degree of thermal conductivity.
High speed fuses are primarily for protecting semiconductors from
short-circuits. Their high operating temperatures often restrict using
element alloys with a lower melting temperature to assist with overload
operation. The result is that high speed fuses are generally not “full
range” (operate on short-circuit and overload conditions) and have more
limited capability to protect against low-level overcurrent conditions.
Many high speed fuses are physically different from branch circuit and
supplemental fuse types, and require additional mounting arrangements
to help prevent installing an incorrect replacement fuse.
Table B1. Factors to consider in high speed fuse selection.

Factors affecting parameter Data provided
Parameter Fuse Diode or thyristor* Fuse Diode or thyristor*
Maximum rated current
Ambient temperature, Ambient temperature, type under specified conditions, Comprehensive curves
attachment, proximity of
Steady state RMS current of circuit, parallel operation, factors for ambient, (mean currents generally
other apparatus and other cooling employed up-rating for forced cooling, quoted)
fuses, cooling employed conductor size
Watts dissipated for steady Maximum quoted for
Function of current Function of current Comprehensive data
state specified conditions
Nominal time/current
Pre-loading, cyclic loading Overload curves, also
Pre-loading, cyclic loading curves for initially cold fuses
Overload capability surges, manufacturing transient thermal
surges – calculation guidelines for
tolerances impedances
duty cycles
AC or DC voltage/short-
Interrupting capacity — Interrupting rating —
circuit levels
Pre-loading; total I²t For initially cold fuses: total
dependent on: circuit I2t curves for worst case Half cycle value or values
I²t ratings impedance, applied voltage, Pre-loading fault duration conditions, pre-arcing I²t for different pulse duration
point of initiation of short- constant fuse clearing time
circuit
Pre-loading; fault current Curves for worst conditions
Peak let-through current (voltage second order Pre-loading fault duration Peak current for fusing
for initially cold fuse-links
effect)
Peak value dependent on:
Peak inverse voltage ratings Maximum peak arc voltages Peak inverse voltage rating
applied voltage, circuit
Arc voltage plotted against applied
impedance, point of (non- repetitive) quoted (non-repetitive)
voltage
initiation of short-circuit
* The protection of transistors is more complex and will be described in the section on IGBT protection.
What is this symbol?
The term “semiconductor fuse” used for high speed fuses is

misleading. Although high speed fuses often display a fuse and diode
symbol on their label (like the one above), there is no semiconductor
material in their construction. The symbol on their label is there solely
to denote their application is for protecting “semiconductor” devices.
B: High speed fuse characteristics
Influencing factors
Ambient temperature Short-circuit performance

Fuses protecting semiconductors may need derating for ambient The fuse’s short-circuit operation zone is usually taken as operating
temperatures above or below 21°C (70°F). Adjusted fuse ratings at other times less than 10 ms (1/2 cycle on 60 Hz supply in AC circuits). It’s
ambient temperatures can be found using derating graphs. in this short-circuit operation zone that high speed fuses are current
Factors affecting ambient temperature include poor fuse mounting, limiting. Since the majority of high speed fuse applications are on AC
enclosure type and proximity to other heat-generating devices and fuses. circuits, their performance data are usually given for AC operation. Where
The maximum high speed fuse rating should be determined for each applicable, prospective RMS symmetrical currents are used.
application using the ambient temperature of the fuse’s installed location
as described in the section on selecting the current rating. I²t ratings
The pre-arcing (melting) I²t tends to be a minimum value when the fuse
Fuse operating temperatures is subjected to high currents (this value is shown in the data sheet). The
Operating temperatures vary by fuse construction and materials. Fiber total clearing I²t varies with applied voltage, available fault current, power
tube fuses tend to run hotter than ceramic body fuses. Generally, for factor and the point on the AC wave when the short circuit initiates. The
fuses with a ceramic body that are fully loaded under IEC conditions, total clearing I²t values shown are for the worst of these conditions.
the temperature rise lies from 70-110°C (158-230°F) on the terminals and The majority of power semiconductor manufacturers give I²t ratings that
from 90-130°C (194-266°F) on the ceramic body. The fuse load constant should not be exceeded for their product during fusing at all times below
for porcelain body fuses is normally 1.0 and with fiber body fuses the 10 ms. These are statistically the lowest values the device has been
factor is normally 0.8. Keep in mind that temperature measurements can tested to.
be misleading when determining whether a particular fuse is suitable for
a given application. For details, see the chapter Determining fuse amp For effective device protection, the total I²t value of the fuse must be
ratings starting on page 11. less than the I²t capability of the device.
Forced cooling Peak fuse currents

To maximize ratings in many installations, diodes or thyristors are force Under short-circuit conditions, high speed fuses are inherently current
cooled by an air stream. Fuses can be similarly uprated if placed in an limiting (the peak let-through current through the fuse is less than
air stream. However, air velocities above 5 m/s (16.5 ft/s) do not provide the peak short-circuit current). The “cut-off” characteristic, (the peak
any substantial increase in the ratings. For further information see the let-through current against prospective RMS symmetrical current)
sections on selecting rated current and data sheets. are shown in the data sheets. Peak let-through currents should be
coordinated with diode or thyristor data in addition to I²t values.
Mean, peak and RMS currents
Arc voltage
Care must be taken in coordinating fuse currents with the circuit
currents. Fuse currents are usually expressed in “Root-Mean Square” The arc voltage produced during fuse opening varies with the applied
(RMS) values, while diodes and thyristors currents are expressed in system voltage. Curves showing variations of arc voltage versus
“mean” values. system voltage are included in the data sheets. Care must be taken in
coordinating the peak arc voltage of the fuse with the semiconductor
Time-current characteristics device’s peak transient voltage limit.
These are the time and current levels needed for a fuse element to melt Conductor size
and open. They are derived using the same test arrangement as the
temperature rise test, with the fuse at ambient temperature before each The RMS current ratings assigned to Bussmann series fuses are based
test. For branch circuit and supplemental fuses, the nominal melting upon standard sized conductors at each end of the fuse during rating
times are plotted against RMS current values down to 10 ms. For high tests. These are based on a current density between 1 and 1.6 A/mm².
speed fuses, the virtual melting time (tv) is used and plotted down to 0.1 Using smaller or larger conductors will affect the fuse’s current rating.
ms. The formula for determining virtual melting time can be found in the
glossary. Package protection
The melting time plus arcing time is called total clearing time, and for Some semiconductor devices are so sensitive to overcurrents and
long melting times the arcing time is negligible. overvoltages that high speed fuses may not operate fast enough to
prevent some or complete damage to the protected device. Regardless,
Cyclic loading/surges high speed fuses are still employed in such cases to minimize the affects
of overcurrent events when the silicon or small connection wires melt.
Effects of cyclic loading, or transient surges, can be taken into account Without using high speed fuses, the packaging surrounding the silicon
by coordinating the effective RMS current values and surge durations may open, with the potential to damage equipment or injure personnel.
with the time-current characteristics. The following conditions should be
accounted for when using published characteristics:
• They are subject to a 10 percent (10%) tolerance on current
• For times below one second, circuit constants and instants of fault
occurrence affect the time-current characteristics. Minimum nominal
times are published according to symmetrical RMS currents.
• Pre-loading at maximum current rating reduces the actual melting
time. Cyclic conditions are detailed in the section on selecting rated
current.
C: Fuse performance data
High speed fuse performance data can be found in various curves and
documents. This information is generally presented in what is called
a data sheet, or spec sheet. The following is a synopsis of what they
contain.
The time-current characteristic curve

The time-current curve, also called the TCC curve, provides vital
information for the selection and determination phase. See Figure C1.
The horizontal axis represents the prospective short-circuit current (Ip)
Virtual pre-arcing time in seconds

A A
in RMS symmetrical amps. The vertical axis represents virtual pre-arcing
time (tv) in seconds, as specified in IEC 60269. The melting time of a
given fuse can be found based on a known available fault current value.
In practice, virtual times longer than 10 ms are equivalent to real time (tr)
where times that are below this value are based upon an instantaneous,
adiabatic fuse interruption derived from minimum pre-arcing values
discussed later in this guide.
It is at these virtual times, using Ip and tv directly from the fuse time-
current curve, that permits calculating its melting integral (Ip² x tv) for the
actual value of prospective current (see Figure C1). The following method Irms1 Irms2
shows two examples (I1 and I2) with guidelines to determine the effect
on a fuse from an overload or short-circuit: Melting point
• First, the actual overload/fault current must be known, either in the
form of a curve (Figure C2, I1=f(tr) and I2=f(tr)), or from Equation C1:
i2dt = Ip2 x tv
tv
t1
i2 dt
0
IRMS (t1) =
t1 Prospective current in RMS symmetrical amps
Figure C1. Time-current curves.
Equation C1. Overload/fault current.

• Calculate the RMS current over time. The RMS value at a given time I
is determined using the formula above. (RMS symmetrical currents for
standard sine waves will be Ipeak/√2).
• Plot the RMS current values as coordinates IRMS, tr onto the fuse time-
current curve as shown in Figure C1
• If the plotted curve crosses the fuse’s melting curve (IRMS in Figure I2 = f(tr)
2
C1), the fuse melts at the time which can be found at the crossing
point in real time (tr)
IRMS2 = f(tr)
If the plotted curve does not cross the fuse’s melting curve (IRMS1 in
Figure C1), the fuse will not open.
In this case, the minimum horizontal distance (expressed in %It)
between the plotted curve and the fuse’s melting curve indicates how
well the fuse will perform when encountering a given overload.
IRMS1 = f(tr)
The above method, together with the guidelines given on overloads
in the section selecting rated current, will determine if the fuse can
withstand the type of overload in question. I1 = f(tr)
This can be done even if the axes of the melting curve are in Ip and tv. It tr
can be shown that a relabeling of the axes designation: Ip = >IRMS and 2 3 4 5
1
tv = >tr can be done without changing the shape of the melting curve.
Figure C2. Overload and fault current curves.
C: Fuse performance data
The A-A curve
1.1
IN X
4
10 1.0
0.9
103 0.8
0.7
0.1 0.2 0.3 0.4 0.5 Cos ϕ
2 62°
10
Figure C5. X factor curve.
A A The fuse I²tcl (based upon 20°C/68°F ambient) should be compared
1
IP x 0.9 with the equivalent 10 ms fusing integral I²t-scr of the semiconductor
10 (normally given at 125°C/257°F) to see if protection is ensured. And even
if I²tcl = I²t-scr, a reasonable safety margin can be expected (cold fuse
versus warm SCR). If the fuse is clearing at a lower voltage than stated
above and at a different power factor, then two correction factors should
IP
1s be used in conjunction with the given I²tcl.
Figure C3. A-A curve. The resultant clearing integral will be equal to:
As a part of the melting curve for Type aR fuses only, an “A-A curve” plot I²tcl x K x X
is given. Melting or loading beyond this point in the melting curve is not
(Factors K and X can be found in Figures C4 and C5)
allowed. This is due to the thermal overload risk that might reduce the
fuse’s interrupting capacity and won’t operate in the A-A zone. The I²t-scr of the device should be compared with this result.
Often, the A-A curve is plotted only by a horizontal line. In order to plot
The I²t curve
the complete A-A curve for a given fuse, the following guidelines should
be observed:
• The prospective short-circuit current (Ip) found for the time equal to
the intersection between the A-A curve and the actual melting curve
should be multiplied by 0.9 (Ip × 0.9) and this point is marked on the
A-A curve (Figure C3)
• From here can be drawn a straight line at sixty two degrees (62°) from
I2t
the A-A curve and melting curve intersection to where the fuse’s rated 100MA2s
current (IN) vertical line is plotted I2t – Clearing = f(Ip)
This completes the A-A curve (Note 62° is only valid if the graph decade
relation is 1:2, which is typical for IEC standard fuses, as opposed to a
1:1 decade relation, which is common for North American fuses). 10ms 7ms 3ms at 900V Ip
Clearing integral information Figure C6. I2t curve.
An I²t curve may also be presented (or available on request). It shows
1.5 the clearing I²t and time as a function of the prospective short-circuit
K
1.0 current for a given system voltage (Figure C6). This can ease the
selectivity coordination between the fuse and the semiconductor to be
protected or other devices in the short-circuit path.
0.5
0.4 Peak let-through
0.3
0.2
Eg 105
100 200 300 400 500 600 660

Peak let-through current
Figure C4. K factor curve.

104
Normally the maximum I²t under short-circuit conditions will be the

10 ms clearing integral I²tcl of the fuse, which is given at the applied
working voltage (Eg) equal to the fuse’s voltage rating (UN) at power-
factor of cos j = 0.15 and at a short-circuit level of 10–15 times the rated 103
current.
Non current Current
limiting limiting
2x102
102 103 104 105 106
Prospective current in amps RMS
Figure C7. Peak let-through curve.
C: Fuse performance data D: Determining fuse voltage ratings
High speed fuses, by their design and purpose, are current-limiting The fuse voltage rating is the maximum AC, DC or AC/DC voltages it is
devices. This means they will reduce the prospective short-circuit designed for. Most commercial fuses are rated for AC RMS voltages
current, and destructive thermal and mechanical forces in equipment to (45-62 Hz), unless otherwise stated on the fuse label.
an acceptable level if a short-circuit should occur. In practice the short-
For proper application, the fuse voltage rating must be less than or
circuit current is given as the symmetrical RMS value of the available
equal to the system voltage. All Bussmann series high speed fuses are
fault current, called Ip. The actual maximum peak (asymmetrical) current
designed to the UL 248-13, IEC 60269 1 and 4, or the BS88 standards.
depends on the circuit’s power factor. For P.F. = cos j = 1.0 to 0.15, or
This allows designers to select a high speed fuse that can be used
100 percent to 15 percent the peak value will lie between:
anywhere around the world.
√2 × Ip and up to 2.3 × Ip
IEC voltage rating
From the peak let-through curve in Figure C7, it can be seen that a
certain magnitude of IP, relative to the fuse’s IN is needed before the IEC requires AC voltage tests to be performed at 110 percent of the
current-limiting effect will take place. rated voltage (with the exception of 105 percent for 690 V), with power
factors between 10 and 20 percent.
The arc voltage curve
This enables the fuse to be used at rated voltage virtually anywhere
without fear of exceeding the maximum tolerances of the test
1.4 conditions. The extra percentages take into account supply voltage
1.2 fluctuations found in some converters.
103
North American voltage rating
8 A North American voltage rating requires that all fuses be tested at their
7 nominal RMS rated voltage only, with power factors between 15 and 20
6 percent. In many instances, a fuse is chosen with a voltage rating well
5 above the system voltage.
4
Under some circuit conditions, there can be normal circuit fluctuations of
±10 percent. It is a good practice to be aware of this when investigating
3
North American style fuses as they are not tested at any voltages above
200 300 400 500 600 660 their rating.
Simple rated voltage determination

Figure C8. Arc voltage curve.
In most converter circuits, the amount and nature of the voltage rating is
The peak arc voltage of the fuse and peak reverse voltage of the evident and the voltage selection can be made right away.
semiconductor should always be coordinated.
Generally, a single fuse on its own should be able to open and clear
An arc voltage is generated due to the specially designed element against the maximum system voltage. If two fuses are applied in series
restrictions (necks) that are packed in arc-quenching sand. This forces the in the same short-circuit path, each fuse must be rated at the system
current to zero during the arcing time and finally, isolation is established. voltage.
This permanent isolation is built up at the restriction sites that are
converted into fulgurite, a composition of metal and sand made during Frequency dependency
the arcing process.
For a given fuse voltage rating, the peak arc voltage UL depends mainly
on the applied working voltage level Eg in RMS, according to Figure C8.
% of rated voltage @ 50 Hz
Watt loss correction curve
1.0
Kp
0.8
0.7
0.6
0.5
0.4
0.3
Voltage frequency (Hz)

0.2
0.15
Ib
IN
0.1 Figure D1. Rated voltage vs. frequency.
30 40 50 60 70 80 90 100%
The stated, AC rated voltage of Bussmann series high speed fuses are
Figure C9. Watts loss correction curve. valid at frequencies from 45 to 1000 Hz. Below 45 Hz please refer to
Figure D1. The interrupting process at lower AC frequencies (beyond the
The rated watt loss is given for each fuse under specified conditions. To scale of Figure D1) tends to behave more like DC voltage and the voltage
calculate the loss at a load current lower than rated current, the rated rating should conform with what is described in DC Applications in this
watt loss is to be multiplied by correction factor Kp. This factor is given guide.
as a function of the RMS load current Ib, in percent of the rated current,
see Figure C9.
D: Determining fuse voltage ratings
Possible AC/DC combinations AC fuses can be used for the protection and isolation of GTOs and IGBTs
on the DC side of voltage commutated inverters (Figure D4).
In case of a DC shoot-through with a very high di/dt of short-circuit
current, it may be possible for the DC rating to be greater than the AC
voltage rating (either IEC or UL).
- For further information, please contact Application Engineering at
FuseTech@eaton.com.
UAC UDC
+ Fuses in series
It is not common to connect fuses directly in series. Under low
overcurrent conditions, only a small variation in fuse performance would
cause one of the fuses to open before the other and thus the opening
fuse should be capable of clearing the full system voltage. Under higher
Figure D2. Six-pulse bridge circuit. fault currents both fuses will open, but it is unlikely the voltage will be
Even in relatively simple converters like the six-pulse bridge, etc. (see shared equally. Therefore, if fuses are connected in series the following
Figure D2) there is the possibility that the fuse’s rated voltage is required should be observed:
to be much higher than the AC supply voltage itself. • Fault currents sufficient to cause melting times of 10 ms or less
This is so, because the converter is regenerative (it is able to return should always be available
energy to the supply). Here, in case of a commutation fault, the AC • The voltage rating of each fuse (UN) should be at least 70 percent of
supply voltage UAC and the output DC voltage will be superimposed. To the system voltage
withstand this increased voltage, the rated voltage UN of the fuse must
be: • If the available fault current can only produce melting times more than
10 ms, then the voltage rating of the fuse must, at a minimum, be the
UN >= 1.8 × UAC same as the applied voltage
For further details please refer to section Fuses protecting regenerative

drives starting on page 20.
AC fuses in DC circuits
+U
-
DC
Figure D3. AC fuses in a DC motor drive circuit.

If AC fuses are used in DC motor and drive circuits, the selection
process becomes more complex (Figure D3).
The determining parameters will be the system DC voltage, the
minimum short-circuit current and the associated maximum time
constant (L/R).
For details, refer to the section on AC fuses in DC circuit applications
starting on page 18.
Fuses under oscillating DC
UDC +
-
Figure D4. AC fuses protecting GTOs and IGBTs on the DC side of

voltage commutated inverters.
E: Determining fuse amp ratings
The fuse’s rated amperage is the RMS current it can continuously carry If two connections are not equal, the equivalent Ke factor can be found
without degrading or exceeding the applicable temperature rise limits using the following formula:
under well defined and steady-state conditions. This is in contrast to
semiconductors, whose rated current is given as a mean or average
value. Many conditions can effect the fuse‘s current carrying capability.
Ke =

(K e1
+ Ke2
2 )
To prevent premature fuse aging, following Parts 1, 2 and 3 below will Where:
allow the rated current selection to be on the safe side.
Ke1 = Thermal correction factor for busbar 1
Part 1 — Basic selection ke2 = Thermal correction factor for busbar 2
This covers the basic selection criteria for only the fuse’s rated amperage Fuse mounting inside an enclosure will reduce the convection cooling
and not the influence from overload and cyclic loading. The actual RMS compared with the IEC test conditions. An additional Ke thermal
steady-state load current passing through the fuse should be lower or connection factor should be chosen here based on judgement. Often,
equal to the calculated maximum permissible load current called Ib. enclosure mounted fuses are given an additional Ke factor of 0.8.
Ib = In × Kt × Ke × Kv × Kf × Ka × Kb
Voltage frequency
Where: Fuses under high frequency loads (like in voltage commutated inverters)
Ib = Maximum permissible continuous RMS load current* call for special attention. At higher frequencies, the fuse’s current carrying
In = Rated current of a given fuse capability can be reduced due to the imposed skin and proximity effect
Kt = Ambient temperature correction factor (Figure E1) on the current-carrying elements inside the fuse. Using the curve given in
Figure E4 normally ensures a sufficient margin (Kf).
Ke = Thermal connection factor (Figure E2)
Kv = Cooling air correction factor (Figure E3) High altitude
Kf = Frequency correction factor (Figure E4)
When fuses are used at high altitudes, the atmosphere’s lower density
Ka = High altitude correction factor (Equation 1) reduces the cooling effect on the fuse. An altitude correction factor (Ka)
Kb = Fuse load constant. (Normally 1.0 for porcelain body fuses and 0.8 should be applied to the fuse’s continuous rating when used above 2000
for fiber body fuses.) m. The correction factor Ka can be determined using Equation E1:
* NB: For any periods of 10 minutes duration or more the RMS value of the load
current should not exceed this. Equation E1
In case of water cooled fuse terminals, please consult Application
Engineering at FuseTech@eaton.com.
I
Ka = =

1-
In 100 100 ( (
h-2000 x 0.5
))
Where:
Busbar current density
I = Current rating at high altitude
The nominal busbar current density on which the fuses are mounted
should be 1.3 A/mm2 (IEC 60269 Part 4 defines 1.0 to 1.6/mm2). If the In = The fuse’s rated current
busbar carries a current density more than this, then the fuse should be h = Altitude in meters
derated. Figure E2 shows the thermal correction factor (Ke).
Kt Kv
1.4 1.30
1.3
1.2 1.25
1.1 1.20
1.0 1.15
0.9
0.8 1.10
0.7 1.05
0.6 1
0.5
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6
Ambient temperature in °C Meter per second (m/sec)
Figure E1. Ambient temperature correction factor. Figure E3. Forced air cooling correction factor.
This curve shows the influence of the ambient temperature on the fuse’s The curve shows the influence of forced air cooling on the fuse.
current-carrying capability.
% of 50Hz
load current Kf
120
100
80
60
100 1000 1 0 ,0 0 0
Frequency in Hz
Percentage of the recommended busbar size
(max permissible load current)
(100% = 1.3 A/mm2)
Figure E2. Thermal connection factor. Figure E4. Voltage frequency correction factor.
Example 1 The percentage factor for each overload should be checked against
the melting curve of the selected fuse in question, based upon the
A 200 A porcelain square body fuse is applied at an ambient temperature
guidelines in Part 1.
of 40°C/104°F, and wired with cables having a 120 mm2 cross section.
Forced air cooling is applied at a rate of 4 m/s. The load current There is a grey area between a sole overload and a pure cyclic load
frequency is 3000 Hz. situation. In particular, the three examples shown in Figure E5 are typical
of this dilemma and for safety, treat this example like a cyclic load based
What is the maximum allowed steady-state RMS current Ib?
upon the guidelines in Part 3 of this section.
To accurately estimate the correct permissible load of the square body Table E1. Influence of overloads.
fuse it is necessary to evaluate each correction factor to the application.
Frequency of occurrence Overloads (>1 sec) Impulse loads (<1 sec)
From the current determining formula given, and the correction factors
shown in Figures E1 through E4, we have: Less than one time per month Imax <80% × It Imax <70% × It
Ib = In x Kt x Ke x Kv x Kf x Ka x Kb Less than twice per week Imax <70% × It Imax <60% × It
Where: Several times a day Imax <60% × It Imax <50% × It
In = 200 A
Kt = 0.9 for 40C° ambient (Figure E1)
Ke = 0.98 at 78% (Figure E2) Electrochemical processes, etc.
Irms
Current density = 200 A/120 mm2 In
= 1.54 A/mm2
150% : 1 min
% Density = 1.3/1.54
= 78%
100%
Kv = 1.2 for 4 m/s forced air cooling (Figure E3)
Kf = 0.85 for a frequency of 3000 Hz (Figure E4) 0 24h T
Ka = 1, at sea level, below 2000 meters (Equation E1)
Kb = 1.0 porcelain body fuse load constant Light industrial and light traction substation service
Irms
That results in: In 200% : 10 s
Ib = 200 x 0.9 x 0.98 x 1.2 x 0.85 x 1 x 1
150% : 1 min
= 180 A RMS
In other words the 200 A fuse should only be subjected to a maximum 100% 70%
180 A RMS under the described steady-state conditions.
0 6h 24h T
Checking permissible load current
Industrial service, heavy duty
A fuse’s maximum permissible steady-state load current (Ib) can be Irms
checked by making simple voltage measurements under actual operating In 200% : 10 s
conditions. This should be done after the fuse is installed in its operating
location and loaded at the calculated Ib value:
E2/E1 × (0.92 + 0.004 × Ta) ≤ N
100% 125% 70%
Where:
E1 = Voltage drop across fuse after 5 seconds 0 6 8 14 16 24h T
E2 = Voltage drop across fuse after 2 hours

Ta = Air temperature at start of test in C° Figure E5. Typical load cycles.
N = Constant (if available, from data sheet, normally 1.5 or 1.6) Example 2
A 200 A fuse has been selected, but is subjected to temporary overloads
Part 2 — Influence of overloads of 300 amps for 5 seconds, and these overloads occur three to five
The maximum overload current Imax that can be imposed on the fuse times a day. From the time-current curve of the fuse found on Figure
found under Example 1 depends upon the duration and frequency of E8, we find the melting current (It) corresponding to the time (t = 5) in
occurrence. seconds of overload duration to be, It = 750 A.
Time durations fall into two categories: From Table E1, the formula for the maximum overload current (Imax) can
be found based on an overload occurring three to five times per day for a
1. Overloads longer than one second duration of 5 seconds and is shown below. Applying the melting current
2. Overloads less than one second (termed impulse loads) (It) of the 200 A fuse will yield the following maximum overload current:
Table E1 gives general application guidelines. In the expression Imax = < 60% x It
Imax < ( percent factor) × It, It is the melting current corresponding to the
< 60% x 750 A
time t of the overload duration as read from the time-current curve of the
fuse. The limits given permit the determination of Imax for a given fuse < 450 A
rating or, conversely, the fuse current rating required for a given overload,
This means that temporary overloads of up to 450 A can be withstood
expressed by:
and the 200 A fuse selected will work in this application.
Imax < (percent factor) × It
Typical examples of load cycles including overload currents are given in
Figure E5.
Part 3 — Cyclic loading and safety margins

Cyclic loading that leads to premature fuse fatigue is defined as regular Duty Class I
or irregular load current variations, each of a sufficient magnitude and G = 1.5
Irms 60 s 15 min
duration to change the temperature of the fuse elements in such a way In
that the very sensitive restrictions (necks) will fatigue. In order to avoid
this condition, calculations can be made to ensure there is an appropriate 100% 150%
safety margin for the selected fuse.
While using the following empirical rules will cover most cyclic loading T
conditions, it is impossible to set up general rules for all applications. Duty Class II
For applications not covered in this section, please contact Application G = 1.6
Engineering at FuseTech@eaton.com. Irms 120 s 15 min
In
Rule 1: Ib > Irms x G
150% 100%
Where:
Ib = The maximum permissible load current based upon the criteria Duty Class III T
presented in Part 1 “Basic selection” G = 1.8
Irms 15 min
IRMS = the RMS value of the cyclic loading condition In
200% 10 sec
G = Cyclic load factor (for most cases a sufficient margin is assured
by using 1.6)
100%
Some cyclic load factors G can be found from the example profiles in
Figure E7, or can be provided upon request. T
Duty Class IV
The required rating for the fuse can, therefore, be found using the G = 1.3
following formula: Irms
IRMS x G In 15 min
In ≥ 120 min.
Kt x Ke x Kv x Kf x Ka x Kb
125% 100%
Rule 2: Ipulse < It x B
Once a fuse has been selected using the above criteria, a check is T
required to see if the individual cyclic load pulses (each expressed in Medium traction substations
Ipulse, tpulse coordinates) have a sufficient safety margin in relation to and mining
G = 2.0
the fuse’s melting current at each pulse duration. It is the fuse’s melting Ib (p.u.) t (min.)
Irms
current corresponding to each pulse (t = tpulse) duration, and the cyclic 0h - 2h 1.3 10
In 200% : 30 s
pulse factor B can be found in Figure E6 for a period (T) of a cyclic 2h - 10h 0.8 15
loading condition. 10h - 12h 1.3 10
12h - 24h 0.7 30
This should ensure a sufficient fuse life when subject to the cyclic Ib:t 150% : 90 s
loadings encountered in an application.
0 T
0.7
0.6 Figure E7. Cyclic loading profile examples and duty class.
0.5
B factor
Example 3
0.4 For a 200 A fuse, there is cyclic loading of 150 A for two minutes
followed by 100 A for 15 minutes.
0.3
This requires a cyclic load factor of G = 1.6 from the example profiles in
0.2 Figure E7. The RMS-value of the cyclic load for a period of T = 17 minutes
1 10 100 1000 is determined by the RMS formula below and expressed as:
Time in minutes
(1502 x 2) + (1002 x 15) ≈ 107Arms
Figure E6. Cyclic pulse factor B. 17
Assuming there aren’t any fuse current derating factors (i.e., Kt x Ke x

Kv x Kf x Ka x Kb = 1), the maximum permissible load current (Ib) for the
fuse’s 200 A rating will be:
I b > Irms x G
> 107 x 1.6
> 171 A
E: Determining fuse amp ratings F: High speed fuse applications
While a 200 A fuse may be sufficient, a safety factor check (B) is needed Power semiconductors protected by high speed fuses are used in many
to ensure that the pulse keeps a sufficient safety distance from the applications such as AC drives, DC drives, traction, soft starters, solid
fuse’s melting curve. This is obtained from the Rule 2 Ipulse equation in state relays, electrolysis, induction furnaces and inverters. The power
Part 3, using Figure E6 for a given total time period T = 17 minutes, then source for these may be supplied by the grid, local generator or batteries.
B = 0.32.
The circuit configurations for these applications vary a lot. Some of the
Given a tpulse of two minutes for the cyclic loading condition, It = 440 A most typical circuits are illustrated on the following page along with
can be found from the time current curve for the 200 A fuse (Figure E8). information on how to find relevant RMS and load current levels for the
fuse installation.
Ipulse < It x B
All of these circuit examples may operate at just a few amps or at many
< 440 A x 0.32
thousands of amps. Regardless, the circuit operating principles are
< 141 A (150 A requirement not met!) usually the same. However, the protection levels involved depend on
multiple needs including protection against:
The result of less than 141 A concludes that the application Ipulse of 150 A
exceeds the fuse’s melting curve and a higher, 250 A fuse rating should • Accidents
be selected. • Injuries to personnel
Fuses in parallel • Integrity of semiconductors and other components, etc.
There are many applications that use fuses in parallel. Some aspects of the example circuits and their protection are common
to many applications. These will be covered here with more specific
As the surface area of two smaller fuses is often greater than a larger,
details covered in following sections.
equally rated fuse, the cooling effect is also greater. The result may
provide a lower I²t solution, providing closer device protection or a lower Applications are broadly grouped into AC and DC current, with many in
power loss (watts loss) solution. modern circuits using both AC and DC currents.
Only fuses of the same part number and rating should be used in parallel The applications that utilize DC to AC inverters (variable speed AC drives
(fuses of the same basic part number and rating, but one with indicator and Uninterruptible Power Supplies (UPS)) can usually have their fusing
as the only difference is considered the exception). requirements considered in two parts. First the AC to DC converter and
then the inverter section. This guide will describe the AC part first and
The fuses must be mounted to allow equal current and heat flow to the
consider the DC rectifier systems and switches second.
connections. In large installations, best practice is to install parallel fuses
as close as possible with equal cold resistance values.
RMS currents in common bridge arrangements
The I²t value of parallel fuses is given by:
The most common circuits involve rectifiers that convert alternating
I²t x N² current (AC) into direct current (DC).
Where: There are a number of ways in which the supply transformers and
rectifying devices may be configured. For the purposes of these
N = The number of fuses connected in parallel
schematic examples, the semiconductor devices are represented by
Mountings should ensure at least 5 mm (0.2”) distance between diodes (although these could also be thyristors or GTOs that would give
adjacent fuses. control over the output voltage or current).
There are common places to apply high speed fuses in rectifier circuits.
The RMS current at these circuit locations varies depending on the
amount of cyclic current that will be flowing. This is described for diodes,
but for controlled circuits (with thyristors or GTOs), these values may
be different. However, they will not exceed those shown, as this is the
Example 3 same as the controlled device being constantly in the ON state.
The most common arrangements are shown here.
A Example 2
The pros and cons of applying high speed fuses in the designated
locations will be considered in detail later.
A
Circuit 1 is not often encountered in power electronics systems. The half
wave output would be inefficient with much distortion reflected to the

supply.
Kb = 1 Prospective current in Arms

N = 1.6
Figure E8. Example 2 and 3, time-current curves.
F: High speed fuse applications
Typical rectifier circuits

Fuses are RMS devices and based upon 100 percent average DC load
current output, the relevant RMS load currents I1, I2 and I3 can be found.
41%
157%
I2
I1
L
O
A 100%
D
I1
Figure F1. Single-phase, half wave L

O
A
D
71% 100%
I2 I1
L
O Figure F6. Six-phase, star
A
D 100%
I1
Figure F2. Single-phase, full wave, center tap

100% 41%
I1 I2
L
58% O
I3 A
D
71%
L
I2 O
A
D
Figure F7. Six-phase parallel (without IPT)

100% 100%
I1
I3
Figure F3. Single-phase, bridge

58% 100%
I2 I1 29%
I2 L
O
41% A
I3 D
L
O
A
D
Figure F4. Three-phase, Wye Figure F8. Six-phase parallel (with IPT)
71%
100% 100% I1
I2
I1
L
O
A
D
58%
I2 Figure F9. Single-phase, anti-parallel, AC control
L 71%
82% O 100% I2 100%
I3 A I3 I1
D
L
O
A
D
Figure F10. Three-phase, anti-parallel, control

Figure F5. Three-phase, bridge
G: Fuse protection for rectifiers
In principle, a fuse should carry all the application’s required continuous Additionally, should the equipment design specify that supply continuity
current and any expected, transient overloads. When a short- must be maintained in the event of one or more semiconductor devices
circuit occurs, the fuse should limit the energy passing through the failing, the “n” in the above formula must be replaced by (n - x), where x
semiconductor device so that it remains undamaged. is the required number of failed semiconductors.
Experience has shown that where “n” is less than four (4) (see Figure
Internal and external faults — high power/high current rectification
G2), protection of the above nature is often difficult to achieve. In
As can be seen in the schematics on the previous page, fuses may be applications utilizing both line and individual device fuses, a check must
placed in different circuit locations. Fuses may be connected in series be made to ensure that when an internal fault occurs, the device fuse
with the semiconductor devices, in the supply lines, and sometimes in selectively coordinates with the line fuse (i.e., the total clearing I2t of the
the output lines. Only the fuses in the bridge legs (or arms) will allow cell fuse must be less than the pre-arcing I2t of the line fuse):
maximum semiconductor steady state current carrying capacity as the
minimum fuse RMS current is in this location. I²t2 < I²t1
In the design of high power rectifier equipment, there are two types of Where:
short-circuits that must be accounted for: I²t2 = Total clearing I²t of cell fuse
• Internal faults — a short-circuit of an individual rectifier cell. Failure I²t1 = Pre-arcing I²t of line fuse
to open in the circuit of a silicon power rectifier is rare. However,
this type of short-circuit can be ascertained by the use of detection DC +
circuitry (see Figures G1 and G2).
• External faults — a short-circuit or excessive load at the output
terminals of the equipment (see Figure G3)
Protection from internal faults L1

L
In order to protect healthy rectifier cells in the event of an internal fault, L2 O
fuses should be connected in series with each rectifier cell. A
D
Consideration for rectifiers with parallel paths L3
It’s important to note that in designing high power, high current rectifier
equipment, continuity of supply in the event of an internal fault is often
a desired feature. The equipment must be designed to provide the
required output under all load conditions with one or more non-operating
semiconductor devices according to the manufacturer’s specification. DC -
This can be done when each arm consists of having multiple rectifier Figure G2. Internal fault, fewer parallel paths.
cells (see Figure G1).
Protection from external faults
To ensure continued operation and output with an internal fault, the fuse
connected in series on the faulted rectifier cell of the arm must open In the event of an external fault, it is undesirable to have all the rectifier’s
and clear without opening the fuses connected in series with other, individual fuses open. Therefore, it’s a good practice to include a fuse, in
functioning rectifier cells within the faulted arm. series, with the supply line (see Figure G3).
In order to satisfy this condition, the total clearing I²t of the single fuse To ensure the line fuse clears before the individual device fuse, the total
must be less than the combined pre-arcing I²t of all the fuses in one arm clearing I²t of the line fuse must be less than the combined pre-arcing I²t
of the equipment’s bridge, expressed as: of the cell fuses used in one bridge arm of the equipment, expressed as:
I²t2< I²t1 x n² I²t1 < I²t2 x n²
Where: Where:
I²t2 = Total clearing I²t of the cell fuse faulted I²t1 = Total clearing I²t of the line fuse
I²t1 = Pre-arcing I²t of each fuse in the arm I²t2 = Pre-arcing I²t of each cell fuse
n = the number of parallel paths in each bridge arm of the equipment n = Device fuses connected in parallel
More precisely, to allow for non-uniform current sharing in the parallel paths,
n should be replaced by n/(1 + S) where S is the uneven sharing, usually
DC +
between 0.1 and 0.2 (10 and 20 percent).
DC +
L1
L
L2 O
A
L1 D
L2 L L3
O
A
D
L3
DC -
Figure G3. External fault.

DC -
Figure G1. Internal fault.
G: Fuse protection for rectifiers H: Fuse protection in DC Systems
Service interruption upon device failure The arc voltage generated by the fuse during operation will also vary
with respect to the system voltage. The arc voltage variation with
The majority of faults in low and medium power rectifying and converting
respect to applied voltage will be different between AC and DC systems.
equipment fall into this category. Fuses connected in series with the
However, in most cases, it is acceptable to use the data provided for AC
semiconductor devices, or in the supply lines, are used to protect against
conditions.
internal and external faults in these common applications:
Unless special design features are included, fuses should not be called
• Variable speed motor drives
upon to clear low overcurrents in DC circuits. The performance in this
• Heater controls area may be a limiting factor on fuse selection.
• Inverters
• Low power rectifiers
With inverter circuits, care must be taken that correct DC voltage ratings
are chosen for each application. DC faults can also occur upon device
failure in bridge circuits when other power sources feed the same 63.2 %
DC bus, or when the load consists of motors, capacitors or batteries.
Current
Example 1 in the worked examples section illustrates the protection of a
typical DC thyristor drive.
Continued service upon device failure

Service interruptions cannot be tolerated in large-scale rectifying
applications such as DC supplies for electrochemical operations. 0 T 2T 3T 4T 5T
Time
As discussed earlier, these applications employ several parallel paths
(n > 4) in each arm of the rectifier. Each of these parallel paths are Figure H1. Time constant (L/R) in DC circuits is 63 percent.
individually fused to isolate faulty devices (see worked example section).
DC fed systems
In applications where many fuses are used, detecting an individual
open fuse is made easier by using indicating fuses that can actuate The vast majority of applications involving DC have an AC supply
microswitches for remote monitoring and warning. that’s rectified to supply a load. This load may be passive such as an
electrolysis cell or as complex as a regenerative drive.
Fuse protection in DC systems There are a number of circuit types requiring special consideration.
The inductance in a DC circuit limits the rate of current rise. The time These include circuits with batteries, capacitors and those where the
required for the current to reach 63 percent of the final value is called the motor drive is regenerative. In large electrolysis systems there are often
“time constant,” and often referred to in terms of L/R (Figure H1). considerations of parallel devices and fuses. Circuits with batteries and
capacitors are covered elsewhere in this guide, as are regenerative
The rate of current rise influences the energy input rate that melts the drives.
fuse’s element. This influences both the fuse’s melting time-current
characteristic and the peak let-through current. For long operating times
(greater than 1 second) the heating effect of an AC current is the same
as DC current and the characteristics will merge. Figure H2 shows a AC 25 ms 80 ms
Melting time in seconds

typical AC peak let-through and time-current curve (red) along with DC
peak let-through and time current curve at time-constants of 25 ms

(green) and 80 ms (blue). Note that higher DC time constants make the
curves shift up for the peak let-through, and right for the time-current Increasing DC
curves. time constant
Many circuits have a time constant ranging between 10 ms and 20

ms. As such, IEC specifications require testing between these values.
Time constants longer than 20 ms are not often encountered outside
of traction third rail applications, where long rail lengths give extremely
high inductance-to-resistance ratios. For short-circuit considerations, the Available current Current
value of the circuit time constant under short-circuit conditions should be Figure H2. AC and DC operating characteristics will merge for operating
used. This may be different than the time constant for normal operating times greater than 1 second.
conditions.
In many rectifier circuits (even under fault conditions), a fuse will be Battery as a load
subjected to an alternating voltage. The voltage will reduce to zero (or In principle, battery-charging circuits are similar to electrolysis systems.
close to zero) on a regular basis as defined by the supply line frequency.
Under these conditions, extinguishing the arc inside the fuse, under fault Standard bridge configurations are commonly used for these systems.
conditions, is assisted by the voltage periodically reducing to zero. Fuses may be located in the AC line, arm or the DC line.
When a fuse is applied in a purely DC application, extinguishing the fuse The use of arm fuses not only provides the closest semiconductor
arc will not be assisted by the reducing voltage or the zero voltages of device protection, but also protects the bridge against internal bridge
alternating current. The inductance in the circuit stores electrical energy. faults and faults in the DC system.
This influences the manner in which the fuse arcing process reduces the In high current circuits, regulating the amount of current is often by
current in the circuit and is beyond the scope of this guide. phase control using thyristors. In lower power systems, the fault current
The voltage under which the fuse can safely operate is dependent on may be limited only by the impedance of the transformer’s secondary
circuit time constants. It should be noted that when the time constant side and the rectifier will consist only of diodes.
is short, it may be possible for the DC voltage rating to be greater than In systems that regulate current by phase control, high fault currents can
the AC voltage rating (to IEC or UL). However, for most fuses, the DC occur if the control to the thyristors fails. Selection of fuses for this type
voltage rating is 75 percent or less than the AC voltage rating, with the of circuit is like that for a DC drive (detailed elsewhere in this guide).
DC rating further decreasing as the circuit time constant increases.
H: Fuse protection in DC systems I: AC fuses in DC circuit applications
However, in a diode-only system, in the event a battery is connected in The following information applies specifically to the 660, 690, 1000 and
reverse polarity, the fault current will pass directly through the diodes. 1250 Vac standard Typower Zilox fuses when they are applied in DC
The resulting fault current will only be limited by the internal impedance applications. These fuses have not specifically been proven and have not
of the battery. Fast isolation is required to protect the diodes and to limit been specifically assigned a DC voltage capability.
the I²t in the diode.
These fuses may be used in circuits where DC faults occur and caution
Attention must also be paid to the possible pulse duty a battery charger must be taken in their selection. It’s recommended to validate the fuses
may be called upon to perform. Many controlled charger circuits have a after following this selection process (this is only a guideline — end
high charge rate for a short time before a lower, continuous charge rate users must validate fuse selection for their application).
is applied. Guidance on this is given in the section on cyclic loads.
The interrupting capability of the fuse depends on a combination of:
Battery as only source • Applied DC voltage
The use of batteries is vast and increasing due to the demands for • Circuit time constant (L/R)
renewable energy where they are common and essential as power
storage devices.
• Minimum prospective short-circuit current (Ipmin) of the circuit
Protecting a battery (or batteries) is particularly difficult under fault

• Pre-arcing I²t of the selected fuse
conditions due to their characteristics. The problem is made more difficult To correctly apply a fuse, a factor (F), relating to the melting I2t to the
by the large number of manufacturers and battery types. prospective current, must be used.
Due to their superior current limiting effect, high speed fuses can be a In order to determine factor F in Figure I3, use the curves in Figure I1
good choice for protecting batteries under short-circuit conditions. or I2 that show the dependency of the maximum applied DC voltage on
However, for a high speed fuse to effectively operate, it requires the L/R, with 3 levels of Ip as a parameter indicated as 1, 2, and 3. Select the
fault current to be high enough to quickly melt the fuse element. The curve 1, 2 or 3 by choosing the curve above the point from the known
fault current’s rate of rise (time constant) has to be fast enough to allow available voltage and circuit time constant.
the fuse to clear the DC arc that’s generated during fault clearing. DC If no curve exists above the voltage-L/R point, then a fuse with a higher
fault conditions are difficult to properly fuse, and misapplication can, in AC rating than 1250 V must be chosen. Contact Application Engineering
some cases, cause a fuse stress failure. Fault current under short-circuit for assistance at FuseTech@eaton.com.
conditions is severely limited by a battery’s internal impedance and state
of charge. If a battery is fully charged there may be sufficient energy to Factor F is found in Figure I3 as a function of the circuit time constant
operate the fuse, but as the battery’s charge reduces, it could be to a L/R and the selected curve 1, 2, or 3 as parameter.
level well under that required by the fuse to open. To check if the minimum level of available current (Ipmin) in the actual DC
As with long time constants typically greater than 15 ms, insufficient circuit is in accordance with the selection made in Figure I1 or I2, the
fault current could cause a similar failure of the fuse. Fault currents following condition must hold true:
applied to the fuse that are above the A-A line (the dotted line area) of Ipmin ≥ F x I2t [A]
the time current curve would be of major concern.
Where I²t is the pre-arcing integral (from cold) in A2s of the fuse in
It is essential that all the possible battery parameters are known before question and, importantly, it‘s capable of interrupting this minimum
attempting to select a fuse. Details of the battery and data sheets should current.
be obtained from the manufacturer. It may be required that the selected
fuse can only be used when the batteries are maintained at or above In Figure I4, the fuse’s worst case peak arc-voltage can be found as a
a certain state of charge, and the manufacturer can guarantee a short- function of applied DC voltage.
circuit time constant in the event of a short-circuit. Note: Where fuses have a reduced AC voltage capability, the DC voltage
A high speed fuse will, of course, only provide short-circuit protection. capability will be reduced by a similar percentage. E.g., a 690 V, 2000 A
For cable protection, a more general purpose fuse should be applied Size 3 fuse has an AC voltage rating of 550 Vac, so the DC voltage rating
that is able to operate under low overload conditions. This causes other will be reduced by 20 percent to 440 Vdc.
problems as branch and supplemental fuses are often not able to handle * These fuses have not specifically been validated and have not been specifically
DC voltages to the same degree as high speed fuses. A sustained low assigned a DC voltage capability.
current overload at high DC voltage may require a fuse that’s specifically
designed for DC applications and will provide safe reliable fuse
protection.
Contact Application Engineering at FuseTech@eaton.com.
I: AC fuses in DC circuit applications
Calculation example
1 2 3 Typower Zilox 170M6149 is:
80
70 • 1250 Vac
60
• 1100 A
50
L / R rms
40
• Size 3/110
30 • 575,000 A2s (I2t pre-arcing integral)
20
The applied voltage E = 500 Vdc
10
0
First, the prospective short-circuit current and time constant (L/R) should
100 200 300 400 500 600 be determined based upon the circuit parameters shown in Figure I1 and
Max applied DC voltage the above ratings for the Typower Zilox 170M6149 fuse.
Prospective current (Ip):
Figure I1. 660-690 Vac Typower Zilox maximum applied DC voltage. Ip = E/R
= 500 V/16 mΩ
1 2 3 = 31.3 kA
80
70 Where:
60
E = 500 Vdc applied voltage
50
L / R rms
40 R = 16 mΩ circuit resistance in Figure I5

30 Time constant (L/R):
20
L/R = 0.64 mH/16 mΩ
10
= 40 ms
0
300 400 500 600 700 800 Where:
Max applied DC voltage
0.64 mH = circuit inductance from Figure I5
16 mΩ = circuit resistance from Figure I5
Figure I2. 1000-1250 Vac Typower Zilox maximum applied DC voltage.
100 3
90
80
Fuse
70
60
F 50
40 E
2
30
20 1
R L
10
0 E = 500 Vdc R = 16 mΩ L = 0.64 mH
10 20 30 40 50 60 70 80
Time constant in ms
Figure I3. Factor F based upon circuit time constant in Figures I1 and I2. Figure I5. Calculation example circuit.
3 1 2 3
2100 80
2000 70
1900
1800 60
1700
1600 50
L / R rms
Volts
1500 40
1400
1300 2 30
1200
1100 20
1
1000 10
900
800 0
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 300 400 500 600 700 800
Max applied DC voltage
DC voltage
Figure I4. Worst case peak arc voltage. Figure I6. Maximum applied DC voltage.
I: AC fuses applied in DC circuit applications J: Fuses protecting regenerative drives
Using Figure I6, at an applied voltage of 500 Vdc and a time constant In principle, the fuse should carry all the required continuous current and
(L/R) of 40 ms, Curve 1 has been passed, meaning that, to be on the any expected transient overloads. When a short-circuit occurs the fuse
safe side, Curve 2 must be used. should limit the energy passing through the semiconductor so that it
remains undamaged.
From Figure I7, we find factor F = 26.5 based upon initially calculated
L/R = 40 ms and Curve 2 selected from the step above. Together with To start, the types of faults that can occur in the equipment must be
the pre-arcing I2t = 575,000 A2s of the selected fuse, this calls for a known before selecting the rated fuse voltage.
minimum prospective current (Ipmin) of:
Fuses could be applied at circuit location F2 only (Figure J1), or at circuit
Ipmin = 26.5 x 575,000 locations F1 + F3.
= 20094.62 In rectifier operation there are three possible fault types: internal faults,
cross-over faults and external faults (Figures J1-J3).
= 20.1 kA
Checking with the actual circuit parameters, it can be seen that the Conclusion on the rectifier mode
interrupting rating of the selected fuse is sufficient for the circuit having With internal, cross-over and external faults, the short-circuit current will
the following main parameters fulfilled: pass through two fuses in series. This means that the two fuses will
• The maximum applied DC voltage is 500 Vdc (up to 280 Vdc could be normally help each other in clearing the fault. Nevertheless, for safety,
allowed at the calculated time constant) at a minimum the rated fuse voltage UN has to be higher than the RMS
AC supply voltage (UN ≥ UAC) (pay attention to the commutation fault
• The time constant L/R is 40 ms (up to 46 ms could be allowed at the situation). When it comes to protecting the semiconductor and the I²t
given maximum applied DC voltage) calculation, it is an advantage to have two fuses in series.
• Minimum Ip of 20.1 kA is needed (actual prospective current is 31.3 In the short-circuit path,if the prospective current is very large, the I²t can
kA) be calculated with almost equal sharing of the fault voltage. At smaller
The fuse’s peak arc voltage can be found in Figure I8 to be lower than fault current levels it is not considered safe to use total equal voltage
1900 V. sharing. Normal procedure is to use 1.3 as a safety factor. Hence, the I²t
values are found at an RMS AC supply voltage of:
Sizing I2t voltage = UAC × 0.5 × 1.3
3
100
≈ UAC × 0.65
90
80 There can also be three fault types while operating in the regenerative
70 mode (Figures J4-J6).
60
F 50
40
F2 F3
2
30
20 1
10
0 F1
10 20 30 40 50 60 70 80 UAC
+
U DC
Time constant (ms)
-
Figure I7. Time constant.
2100
2000
1900
1000-1250V
1800
1700
Figure J1. Internal fault — rectifier mode.
1600 This fault is due to a thyristor losing its blocking capacity, leading to an AC
1500 line-to-line short-circuit.
Volt
1400
1300
1200
1100
660-690V
1000
900
800
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900
Maximum DC voltage
+
Figure I8. Worst case arc voltage. UAC UDC
-
Figure J2. Cross-over fault — rectifier mode.

This fault occurs when a misfiring of one of the thyristors in the inverter
bridge results in an AC line-to-line short-circuit.
J: Fuses protecting regenerative drives
As a general rule, the fault voltage is half a sine wave at a lower

frequency. The RMS value of the fault voltage will be:
RMS fault voltage = 2.58 × UAC × 1/ 2
UAC + ≈ 1.8 × UAC

UDC
- Though this type of fault is very rare, it will require derating the fuse’s
rated voltage. This means the rated fuse voltage should be in accordance
with:
UN ≥ 1.8 × UAC
Figure J3. External fault — rectifier mode. Where:

This fault is due to a short-circuit on the DC output side (motor flash-over for UN = Fuse voltage rating
example). The applied fault voltage is again equal to the AC line-to-line voltage.
If an I²t calculation is needed (mainly for internal fault only), the sizing I²t
voltage having two fuses in the same short-circuit path will give:
Sizing I2t voltage = 1.8 x 0.5 x 1.3 UAC
≈ 1.2 × UAC
+
UAC UDC For the other three inverter fault types, the fault voltage will be a pure
- DC voltage and the maximum DC fault voltage will be:
UDC = 0.866 x 1.35 x UAC
≈ 1.1 x UAC
A normal AC fuse can operate under DC conditions with some limit
Figure J4. Commutation fault — rectifier mode. to the line voltage, the minimum available fault current and the time
This fault is due to a thyristor losing its blocking capability while there is a constant.
direct line-to-line voltage across it. This leads to a short-circuit where the AC
voltage is superimposed on the DC voltage. Please refer to the section AC fuses in DC circuit applications.
During the DC shoot-through fault (Figure J6), the only impedances
in the circuit are in the motor and inverter branch. The minimum
prospective fault current is normally very large and the time constant
in the circuit is small (e.g., 10 to 25 ms). Under this condition, having
+ two fuses in series, the I²t value is normally equal to the value obtained
UDC under an AC sizing I2t voltage of:
-
Sizing I2t voltage = UDC x 1/ 2 x 0.5 x 1.3
= 1/ 2 x 1.1 x 0.5 x 1.3 x UAC
≈ 0.5 x UAC
Figure J5. Loss of AC power — regenerative mode.
If the AC voltage fails, a short on the motor acting as a generator occurs
In order to be certain, all data should be available for the motor and other
through the thyristors and the transformer. impedance in the circuit.
In case of a reduced or total loss of the AC power, the condition is worse
(Figure J5). The fault current level can be very low and the impedance of
the transformer gives large time constants.
In order to select fuses that can function under these conditions it is
Va necessary to have information not only on the motor and the inverter
UAC + impedance, but also on the transformer.
UDC
-
Summary of voltage selection for regenerative drives
Combination of line voltage and load voltage requires:
Fuse voltage UN ≥ 1.8 × UAC (line-to-line) — e.g.:
Figure J6. DC shoot-through — regenerative mode. • 110 V system: 200 V fuse

This fault occurs due to one thyristor misfiring and leads to a DC short-circuit. • 380 V system: 690 V fuse
Conclusion on the regenerative mode • 690 V system: 1250 V fuse
As it can be seen from the fault circuit diagrams (Figures J4-J6), there For further guidance, please contact Application Engineering at
will also be two fuses in series, but the fault voltage greatly differs. FuseTech@eaton.com.
During the commutation fault, the fault voltage is the AC voltage added
to the DC voltage. In the worst case (assuming a minimum firing angle
of 30 degrees), the peak voltage will be:
Peak fault voltage = 0.866 x 1.35 × UAC + UAC × 2
≈ 2.58 × UAC
Where:
UAC = RMS AC line-to-line supply voltage
K: Fuses protecting inverters
There are many inverter types. Some simply convert DC current to AC To ensure device protection a fuse selected for lowest I²t that will meet
current (e.g., PV inverters) or AC current to DC current (this may also be the current sizing requirements will be the best way. Even if device
accomplished with a rectifier) or that convert AC current to DC current protection is not ensured, this fuse selection will certainly limit the
and back to AC current (e.g., VFD variable speed motor drives and UPS damage to all the circuit components.
uninterruptible power supplies).
It is especially important to select a low I²t fuse if the capacitor is a low
VFD and UPS inverters work by switching DC current ON and OFF in value. When a short-circuit occurs in the inverter, the current rises rapidly
a predetermined manner. Early inverters using thyristors were often of to a peak and will then decay, displaying a waveform that is classical of
the McMurray form (Figure K1). Once turned ON, thyristors continue to capacitor discharge. It is important that the fuse has opened and cleared
pass current until the voltage across them is reversed using numerous before the voltage on the capacitor has decayed to a low value. If the
components to commutate the devices. The commutation thyristors also fuse was to operate at a low voltage on the capacitor, the fuse may
require protection. not have developed sufficient insulation resistance to withstand the DC
circuit voltage when it is replenished from the supply.
Filter
inductor
F3 L1
DC supply
D1
Thy1
Filter
capacitor Commutation F1
LOAD
components F2
Thy2
D2
L2
Figure K2. GTO inverter.

LOAD
Typical thyristor inverter (one phase of three-phase unit)
With the development of Gate Turn-Off thyristor (GTO), it was possible to
switch the DC current OFF without the use of commutation components
Figure K1. McMurry inverter. (as required in the McMurry inverter using thyristors). It should be noted
that by reducing the complicated trigger (firing) circuits, considerable
Even with fuse protection in the DC circuit at fuse location F3, it is best space and costs savings were made and energy losses were reduced,
to use device protection for the thyristors at fuse positions F1 and F2. To too.
ensure protection in these circuits, it is essential to use the fastest fuses
available (and still meet all the current sizing) which are also rated with a Although GTOs are more expensive than thyristors, the additional cost
DC voltage capability at least as high as the DC circuit voltage. is more than offset by the component reduction. In terms of protection,
there is little difference in the selection parameters between an inverter
The key to fuse selection for inverters is to select the highest speed using thyristors and one using GTOs. However, the GTO circuits are
available that will meet the current and voltage sizing requirements. inherently more reliable with fewer power components to protect.
Voltage selection IGBT as switching device
Fuses in the inverter must have a DC voltage rating of at least the supply The advent of the Insulated Gate Bipolar Transistor (IGBT) as a switching
circuit voltage. Even though in most fault conditions there will be two device has made control circuits much easier and power dissipation in
fuses in series, these will not share the voltage equally. Also, in some the power switching sections reduced. The higher switching frequency
fault situations the voltage on the DC circuit may exceed the nominal capability and ease of control allows more efficient use of the pulse
value for a short time by up to 30 percent. width modulation techniques, as well as improved quality of the
output waveform. However, the IGBT circuit poses different protection
Current selection problems.
As shown in the inverter circuit schematics, there are several locations to
place fuses. As with DC drive circuits, the use of DC line fuses results in
the highest current rating and closest protection is determined by where
individual fuses are located in the circuit.
As inverter circuits contain high frequency components to carry the
current, and the physical arrangements are compact, proximity effects
may influence the fuses, and further allowance must be made for current LOAD
carrying capability.
I²t selection
Due to the magnitude of the fault current from the capacitor and
small inductance in the circuit, the current rise rate may be very high.
Selection of suitable I²t criteria is not easy. Device data may not be
Figure K3. IGBT inverter.
available for times below 3 ms, nor fuse information for these conditions.
Fuse performance will also vary slightly depending on the capacitor size, To reduce switching losses, the inductance of the filter capacitor and
the circuit inductance and resistance, and DC link voltage. IGBTs has to be as low as possible. This is achieved by careful busbar
arrangements that often preclude using fuses.
L: Worked examples
Due to the silicon switching element design, an IGBT module can limit Basic design
current for a short period. In addition, it is often possible to detect fault
For optimal protection, device protection with six fuses will be
currents and switch the IGBT OFF before damage occurs. However, if
considered (one per thyristor).
the IGBT is not switched OFF before the device is damaged, the silicon
will melt and vaporize. Since the fuses are for short-circuit protection only, this is simply a
question of coordinating the I2t, peak current and maximum RMS fuse
There is another failure mode with plastic IGBT modules that occurs
current ratings. Maximum RMS current through each thyristor is given
before the silicon melts. The internal conductors to the IGBTs and other
by the appropriate factor for the circuit layout, multiplied by the DC load
components are thin aluminum wires. These wires melt and arc under
current (see Typical rectifier circuits, Figure F5, circuit location I2):
fault conditions, causing the module case to become detached from the
base. Therefore, fuse protection must also include protecting the wires = 0.58 x 600 A
and the module case as well as the devices. Unfortunately, there is often
no I2t data provided for IGBT modules. = 348 A
For this example a fuse from the square body fuse type is required. From
Protecting drive circuits the catalog for a square body, 690 V, 400 A and size 00, the Bussmann
If damage is caused to the IGBT device or connecting leads, the gate series 170M2621 fuse is initially selected. From the temperature rating
control circuits may become involved with the high voltage and current graph, Figure E1 in the section Determining fuse amp ratings, a derating
of the power circuit. To avoid, or limit, damage to the control circuits, to 90 percent is required at 40°C/104°F. No other thermal derating factors
miniature fuses with a high interrupting rating should be used in the drive will be required.
circuits. Low interrupting capacity glass tube fuses are not suitable. 0.9 x 400 A = 360 A
Bipolar power and Darlington pair transistors As shown above, the maximum current through each thyristor is 348 A
and the maximum permissible current rating for the selected fuse would
It is difficult to protect power transistors with fuses. The power transistor be appropriate.
usually operates extremely close to its power limits of current and
voltage. Only a short excursion beyond the safe operating area will Next, the fuse I2t has to be confirmed to be less than the I²t withstand of
damage the functional aspect of the transistor and even a high speed the device.
fuse will not react fast enough to protect the device. However, like For the Bussmann series 170M2621 fuse selected above, the total I2t
IGBTs, when the function of the transistor is lost the current is limited is 125,000 A2s at 660 V. By observing the factor for I2t with respect to
only by the damaged silicon’s low resistance. This results in very high applied voltage on the data sheet the adjusted I2t at 480 V (Figure L1)
currents. These will melt any connecting wires and will, in the case of a can be found to be less than the I2t withstand of the thyristor:
press pack configuration, eventually melt the silicon. The resulting arcs
will cause the packaging to fail with catastrophic results. Even though I2t at 480 V = K x I2t at fuse voltage rating
device protection cannot be offered by fuses, it is still essential to use = 0.7 (70%) x 125,000 A2s
fuses to prevent case rupture and isolate the circuit.
= 87,500 A2s (well below the thyristor withstand)
Worked examples
The previous information can be best understood by studying typical
1.5
examples for selecting an appropriate Bussmann series fuse that meets K
the protection requirements. 1.0
Example 1: DC Thyristor drive
Basic information
0.5
Note: no cyclic loading details are included for this example.
0.4
• 500 Hp VFD
0.3
• Motor: nominal voltage 660 Vdc, maximum current 600 A DC
• Supply transformer 750 kVA, 5% impedance. 0.2
• Supply voltage - 480 Vac RMS.
Eg
• Overload protection is provided by a current limit circuit (direct control 100 200 300 400 500 600 690
of thyristor firing) with a response time of 25 ms
• Maximum 40°C/104°F ambient, convection ventilation Figure L1. Reference: Bussmann series product data sheet no. 720013, total
• The circuit is a 3-phase thyristor bridge, one thyristor per leg clearing I2t at rated voltage.
• Thyristor particulars:
• I²t 120,000 A2s
• Peak reverse voltage withstand (Urrm) 1600 V
L: Worked examples
A check of the arc voltage shown on the graph in Bussmann series data As six fuses are used in this example, the total power saved by using the
sheet 720013 (Figure L2) confirms that the fuse arc voltage of 1000 V physically larger, Size 2 fuses will be:
will be below the 1600 V peak reverse voltage rating of the thyristor:
1.4 = (53-34) x 6
1.2 UL = 114 watts
103
9 Example 2: High power/high current DC supply with redundant
8 diodes
7
A rectifier is to provide a 7500 A, 80 Vdc supply from a 50 Hz source
6
5 Basic information
4 • Nominal 80 Vdc voltage supply

Eg • Maximum 7500 A current output
3
200 300 400 500 600 690 • 3-phase diode bridge with six parallel diodes per leg
Figure L2. Reference: Bussmann series product data sheet no. 720013, • 200% overload for 1 minute, once a month
peak arc voltage at applied voltage. • Maximum 55°C/131°F ambient with 4 m/s forced air cooling
Thus the Bussmann series 170M2621 fuse is confirmed to work in this • Busbars based on 1 A/mm²
application. If the power dissipation (watts loss) of the equipment is
critical and there are no physical space constraints, it may be possible to • Diode rating:
utilize an alternative solution. • Maximum mean rating (free convection, specified heat sink) 1000 A
By selecting a fuse with a higher current rating and using it at a current • I²t rating, 10 ms, 1,000,000 A²s, peak reverse voltage withstand
well below its capability, the power dissipation (watts loss) will be (Urrm) 500 V
considerably lower. For this example we can choose a square body, 690
V, 500 A Size 2, fuse (Bussmann series 170M5010 fuse for example). • Maximum prospective AC fault current = 125,000 A RMS
Although the I2t is 145,000 A2s at 660 V, this will be reduced to 101,000 symmetrical
A2s at 480 V, which is also below the I2t withstand of the device.
Protection requirements
The power dissipation (watts loss) of the Bussmann series 170M2621
fuse, 400 A fuse applied at 348 A is calculated below: Fuses must protect the diodes from internal faults and isolate faulty
diodes without interrupting the power supply.
% Load current = load current/current rating x 100
= 348 A/400 A x 100 Design details
= 87% Protection requirements for this application requires device fuses and the
equipment design specifies continuity in the event of one semiconductor
Watts loss at 87% = Kp x Watts loss at 100% fuse current rating device failing in one leg of the bridge. Maximum RMS current through
= 0.75 (75%) x 70 W (Figure L3) each arm of the bridge is given by the appropriate factor for the circuit
layout, multiplied by the DC load current (see Typical Rectifier Circuits,
= 53 W Figure E5, circuit location I2):
If the 500 A, square body, Size 2 fuse is used at 348 A, then the actual = 0.58 x 7500 A
watts loss calculated below will be reduced to below the adjusted power
loss found for the 400 A, Bussmann series 170M2621 fuse: = 4350 A (per arm of bridge rectifier)
% Load current = load current/current rating x 100 The maximum RMS current through each diode (or fuse) in one leg of
the rectifier can now be calculated allowing for one defective diode, non-
= 348 A/500 A x 100 uniform current sharing of 10 percent, and six parallel diodes per leg of
= 70% the bridge:
Watts loss at 70% = Kp x Watts Loss at 100% fuse current rating = 4350 A / ((n-1) / (1+s))
= 0.45 (45%) x 75 W (Figure L3) = 4350 A / ((6-1) / (1.0+0.1))
= 34 W = 966 A (per diode of each arm of bridge rectifier)

The selected fuse must have a current rating above 966 A after thermal
1.0 deratings (to account for the high ambient temperature and forced air
Kp
0.8 cooling) are applied. From the section on Determining fuse amp ratings
400A, size 00 75%
(page 11) we find the following correction factors based on the given
0.6
application information:
0.5
500A, size 2 45% Ke = 1 (at 1 A/mm2 current density, no adjustment is needed)
0.4
0.3 Kt = 0.85 (Figure E1 at ambient temperature of 55°C/131°F)
Kv = 1.2 (Figure E3 at 4 m/s forced air flow)
0.2
Ib
0.1
30 40 50 60 70 80 90 100%
Figure L3. Reference: Bussmann series product data sheet no. 720013,
720014, watts loss at % load current of rated current.
L: Worked examples
The selected fuse’s rated current (In) must be greater than: Arc voltage
In ≥ Irms/(Ke x Kt x Kv) From the Bussmann series FWA-1200AH data sheet, (Figure L5) the arc
voltage of 190 V can be seen to be less than the 500 V reverse voltage
≥ 966 A/(0.85 x 1.2 x 1)
capability of the diodes chosen.
= 947 A
UL
For this low voltage application, with a low peak reverse voltage diode, a
fuse from the low voltage UL branch or supplemental or British range is
required. 300
Based on a power source of 950 A, 240 Vac a UL or British fuse series

would require using three, 350 A fuses in parallel (Note: NEC 240.8 200
190 V
precludes field installation of parallel fusing). To avoid parallel fusing, the
Bussmann series FWA North American style high speed fuse should be 100
chosen. It’s this fusing option that will be considered for the overload
considerations. Eg
50 100 150
Overload
Figure L5. Reference: Bussmann series product data sheet no. 720001, arc
The selected fuse must also withstand a 200 percent (twice the voltage at applied voltage.
continuous load current) overload for 60 seconds, once a month. Using
Table E1 from Part 2 on page 12 of this application guide, it should be Short-circuit protection
possible to select a fuse up to 80 percent of the time-current curve at
the 60 second operating time: The Bussmann series FWA-1200AH fuse I2t is 730,000 A2s at 130 V,
which will reduce in the following manner (see Figure L6):
Imax < 80% x It
I2t at 80 V = K x I2t at fuse voltage rating
< 80% x 3000 A
= 0.75 (75%) x 730,000 A2s
< 2400 A
= 548,000 A2s (well below the thyristor withstand)
Or
It at 60 sec > Irms x 2 / 80% 1.5
K
3000 A > 2415 A TRUE! 1.0
75%
104
6
4 0.5
2
103 0.3
6 0.2
4 FWA-1000AH
FWA-1200AH
2 FWA-1500AH 0.15 Eg
FWA-2000AH
102 26 65 104 130
FWA-2500AH
6
FWA-3000AH
4 Figure L6. Reference: Bussmann series product data sheet 720001, total
FWA-4000AH
2 clearing I2t at applied voltage.
101 To ensure continuity of supply when a device fails, the total clearing I2t of
6 the fuse in series with the faulty device must be less than the combined
4
pre-arcing I2t (270,000 A2s each) of all the six fuses in series with the
2 fault (in a different arm of the bridge):
0
10 I²t clearing < I²t pre-arc x n2
6
4 548,000 < 270,000 A2s x 62
2
< 9,720,000 A2s
10–1
6
This confirms the selected fuse will protect the devices suggested.
4
Example 3: Regenerative drive application
2
10–2
6
4
10–3 +
6
UAC UDC
4 -
2
10–4
2 4 6 8 2 4 6 8
103 104 105
Prospective current in amp RMS
Figure L4. Reference: Bussmann series product data sheet 35785301, Figure L7. Regenerative drive circuit example (see Figures J1 to J6 on
time-current curve; 60 second operating time. pages 20 to 21).
L: Worked examples Appendix 1: International standards
Basic information (Figure L7) For many years high speed fuses did not have any international
standards. As more manufacturers produced these fuses, many
• High inertia drive, 500 Hp DC motor
dimensional arrangements became commonplace. High speed fuses
• Motor: 500 Vdc nominal voltage, maximum current 750 A DC are now a mature product with international standards covering test
methods and dimensions.
• Power is supplied from a 380 Vac* three-phase grid
• The busbars are rated between 1 and 1.6 A/mm2 In the United States
• An air cooled system with ambient temperature of 35°C/95°F with no Common dimensions became an “industry norm” but until they were
forced air cooling included in EN 60269, they were not a part of any published standard.
Testing was performed to either customer requirements, or when a UL
Recognized fuse was required and the tests performed would be similar
Although expected overloads will be cyclic, regenerative drives would not to those of other UL standards.
be cost effective if the load was not regularly stopped.
The UL Standard 248-13 now defines test conditions and methods.
For simplicity, the cyclic loading details will not be included in this Although these UL and EN (IEC) standards are similar, there are some
example. In practice, the rules for cyclic loading explained in this guide small differences that are beyond the scope of this guide.
should be followed and applied to the current rating as well as the
ratings described in this example. A 380 Vac supply will give a nominal The major difference between the UL and IEC standards is voltage
DC voltage of 500 V (typically 135 percent of the RMS line voltage) from rating. This difference is common to many electrical specifications and
a six-pulse bridge. is based upon a long historical background. Briefly, European standards
require voltage testing at some tolerance above the fuse’s rated voltage,
To maximize the degree of protection in this application, the best place thus providing a safety margin. Practice in the United States requires
to install the fuses is in series with each semiconductor (device or arm fuse testing at the rated voltage. Hence, it is design practice to use
fusing). The maximum RMS current through each arm of the bridge is the maximum voltage available for determining the rated voltage of
given by the appropriate factor for the circuit layout, multiplied by the DC components.
load current (see Typical rectifier circuits, Figure F5, circuit location I2):
= 0.58 x 750 A In Europe
= 435 A The test requirements of BS88 Part 4 (1976) are the same as IEC 60269-
4, with dimensions included for high speed fuses in common use in the
The only consideration for permissible maximum continuous current will UK. IEC 60269-4 included test conditions for AC and DC circuits that
be ambient temperature derating where we find Kt, which adjusts the are more suitable for high speed fuses than fuses for industrial circuits.
minimum fuse current rating: The German VDE specification 0623 Part 23 is specific to the testing
Kt = 0.94 (Figure E1 at ambient temperature of 35°C/95°F) of high speed fuses. Dimensions are included in DIN 43620 (the same
as industrial fuses) and DIN 43653 (European high speed square body).
In = Ib/Kt Cylindrical fuses are usually dimensioned to French NF C63211.
= 435/0.94 The latest version of EN 60269-4 includes dimensions from all previous
= 462 A European and United States high speed fuse standards and includes
standardized testing for fuses used in voltage sourced inverters (VSI).
The next rating available fuse above this should be chosen. In most This standard now supersedes all previous standards.
product ranges this will be 500 A.
Bussmann series product range
Voltage rating consideration
Various fuse constructions originate from different parts of the world.
The worst case voltage rating in a regenerative drive results from a As a result, Bussmann series high speed fuses can be grouped in four
commutation fault. Therefore, the fuse will require an AC voltage rating world-recognized standards:
of at least:
• US Style — North American blade and flush-end style
= (0.866 x 1.35 x Vac + 1.414 x Vac) x 1/√2
• European standard — square body
= 1.8 x 380 Vac
• British Standard — round body BS88
= 684 Vac
• Ferrule fuses — cylindrical
For this we would select a 690 Vac fuse.
US Style — North American blade and flush-end style
See section on Fuses protecting regenerative drives on page 20.
Selection of fuse is then based on mounting arrangements, physical
constraints and approvals required, etc.
Note on voltage rating

If a drive system is to align with the international standard voltage (not
the old voltages) the drive should be rated for a 400 Vac supply with the
DC voltage maintained at the same voltage using phase-angle control
of the bridge devices. In this case, using a higher voltage fuse may be
required.
Over the years, the North American market has adapted its own
mounting styles for high speed fuses. Although no published standard
exists for these as yet, the industry has standardized on mounting
centers that accept Eaton’s Bussmann series fuses.
Appendix 1: International standards
In many ways, US Style fuses are similar to the European Style. They British Standard — BS88
are made in both blade and flush-end versions, but with two major
differences: US Style fuses are usually made in mineral fiber tubes and
the fixing centers will vary depending on both rated voltage and rated
current.
European standard
DIN 43620 — bladed fuse
Not surprisingly, this mounting type has found its use mainly, but not
exclusively, in the United Kingdom and British Commonwealth countries.
Also, North American manufacturers have begun to specify British style
fuses (particularly in applications like UPS equipment with voltages of
240 V or less) due to their size, performance and cost advantages. The
DIN 43653 — botled tag fuse dimensions given in the BS88 Standard for high speed fuses are not
physically interchangeable with industrial fuse standard.
In Europe, outside of the United Kingdom, two mounting types are Cylindrical/ferrule fuses
preferred for high speed fuse applications - blade type and flush-end.
Blade type fuses

In Europe, two German standards cover most fuse mounting for normal
styles of Bussmann series blade type high speed fuses.
They are:
• The DIN 43620 style is used for gG fuses (previously referred to as
gL). It is also used for high speed fuses. However, a high speed fuse
typically reaches a higher temperature during continuous operation
than a normal gG fuse. As a result, the DIN 43620 style high speed
fuse cannot get a sufficient rating if its holder temperature limits are
not to be exceeded. Knives with holes for mounting fuses directly on
the busbar are the solution to this issue.
Often referred to as ferrule fuses, this style is internationally used and
• The DIN 43653 standard came in 1973 with the possibility of mount- accepted. In most cases, the Bussmann series cylindrical high speed
ing the fuse directly on the busbar. New holders also appeared at the fuses have dimensions that meet IEC Standard 60269. These fuses have
same time. For the most common voltage ratings, fuses with blades proven very popular for applications with ratings up to
according to DIN 43653 will always have fixing centers of 80 mm or 660 V/100 A, due to their easy installation. The standard dimensions are
110 mm. 8x31 mm, 10×38 mm, 14×51 mm and 22×58 mm, and can be installed
in available Bussmann series fuse blocks and holders.
Flush-end contact type
Like the DIN 43653 style, the flush-end style has become a very efficient
and popular high speed fuse style due to its installation flexibility. This
style is also selected because the current carrying capacity is the most
efficient of all fuse types. This is now an industry standard style and is
included in the international standard IEC 60269-4-1.
Appendix 2: Fuse reference system
With the many Bussmann series high speed fuse varieties, our reference Position 2 — Body size
system is complex. The use of one reference system in Europe (outside
the UK), another one in the UK and a third in the US has become a
fact of life. Discussions have been held on replacing them all with
one. However, all reference systems are so well established in their
respective markets, the decision was made to maintain the existing
systems.
The following describes the Bussmann series reference systems in
detail.
European high speed fuses

A typical fuse from the Bussmann series range of European square
body fuses could have a catalog number like 170M3473. However, this
will not give any guide to what ratings or mounting this fuse has. Here,
the user will first need to know the ratings. However, the mounting is
also of interest, so we use a Type Description to determine what style Position 3 — Optional
is in question. Fuses according to the German DIN 43620 standard are Over the years, many square body fuses have been adapted to specific
always listed by size. For example, DIN 3, DIN 00, etc. For other fuses, customer needs. Therefore, a lot of special, customized fuses are now a
according to DIN 43653, flush-end types or special types, this description part of the product offering. Position 3 in the Type Code might therefore
will reveal the actual type in question. For the reference given above, the be an S for special. For all such references, please consult the Bussmann
type designation will be the following: Division for a mechanical drawing, if this is not already at your disposal.
1*BKN/50
To interpret this Type Code we have made the following general
guideline, that will cover most of the European fuses.
Fuse type code positions

1 2 3 4 5 6 7
1* B K N 80
Primary
code
Optional Indicator
type
Center
distance
Position 4 — Mechanical fixing
Body Mechanical Indicator
size fixing position
The following tables show the various options for all characters in the
above Type Code:
Position 1 — Primary code
The primary code can be one of the following values:
Position 5 — Indicator type BS88 high speed fuses

Often a fuse will have an indicator to show if it has opened. Some Since fuses were first produced in the dimensions that became
indicators are built in and some have to be externally fitted. Indicators standardized in BS88 Part 4, fuse technology has improved. It is
may also trigger optional microswitches for remote indication. On the now possible to manufacture fuses with many different operating
indicator Position 5 in the Type Code, the following options are standard: characteristics. In these dimensions, Bussmann series high speed fuses
are available in two speed ratings: T range and F range. Fuses can be
selected according to the following codes.
For T range fuses
BS88 fuse code positions

1 2 3 4
80 L E T
Current Body
rating size
Voltage T Range
or
style
Position 1 — Current rating

The continuous current rating in amps.
Position 2 — Voltage or style
Position 6 — Indicator position

The indicator position may vary from fuse to fuse. Standard mounting is
the so called Position N (North) and alternative positions are E (East), W
(West), and S (South):
Position 3 — Body style

In BS88 Part 4, fuses have three diameters. The letter in Position 3
indicates the fuse’s diameter. To achieve a greater fuse current rating, it
is possible to place two fuses in parallel. With such fuses, and to indicate
that two fuse barrels are used, the letter indicating the diameter is
repeated (e.g., two M diameter fuses in parallel is LMMT).
Position 7 — Center distance

Indicates center distance for mounting, or overall length of fuses with
flush-end contacts, stated in millimeters.
Position 3 — F Range
The Bussmann series F range fuse (these are faster acting than the T
range) has an F in the third position.
Position 4 — Body style
In BS88 Part 4, fuses have three diameters. The fuse diameter is
indicated by means of a letter in Position 4. To achieve additional fuse
current ratings, it is possible to place two fuses in parallel. To indicate
that two fuse barrels are used, the letter indicating the diameter is
repeated (e.g. two FM fuses in parallel is FMM).
For example, 80FE is an 80 amp, 660 volt fuse, 18 mm diameter.
Position 4 — “T Range”
The Bussmann series T range fuse has a “T” in the fourth position.
Some special purpose fuses in “standard” dimensions or with special
fixing arrangements may have an alternate letter in this position. For
example, 80LET is an 80 amp, 240 volt fuse, 18 mm diameter. 160AEET
is a 160 amp, 660 volt fuse with two 18 mm diameter barrels and US high speed fuses
80 mm mountings.
Like the European square and round body fuses, US fuses also have
For F range fuses descriptive part numbers. While there is no recognized US dimensional
standard for high speed fuses, there are accepted industry standards
BS88 fuse code positions that Bussmann series fuses meet.
1 2 3 4 The following tables show the various options for all positions in the Type
Code.
80 A F E Standard fuses — Type FW
Current F Range
rating
Voltage Body
Fuses can be selected by the following codes:
or size
style
Fuse type FW code positions
Position 1 — Current rating 1 2 3 4 5 6
The continuous current rating in amps.
FW X - 1000 A H I
Position 2 — Voltage or style
Primary Technical Indicator
code revision type
Voltage Current Fixing
rating rating style
Position 1 — Primary code

All Bussmann series US style high speed fuses in the standard program
are designated by the prefix FW.
Position 2 — Voltage rating
The AC voltage rating of the fuse.
Letter code Volts Example

A 130 or 150 FWA-80A
X 250 FWX-1A14F
H 500 FWH-175B
C 600 FWC-12A10F
P 700 FWP-15A14F
K 750 FWK-5A20F
J 1000 FWJ-20A14F
L 1250 FWL-20A20F
S 1500 FWS-15A20F
Position 3 — Current rating Special fuses - Type SF and XL

For Bussmann series high speed fuses this is usually the continuous In addition to the standard FW fuses, special purpose fuses are offered
current rating. along with higher speed versions as an alternative to some of the FW
range. These special fuses can be selected by the following codes.
Position 4 — Technical revision
Resulting from continuous improvement, Bussmann series FW fuses Fuse type SF an XL code positions
also represent a consolidation after several acquisitions. When this 1 2 3 4 5 6 7
occurs, it is necessary to distinguish each technical revision without
changing the existing part numbers. In common with the semiconductor SF 75 X 1000 H I
industry, a letter code is used for this purpose. For technical reasons, Primary Style Technical Indicator
it may be necessary to maintain more than one of these revisions for code
Voltage Current
revision
Fixing
type
some applications, but most applications should use the latest revision. rating rating style
Position 4 Description Position 1 — Primary code

No mark The first version of this product Bussmann series US style high speed and special purpose fuses are
A, B, C, etc. Later improved version FWP-10B designated by the prefix SF or XL.
Position 5 — Fixing style Position 2 — Voltage rating
Most of the FW fuses have center blades with mounting holes. Generally, this is one tenth of the AC voltage rating of the fuse. For
However, flush end mounting (often called a “Hockey Puck”) are special purpose fuses, please check with Application Engineering at
common and so are the cylindrical or ferrule types. FuseTech@eaton.com.
Note: Where F is in Position 5, the first version of the product will be designated Position 3 — Style
with an A
Position 3 Description
High speed performance. This often also means good
F DC voltage performance
X Slow speed, often for traction applications
This is only an indication of the letters used; others may also be used.
Position 4 — Current rating
On standard high speed fuses this is usually the continuous current
rating. For special types, this position may only be an indication of
capabilities, as many of these designations are agreed upon with OEMs
for special applications.
Position 5 — Technical revision
When a technical revision occurs with products outside our main fuse
offerings, it is necessary to distinguish each technical revision without
changing the existing part numbers. In common with the semiconductor
industry, a letter code is used for this purpose. For technical reasons
it may be necessary to maintain more than one of these revisions for
some applications while most applications should use the latest revision.
Empty The first version of this product
A, B, C, etc. Later improved version
Position 6 — Fixing style
Most of the SF and XL type fuses have center blades with mounting
holes.
Position 6 — Indicator
Empty Standard blade
As standard, the Bussmann series FW fuses do not have visual
HP Flush end fixings — unified thread
indication of fuse operation.
BB Flush end fixings — metric thread
Position Others Agreed with OEM
Description
6
Position 7 — Indicator
Empty Standard product
Indication by additional external type TI (Trip Indicator) Position 7 Description
I indicating fuse that also takes the MAI or MBI type
microswitches (see BS style accessories) Empty Standard product
Indication by external indicator that also takes the Indication by additional external type TI (Trip Indicator)
SI I indicating fuse that also takes the MAI or MBI type
170H0069 microswitch
microswitches (see BS style accessories)
M Microswitch fitted
Appendix 3: Installation, maintenance, environmental and storage
High speed fuses are highly sophisticated and require proper installation
and maintenance. Doing so will help ensure reliable performance
throughout the fuse’s life. This section will cover the following topics:
F
• Tightening torque and contact pressure
F
• Mounting alignment F
F F
• Surface materials of contacts
• Resistance to vibration and shock
• Service/maintenance
• Environmental issues
F: Establish balance
Tightening torque and contact pressure
F
High speed fuses are electromechanical devices. Their proper function
Fuses with contact knives
depends on the contact quality between the fuse and the connecting
cables/busbars, or between the fuse and fuse holder. This is not only Generally this fuse type is divided into two main groups: fuses with
important for proper electrical contact, but also heat dissipation because slotted knives according to DIN 43653 for mounting directly on busbars
high speed fuses generate a lot of heat that is partially removed via (or in special fuse holders) and fuses with solid knives according to DIN
thermal conduction through the fuse’s connections. A poor thermal 43620 for mounting in spring-loaded fuse holders.
connection can result in the fuse overheating and a reduced service
life. Therefore it is important to apply the right tightening torque when DIN 43653 bolted tag fuses on busbars
mounting fuses.
Fuses for mounting on busbars are to be tightened with the
largest possible bolts/studs, nuts and washers. Use of washers is
Flush-end contact fuses recommended. The bolts/nuts are tightened with a torque appropriate to
their size and tensile strength. E.g., M8 Type 8.8 30 N•m (with lubricant)
For all kinds of flush-end fuses, grade 8.8 steel socket set screws or 50 N•m (without lubricant).
according to ISO 4026/DIN 913 or ISO 4029/DIN 916 are recommended.
The studs must be tightened carefully applying a torque of 5-8 N•m. As a
rule, the torque on the nuts relates to the threaded hole dimension in the DIN 43653 bolted tag fuses in blocks
fuse contact. A calibrated torque wrench with a tolerance of maximum
± 4 percent is recommended. The following provides the recommended Fuses mounted in special-made fuse blocks must be tightened according
nut tightening torques: to the specification provided with the blocks.
Thread hole Torque N•m Maximum tightening torque for some Bussmann series blocks are given
below:
Size/type mm Inches Ungreased Greased †
00B M8 — 20 10 Catalog number Torque N•m (lb-in)

1*B – 1*G M8 5/16 20 10 Bolts for holder* Bolts for cables/
1B – 1G M8 5/16 20 10 fuses
2B – 2G M10 3/8 40 20 170H1007** 4 (35) — M6 —
3B – 3G M12 1/2 50 40 170H3003 – 170H3006** 10 (88) — M8 —
23B – 23G 2 × M10 2 × 3/8 40 20 Conductor set Fuse mounting
screw bolt
4B – 4G 4 × M10 4 × 3/8 40 20
1BS101 13 (120) 8 (70)
24B – 24G 3 × M12 3 × 1/2 50 40
1BS102 31 (275) 13 (120)
5B – 5G 5 × M12 5 × 1/2 50 40
1BS103 31 (275) 19 (170)
FWX, FWA, KBC — 3/8 40 20
1BS104 42 (375) 19 (170)
† Greased with Rhodorsil paste 4. BH-1, 2, 3 — —
* Thread greased with Rhodorsil paste 4 (Rhone-Poulenc), etc.

Special flush-end types ** For 170Hxxxx holders, the above values can be increased by 25 percent if no
plastic parts are stressed.
Special types like 4SB or 24SB normally have threaded holes in only one
end and a plate on the other for mounting on (water cooled) busbars. In
DIN 43620 bladed fuses in blocks
such cases, the screw-in studs and nuts for the threaded hole use the
values in the torque table while the plate is mounted on the busbars This kind of holder is equipped with one or more springs to provide the
with 50 N•m of torque. correct contact pressure on the fuse’s blades. No tightening is possible
or recommendations given. When mounting Bussmann series holder
170H3040-47 onto the equipment, use a maximum tightening torque
of 10 N•m to mount it. Note: a fuse holder amp rating may not always
match up with the fuse amp rating. In some cases you might be able to
match the fuse’s rated watts loss and maximum permissible load current
to arrive at the correct holder.
Appendix 3: Installation, maintenance, environmental and storage
Press Pack fuses Surface material

Some of the most common semiconductors can be stack-mounted The electrical conducting parts of Bussmann series high speed fuses
under an applied clamping force. A range of so-called “Press Pack” are usually plated to maintain an acceptable surface condition. Tin is the
fuses in body type 3P and 4P are available, and allow the user to reduce most common material for the fuse contacts.
the required number of components. This can be achieved by clamping
Tin-plated contacts
the semiconductor and the fuse together with a water cooling box in a
single mounting arrangement. The maximum clamping force a fuse can
Concentration – duration
withstand depends on many factors such as:
PPM – h According to standard
• Fuse body length and cross section area H2O 12.5 ppm – 96 h IEC 68-2-43 Kd
• Temperature gradient between the fuse contacts SO2 25 ppm – 504 h IEC 68-2-42 Kd
• Electrical load conditions
Most Bussmann series high speed fuse contact surfaces are
When clamping a fuse into an application, these requirements need to electroplated with a 5 µm layer of tin. This plating provides an excellent
be considered: electrical and thermal interface with holders or cables/busbars of either
pure copper or copper/aluminum plated with tin/nickel or silver.
• The maximum clamping force applied to a press pack fuse should not
exceed the stated value (see table below) as this may damage the Many tests and more than 30 years of experience have shown that a
ceramic body tin, nickel or silver-plated surface is both mechanically and electrically
stable in the entire high speed fuse operating temperature range (typical
• To ensure safe electrical and thermal contact between fuse contact
maximum temperature rise of 130°C/266°F).
and the water cooling box or busbar, a minimum force of 2 N/mm²
needs to be applied to the contact area of the fuse
• A maximum for of 15 N/mm² can be applied to the minimum contact Vibration and shock resistance
area of the fuse, to ensure safe thermal contact pressure (note that High speed fuses should not be submitted to excessive vibration.
the total pressure should not exceed the amounts stated in the table However, standard high speed fuses can withstand vibration with a
below) maximum 5 g magnitude for a long-time basis and 7 g for short periods
(shocks). Before using fuses in applications with stronger vibration,
Example maximum clamping force values consult Application Engineering at FuseTech@eaton.com.
Size Single-sided cooling kN Double-sided cooling kN
3P/55 22 30 Service and maintenance
4P/60 40 50 The following points should be observed and checked during
3P/80 30 40 maintenance of electrical cabinets and switchgear:
4P/80 50 60 • Check tightening torques and examine ceramic fuse bodies for visible
Note: Greater permissible clamping force can be applied on some press pack cracks. Tighten or replace as needed.
fuses, please consult Application Engineering at FuseTech@eaton.com.
If a Press Pack fuse is water cooled at one end and not at the other, • Check all fuse indicators. In case of any fuse opening, replace all
there will be a temperature difference (thermal gradient) between each opened AND unopened fuses that have been subjected to the
contact end. If the difference in temperature at each end is greater than same fault current or any part of it. Even if the resistance (Ω) of the
55°C then the clamping values in the above table are invalid. For fuses unopened fuses is unchanged, the fuses may be damaged by the fault
that use double-sided water cooling, the temperature difference between current and must be replaced to avoid nuisance openings.
the fuse contacts is expected to be negligible, and the above table
values remain valid.
Environmental issues
There are Bussmann series double body Press Pack fuses (24B and
24+B). Consult Application Engineering (FuseTech@eaton.com) when Generally, high speed fuses are made from the following materials:
using these fuses in your application. • Ceramic
Mounting alignment • Fiberglass
Bussmann series high speed fuses are generally supplied in a ready-to- • Silver
install condition. • Copper
The fuses are not meant as mounting isolators. Excessive tension, • Brass
compression and torque from misalignment between fuse blades and
busbars (see example below) should be avoided. If possible, mounting • Steel
should start with the fuse followed by the necessary adjustment and • Silica sand
tolerance utilization of busbar components.
Accessories like microswitches and fuse holders are partly made of
various plastic materials. For further information on fuse materials,
contact Application Engineering at FuseTech@eaton.com.
Storage
Front view Side view Front view Side view
Fuses should be stored in their original boxes under typical warehouse
conditions for electromechanical products (free from any dirt and dust).
Storage conditions should be no more than 70 percent relative humidity
and in the -40°C to +85°C (-40°F to +185°F) range.
Correct: end tag is not distorted Incorrect: end tag is distorted
Glossary
Arcing I²t Continuous current rating
Value of the I²t during the arcing time under specified conditions. The current level that causes the fuse to operate in a time of four hours
is called the continuous current rating.
Amp (Ampere)
Current-limitation
The measurement of intensity of rate of flow of electrons in an electric
circuit. An amp is the amount of current that will flow through a A fuse operation relating to short-circuits only. When a fuse operates
resistance of one ohm under a pressure of one volt. in its current-limiting range, it will clear a short-circuit before the first
peak of the current. Also, it will limit the instantaneous peak let-
Amp rating
through current to a value substantially less than that obtainable in the
The current-carrying capacity of a fuse. It is given in amps RMS (root same circuit if that fuse were replaced with a solid conductor of equal
mean square, also called the effective value). impedance.
I²t (amp squared seconds) Cut-off current/peak let-through current
The measure of heat energy developed within a circuit during the The maximum value reached by the fault current during the interrupting
fuse operation. I stands for effective let-through current (RMS), which operation of a fuse. In many cases the fuse will be current-limiting.
is squared, and t stands for time of opening, in seconds. It can be
Electrical load
expressed as Melting I²t, Arcing I²t or the sum of them as Clearing I²t.
That part of the electrical system which actually uses the energy or does
Arcing time
the work required.
The amount of time from the instant the fuse has melted until the
Fast-acting fuse
overcurrent is safely interrupted (cleared).
A fuse which opens on overload and short-circuits very quickly. This
Arc voltage
type of fuse is not designed to withstand temporary overload currents
This is the voltage, which occurs between the terminals of a fuse during associated with some electrical loads, when sized near the full load
operation. The size of the arc voltage for a given fuse is supply voltage current of the circuit.
dependent.
Fulgurite
Breaking capacity
In the context of fuses, the non-conductive, rock like substance that
This is the maximum value of prospective current, RMS symmetrical,
forms during a fuse’s short-circuit interruption when the element material
which a fuse is capable of interrupting at stated conditions.
vapor fuses with the quartz sand fill.
Class of fuses/fuse class
Fuse
National and international standards have developed basic physical
An overcurrent protective device with a fusible link that operates and
specifications and electrical performance requirements for fuses with
opens the circuit on an overcurrent condition.
voltage ratings that pertain to specific countries.
The fuse class refers to the designed interrupting characteristic of the Fusing factor
fuse. The following fuse class found in IEC 60269 applies to high speed
The ratio of minimum fusing current to the rated current.
fuses.
High speed fuses
• aR - Partial-range interrupting capacity (short-circuit protection only) for
the protection of power semiconductors (IEC Utilization category). Fuses with no intentional time-delay in the overload range and designed
to open as quickly as possible in the short-circuit range. These fuses are
Other classes are: often used to protect solid-state devices.
• gG (gL) — Full-range interrupting capacity (overload and short-circuit
protection) for general applications (IEC Utilization category). I2t
• gM — Full-range interrupting capacity (overload and short-circuit Also referred at as the Joule integral, I2t is the integral of the square
protection)for the protection of motor circuits (IEC Utilization category). of the current over a given time interval. Pre-arcing I2t is the I2t integral
extended over the pre-arcing time of the fuse. Operating I2t is the I2t
• aM — Partial-range interrupting capacity (short-circuit protection only) integral extended over the operating time of the fuse.
for the protection of motor circuits (IEC Utilization category).
IEC
• gR — Full-range interrupting capacity (overload and short-circuit
protection) for the protection of Power Semiconductors (pending). IEC stands for the International Electrotechnical Commission. It is
a non-profit, non-governmental international standards organization
• gPV — gPV – Full-range interrupting capacity (overload and short-circuit that prepares and publishes International Standards for all electrical,
protection) for the protection of Photovoltaic (PV) systems” below gR electronic and related technologies – collectively known as
class. "electrotechnology."
Clearing (total operating) time
Inductive load
The total time between the beginning of the overcurrent and the final
opening of the circuit at system voltage. Clearing time is the total of the A load which has inductive properties. Common forms are motors,
melting time and the arcing time. transformers, wound control gear. This type of load pulls a large amount
of current when first energized.
Commutation fault
Interrupting capacity/rating
A fault that occurs on a regenerative DC drive due to a thyristor losing
its blocking capability while there is a direct line-to-line voltage across it, Refer to breaking capacity.
which leads to a short-circuit where the AC voltage is superimposed on Melting time
the DC voltage.
The amount of time required to melt the fuse element during a specified
overcurrent. (See arcing time and clearing time.)
Glossary
Ohm Time-current characteristics
The unit of measure for electric resistance. An ohm is the amount of These are the time and current levels needed for a fuse element to melt
resistance that will allow one amp to flow under a pressure of one volt. and open. They are derived using the same test arrangement as the
temperature rise test, with the fuse at ambient temperature before each
Overload
test.
This is a condition in which an overcurrent exceeds the normal full load
Time-delay fuse
current of a circuit that is in an otherwise healthy condition.
A fuse with a built-in delay that allows temporary and harmless inrush
currents to pass without opening, but is so designed to open on
The instantaneous value of peak current let-through by a current-limiting sustained overloads and short-circuits.
fuse, when it operates in its current-limiting range.
Total clearing time
Power factor
Also referred to as total clearing I2t, it is the total measure of heat energy
The ratio of active power (kW) to apparent power (kVA) drawn by a load.
developed within a circuit during the fuse’s clearing of a fault current.
It corresponds to the cosine of the phase angle between the voltage and
Total clearing I2t is the sum of the melting I2t and the arcing I2t.
current (cos).
Virtual melting time
Power losses/watts losses
Is a method of presenting melting times in a manner independent of the
The power released in a fuse when loaded according to stated
current waveform. It is the time that it would take a DC current equal to
conditions.
IP to generate the melting l²t. For high speed fuses, the virtual melting
time (tv) is used and plotted down to 0.1 ms. The formula for determining
Pre-arcing time
time-current characteristics is:
The time taken from the initiation of the fault to the element melting
Prospective short-circuit current
tv =
∫
i dt
2
Ip2
This is the current that would flow in the fault circuit if the fuse was Where:
replaced by a link with an infinitely small impedance. Normally it is given
as symmetrical RMS value, also called IP. tv = Virtual pre-arcing time
Recovery voltage i2 = Applied fuse current squared
This is the voltage which can be measured across the fuse connections dt = Change in time
after operation.
Ip = Prospective short-circuit current
Resistive load
An electrical load which is characteristic of not having any significant
Total operating (Clearing) I²t
inductive or capacitive component. When a resistive load is energized,
the current rises instantly to its steady-state value, without first rising to The total operating I²t value is the total of the pre-arcing and the arcing
a higher value. I²t values under specified conditions.
RMS current UL
Also known as the effective value, it corresponds to the peak UL stands for Underwriters Laboratories, Inc., an independent, non-
instantaneous value of a sinusoidal waveform divided by the square profit, and non-governmental organization focusing on product safety. UL
root of two. The RMS value of an alternating current is equivalent to the issues standards, and provides third party testing mainly for US markets.
value of direct current which would produce the same amount of heat or
Voltage rating
power.
The maximum open circuit RMS voltage in which a fuse can be used,
Semiconductor fuses
yet safely interrupt an overcurrent. Exceeding the voltage rating of a fuse
Fuses used to protect solid-state, semiconductor devices. Commonly impairs its ability to clear an overload or short-circuit safely.
referred to as high speed fuses or less commonly I2t fuses. See high
Withstand rating
speed fuses.
The maximum current that an unprotected electrical component
Short-circuit current
can sustain for a specified period of time without the occurrence of
Can be classified as an overcurrent which exceeds the normal full load extensive damage. See short-circuit current rating (SCCR).
current of a circuit by a factor many times.
Short-Circuit Current Rating (SCCR)
The maximum short-circuit current an electrical component can sustain
without the occurrence of excessive damage when protected with an
overcurrent protective device.
Threshold current
The symmetrical RMS available current at the threshold of the current
limiting range, where the fuse becomes current limiting when tested to This application guide is intended to clearly present comprehensive
the industry standard. This value can be read off of a peak let-through technical information that will help the end user with design application.
chart where the fuse curve intersects the A-B line. A threshold ratio is Eaton reserves the right to change design or construction of any
the relationship of the threshold current to the fuse’s continuous current products.
rating. This current is used during testing to UL specifications
Eaton also reserves the right to change or update, without notice, any
Time constant technical information contained in this application guide.
The inductance in a DC circuit limits the rate of current rise. The time Once a product has been selected, it should be tested by the user in all
required for the current to reach 63 percent of the final value at rated possible applications.
voltage is called the “time constant,” and is often referred to in terms of
L/R where L is inductance in Henries and R is resistance in ohms.
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114 Old State Road CSA is a registered trademark of the Canadian
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© 2016 Eaton Fire Protection Association, Inc. Follow us on social media to get the
All Rights Reserved NEMA is a registered trademark of the latest product and support information.
Printed in USA National Electrical Manufacturers Association.
Publication No. 10507 — BU-MC16053 UL is a registered trademark of the
June 2016 Underwriters Laboratories, Inc.

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Application Guide 10507 Effective June 2016

A: Introduction 3 J: Fuses protecting regenerative drives 20

Bussmann® series high

About this guide

The most common use for fuses is in electrical distribution systems

Typical fuse construction

How high speed fuses are different Application factors

Table B1. Factors to consider in high speed fuse selection.

What is this symbol?

The term “semiconductor fuse” used for high speed fuses is

Ambient temperature Short-circuit performance

Forced cooling Peak fuse currents

The time-current characteristic curve

Virtual pre-arcing time in seconds

Equation C1. Overload/fault current.

The A-A curve

100 200 300 400 500 600 660

Figure C4. K factor curve.

Normally the maximum I²t under short-circuit conditions will be the

Figure C7. Peak let-through curve.

Simple rated voltage determination

Watt loss correction curve

Voltage frequency (Hz)

For further details please refer to section Fuses protecting regenerative

Figure D3. AC fuses in a DC motor drive circuit.

Fuses under oscillating DC

Figure D4. AC fuses protecting GTOs and IGBTs on the DC side of

Ambient temperature in °C Meter per second (m/sec)

E2 = Voltage drop across fuse after 2 hours

Part 3 — Cyclic loading and safety margins

Assuming there aren’t any fuse current derating factors (i.e., Kt x Ke x

wave output would be inefficient with much distortion reflected to the

Kb = 1 Prospective current in Arms

Figure E8. Example 2 and 3, time-current curves.

Typical rectifier circuits

Figure F1. Single-phase, half wave L

Figure F2. Single-phase, full wave, center tap

Figure F7. Six-phase parallel (without IPT)

Figure F3. Single-phase, bridge

Figure F10. Three-phase, anti-parallel, control

Protection from internal faults L1

Figure G3. External fault.

Figure G1. Internal fault.

Continued service upon device failure

Melting time in seconds

peak let-through and time current curve at time-constants of 25 ms

Many circuits have a time constant ranging between 10 ms and 20

Protecting a battery (or batteries) is particularly difficult under fault

40 R = 16 mΩ circuit resistance in Figure I5

Figure J2. Cross-over fault — rectifier mode.

As a general rule, the fault voltage is half a sine wave at a lower

UAC + ≈ 1.8 × UAC

Figure J3. External fault — rectifier mode. Where:

Figure J6. DC shoot-through — regenerative mode. • 110 V system: 200 V fuse

Figure K2. GTO inverter.

4 • Nominal 80 Vdc voltage supply

= 0.45 (45%) x 75 W (Figure L3) = 4350 A / ((6-1) / (1.0+0.1))

= 34 W = 966 A (per diode of each arm of bridge rectifier)

Based on a power source of 950 A, 240 Vac a UL or British fuse series