Next

CIRCUIT PROTECTION TECHNOLOGIES

The application guidelines and product data in this guide are intended to provide technical information that will help with
application design. Since these are only a few of the contributing parameters, application testing is strongly recommended
and should be used to verify performance in the circuit/application. In the absence of special requirements, Littelfuse reserves
the right to make appropriate changes in design, process, and manufacturing location without notice.
The purpose of the Fuseology Section is to promote a better understanding of both fuses and common application details.
The fuses to be considered are current sensitive devices which are designed as the intentional weak link in the electrical
circuit. The function of the fuse is to provide protection of discrete components, or of complete circuits, by reliably melting
under current overload conditions. This fuseology section will cover some important facts about fuses, selection considera-
tions, and standards.

FUSE FACTS
The following fuse parameters or application concepts DIMENSIONS: Unless otherwise specified, dimensions are
should be well understood in order to properly select a fuse in inches. The fuses in this catalog range in size from the
for a given application. approx. 0603 chip size (.063"L x .031"W x .018"H) up to the
AMBIENT TEMPERATURE: Refers to the temperature of 5 AG, also commonly known as a “MIDGET” fuse (13/32"
the air immediately surrounding the fuse and is not to be dia. x 11/2" length). As new products were developed
confused with “room temperature.” The fuse ambient throughout the years, fuse sizes evolved to fill the various
temperature is appreciably higher in many cases, because electrical circuit protection needs. The first fuses were simple,
it is enclosed (as in a panel mount fuseholder) or mounted open-wire devices, followed in the 1890’s by Edison’s enclo-
near other heat producing components, such as resistors, sure of thin wire in a lamp base to make the first plug fuse.
transformers, etc. By 1904, Underwriters Laboratories had established size and
rating specifications to meet safety standards. The renewable
BREAKING CAPACITY: See Interrupting Rating. type fuses and automotive fuses appeared in 1914, and in
CURRENT RATING: The nominal amperage value marked 1927 Littelfuse started making very low amperage fuses for
on the fuse. It is established by the manufacturer as a value the budding electronics industry.
of current which the fuse can be loaded to, based on a con- The fuse sizes in the chart below began with the early
trolled set of test conditions (See RERATING). “Automobile Glass” fuses, thus the term “AG”. The numbers
Catalog Fuse part numbers include series identification and were applied chronologically as different manufacturers
amperage ratings. Refer to the FUSE SELECTION GUIDE started making a new size: “3AG,” for example, was the
section for guidance on making the proper choice. third size placed on the market. Other non-glass fuse sizes
and constructions were determined by functional require-
RERATING: For 25°C ambient temperatures, it is recom- ments, but they still retained the length or diameter dimen-
mended that fuses be operated at no more than 75% of the sions of the glass fuses. Their designation was modified to
nominal current rating established using the controlled test AB in place of AG, indicating that the outer tube was con-
conditions. These test conditions are part of UL/CSA/ANCE structed from Bakelite, fibre, ceramic, or a similar material
(Mexico) 248-14 “Fuses for Supplementary Overcurrent other than glass. The largest size fuse shown in the chart is
Protection,” whose primary objective is to specify common the 5AG, or “MIDGET,” a name adopted from its use by the
test standards necessary for the continued control of manu- electrical industry and the National Electrical Code range
factured items intended for protection against fire, etc. which normally recognizes fuses of 9/16" x 2" as the
Some common variations of these standards include: fully smallest standard fuse in use.
enclosed fuseholders, high contact resistances, air move-
ment, transient spikes, and changes in connecting cable FUSE SIZES
size (diameter and length). Fuses are essentially tempera- DIAMETER LENGTH
ture-sensitive devices. Even small variations from the SIZE (Inches) (Inches)
controlled test conditions can greatly affect the predicted life 1AG 1/4 .250 5/8 .625
of a fuse when it is loaded to its nominal value, usually 2AG — .177 — .588
expressed as 100% of rating. 3AG 1/4 .250 11/4 1.25
4AG 9/32 .281 11/4 1.25
The circuit design engineer should clearly understand that 5AG 13/32 .406 11/2 1.50
the purpose of these controlled test conditions is to enable 7AG 1/4 .250 7/8 .875
fuse manufacturers to maintain unified performance stan- 8AG 1/4 .250 1 1
dards for their products, and he must account for the vari-
able conditions of his application. To compensate for these TOLERANCES: The dimensions shown in this catalog are
variables, the circuit design engineer who is designing for nominal. Unless otherwise specified, tolerances are applied
trouble-free, long-life fuse protection in his equipment gener- as follows:
ally loads his fuse not more than 75% of the nominal rating ± .010" for dimensions to 2 decimal places.
listed by the manufacturer, keeping in mind that overload ± .005" for dimensions to 3 decimal places.
and short circuit protection must be adequately provided for. The factory should be contacted concerning metric system and
The fuses under discussion are temperature-sensitive fractional tolerances. Tolerances do not apply to lead lengths.
devices whose ratings have been established in a 25°C
FUSE CHARACTERISTICS: The characteristic of a fuse
ambient. The fuse temperature generated by the current
design refers to how rapidly the fuse responds to various
passing through the fuse increases or decreases with
current overloads. Fuse characteristics can be classified
ambient temperature change.
into three general categories: very fast-acting, fast-acting,
The ambient temperature chart in the FUSE SELECTION or Slo-Blo® Fuse. The distinguishing feature of Slo-Blo® fuses
GUIDE section illustrates the effect that ambient temperature is that these fuses have additional thermal inertia designed
has on the nominal current rating of a fuse. Most traditional to tolerate normal initial or start-up overload pulses.
Slo-Blo® Fuse designs use lower melting temperature
materials and are, therefore, more sensitive to ambient FUSE CONSTRUCTION: Internal construction may vary
temperature changes. depending on ampere rating. Fuse photos in this catalog
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CIRCUIT PROTECTION TECHNOLOGIES

FUSE FACTS
show typical construction of a particular ampere rating Resistance data on all of our fuses is available on request.
within the fuse series. Fuses can be supplied to specified controlled resistance
tolerances at additional cost.
FUSEHOLDERS: In many applications, fuses are
installed in fuseholders. These fuses and their associated SOLDERING RECOMMENDATIONS: Since most fuse con-
fuseholders are not intended for operation as a “switch” for structions incorporate soldered connections, caution should
turning power “on” and “off ”. be used when installing those fuses intended to be soldered
in place. The application of excessive heat can reflow the
INTERRUPTING RATING: Also known as breaking capac-
solder within the fuse and change its rating. Fuses are heat-
ity or short circuit rating, the interrupting rating is the maxi-
sensitive components similar to semi-conductors, and the
mum approved current which the fuse can safely interrupt
use of heat sinks during soldering is often recommended.
at rated voltage. During a fault or short circuit condition, a
fuse may receive an instantaneous overload current many TEST SAMPLING PLAN: Because compliance with certain
times greater than its normal operating current. Safe opera- specifications requires destructive testing, these tests are
tion requires that the fuse remain intact (no explosion or selected on a statistical basis for each lot manufactured.
body rupture) and clear the circuit.
TIME-CURRENT CURVE: The graphical presentation of
Interrupting ratings may vary with fuse design and range the fusing characteristic, time-current curves are generally
from 35 amperes AC for some 250V metric size (5 x 20mm) average curves which are presented as a design aid but
fuses up to 200,000 amperes AC for the 600V KLK series. are not generally considered part of the fuse specification.
Information on other fuse series can be obtained from Time-current curves are extremely useful in defining a fuse,
the factory. since fuses with the same current rating can be represent-
Fuses listed in accordance with UL/CSA/ANCE 248 are ed by considerably different time-current curves. The fuse
required to have an interrupting rating of 10,000 amperes, specification typically will include a life requirement at 100%
with some exceptions (See STANDARDS section) which, in of rating and maximum opening times at overload points
many applications, provides a safety factor far in excess of (usually 135% and 200% of rating). A time-current curve
the short circuit currents available. represents average data for the design; however, there
may be some differences in the values for any one given
NUISANCE OPENING: Nuisance opening is most often
production lot. Samples should be tested to verify perfor-
caused by an incomplete analysis of the circuit under consid-
mance, once the fuse has been selected.
eration. Of all the “Selection Factors” listed in the FUSE
SELECTION GUIDE, special attention must be given to UNDERWRITERS LABORATORIES: Reference to “Listed by
items 1, 3, and 6, namely, normal operating current, ambient Underwriters Laboratories” signifies that the fuses meet the
temperature, and pulses. For example, one prevalent cause requirements of UL/CSA/ANCE 248 “Fuses for
of nuisance opening in conventional power supplies is the Supplementary Overcurrent Protection”. Some 32 volt fuses
failure to adequately consider the fuse’s nominal melting I2t (automotive) in this catalog are listed under UL Standard 275.
rating. The fuse cannot be selected solely on the basis of Reference to “Recognized under the Component Program of
normal operating current and ambient temperature. In this Underwriters Laboratories” signifies that the item is recog-
application, the fuse’s nominal melting I2t rating must also nized under the component program of Underwriters
meet the inrush current requirements created by the input Laboratories and application approval is required.
capacitor of the power supply’s smoothing filter. The proce-
dure for converting various waveforms into I2t circuit demand is VOLTAGE RATING: The voltage rating, as marked on a
given in the FUSE SELECTION GUIDE. For trouble-free, fuse, indicates that the fuse can be relied upon to safely
long-life fuse protection, it is good design practice to select a interrupt its rated short circuit current in a circuit where the
fuse such that the I2t of the waveform is no more than 20% of voltage is equal to, or less than, its rated voltage. This sys-
the nominal melting I2t rating of the fuse. Refer to the sec- tem of voltage rating is covered by N.E.C. regulations and
tion on PULSES in the FUSE SELECTION GUIDE. is a requirement of Underwriters Laboratories as a protec-
tion against fire risk. The standard voltage ratings used by
RESISTANCE: The resistance of a fuse is usually an fuse manufacturers for most small-dimension and midget
insignificant part of the total circuit resistance. Since the fuses are 32, 63, 125, 250 and 600.
resistance of fractional amperage fuses can be several In electronic equipment with relatively low output power
ohms, this fact should be considered when using them in supplies, with circuit impedance limiting short circuit cur-
low-voltage circuits. Actual values can be obtained from the rents to values of less than ten times the current rating of
factory. Most fuses are manufactured from materials which the fuse, it is common practice to specify fuses with 125 or
have positive temperature coefficients, and, therefore, it is 250 volt ratings for secondary circuit protection of 500 volts
common to refer to cold resistance and hot resistance (volt- or higher.
age drop at rated current), with actual operation being some-
where in between. Cold resistance is the resistance obtained As mentioned previously (See RERATING), fuses are sen-
using a measuring current of no more than 10% of the fuse’s sitive to changes in current, not voltage, maintaining their
nominal rated current. Values shown in this publication for “status quo” at any voltage from zero to the maximum
cold resistance are nominal and representative. The factory rating of the fuse. It is not until the fuse element melts and
should be consulted if this parameter is critical to the design arcing occurs that the circuit voltage and available power
analysis. Hot resistance is the resistance calculated from the become an issue. The safe interruption of the circuit, as it
stabilized voltage drop across the fuse, with current equal to relates to circuit voltage and available power, is discussed
the nominal rated current flowing through it. in the section on INTERRUPTING RATING.

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CIRCUIT PROTECTION TECHNOLOGIES

FUSE FACTS
To summarize, a fuse may be used at any voltage that is is repeated until melting of the fuse element is confined to
less than its voltage rating without detriment to its fusing within about 8 milliseconds. The purpose of this procedure
characteristics. Please contact the factory for applications is to assure that the heat created has insufficient time to
at voltages greater than the voltage rating. thermally conduct away from the fuse element. That is, all
of the heat energy (I2t) is used, to cause melting. Once the
DERIVATION OF NOMINAL MELTING I2t: Laboratory tests measurements of current (I) and time (t) are determined, it
are conducted on each fuse design to determine the is a simple matter to calculate melting I2t. When the melting
amount of energy required to melt the fusing element. This phase reaches completion, an electrical arc occurs immedi-
energy is described as nominal melting I2t and is expressed ately prior to the “opening” of the fuse element. Clearing
as “Ampere Squared Seconds” (A2 Sec.). A pulse of current I2t = Melting I2t + arcing I2t. The nominal I2t values given in
is applied to the fuse, and a time measurement is taken for this publication pertain to the melting phase portion of the
melting to occur. If melting does not occur within a short “clearing” or “opening”.
duration of about 8 milliseconds (0.008 seconds) or less,
the level of pulse current is increased. This test procedure

FUSE SELECTION GUIDE

The application guidelines and product data in this guide are intended to provide technical information that will help with
application design. Since these are only a few of the contributing parameters, application testing is strongly recommended
and should be used to verify performance in the circuit/application.
Many of the factors involved with fuse selection are listed CHART SHOWING EFFECT OF AMBIENT TEMPERATURE
below: ON CURRENT-CARRYING CAPACITY (TYPICAL)
Selection Factors KEY TO CHART:
1. Normal operating current Curve A: Thin-Film Fuses and 313 Series (.010 to .150A)
2. Application voltage (AC or DC) Curve B: Very Fast-Acting, Fast-Acting, and Spiral Wound
Slo-Blo® Fuses
3. Ambient temperature
Curve C: Resettable PTC’s
4. Overload current and length of time in which the fuse
must open. C
140
5. Maximum available fault current
6. Pulses, Surge Currents, Inrush Currents, Start-up 120
A
PERCENT OF RATING*

Currents, and Circuit Transients B

7. Physical size limitations, such as length, diameter, or 100
B
height
8. Agency Approvals required, such as UL, CSA, VDE, 80

or Military A
60
9. Considerations: mounting type/form factor, ease of
removal, axial leads, visual indication, etc. 25°C
40
10. Fuseholder features: clips, mounting block, panel C

mount, p.c. board mount, R.F.I. shielded, etc.

20
NORMAL OPERATING CURRENT: The current rating of a -60°C -40°C -20°C 0°C 20°C 40°C 60°C 80°C 100°C 120°C
fuse is typically derated 25% for operation at 25°C to avoid -76°F -40°F -4°F 32°F 68°F 104°F 140°F 176°F 212°F 248°F
nuisance blowing. For example, a fuse with a current rating AMBIENT TEMPERATURE
of 10A is not usually recommended for operation at more
than 7.5A in a 25°C ambient. For additional details, see *Ambient temperature effects are in addition to the normal
RERATING in the previous section and AMBIENT derating, see example.
TEMPERATURE below.
VOLTAGE: The voltage rating of the fuse must be equal to,
or greater than, the available circuit voltage. For excep-
tions, see VOLTAGE RATING. Example: Given a normal operating current of 1.5 amperes in
AMBIENT TEMPERATURE: The current carrying capacity an application using a traditional Slo-Blo® fuse at room tem-
tests of fuses are performed at 25°C and will be affected by perature, then:
changes in ambient temperature. The higher the ambient Normal Operating Current
Catalog Fuse Rating =
temperature, the hotter the fuse will operate, and the short- 0.75
er its life will be. Conversely, operating at a lower tempera-
or
ture will prolong fuse life. A fuse also runs hotter as the nor-
mal operating current approaches or exceeds the rating of 1.5 Amperes
= 2.0 Amp Fuse (at 25°C)
the selected fuse. Practical experience indicates fuses at 0.75
room temperature should last indefinitely, if operated at no
more than 75% of catalog fuse rating.

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CIRCUIT PROTECTION TECHNOLOGIES

FUSE SELECTION GUIDE

Similarly, if that same fuse were operated at a very high category of “pulses.” Laboratory tests are conducted on
ambient temperature of 70°C, additional derating would be each fuse design to determine its nominal melting I2t rating.
necessary. Curve “A” (Traditional Slo-Blo® Fuse) of the The values for I2t given in this publication are nominal and
ambient temperature chart shows the maximum operating representative. The factory should be consulted if this
“Percent of Rating” at 70°C to be 80%, in which case; parameter is critical to the design analysis. The following
Nominal Operating Current example should assist in providing a better understanding
Catalog Fuse Rating = of the application of I2t.
0.75 x Percent of Rating
EXAMPLE: Select a 125V, very fast-acting PICO® fuse
or that is capable of withstanding 100,000 pulses of current (I)
1.5 Amperes
= 2.5 Amp Fuse (at 70°C) of the pulse waveform shown in Figure 1. The normal oper-
0.75 x 0.80 ating current is 0.75 ampere at an ambient temperature
of 25°C.
OVERLOAD CURRENT CONDITION: The current level for
which protection is required. Fault conditions may be speci- Step 1 — Refer to Chart I (page #6) and select the appro-
fied, either in terms of current or, in terms of both current priate pulse waveform, which is waveform (E) in this
and maximum time the fault can be tolerated before dam- example. Place the applicable value for peak pulse current
age occurs. Time-current curves should be consulted to try (ip) and time (t) into the corresponding formula for wave-
to match the fuse characteristic to the circuit needs, while shape (E), and calculate the result, as shown:
keeping in mind that the curves are based on average data. 1
I2t = (ip)2 t
MAXIMUM FAULT CURRENT: The Interrupting Rating of a 5
fuse must meet or exceed the Maximum Fault Current of
1
the circuit. =
x 82 x .004 = 0.0512 A2 Sec.
5
PULSES: The general term “pulses” is used in this context This value is referred to as the “Pulse I2t”.
to describe the broad category of wave shapes referred to
as “surge currents”, “start-up currents”, “inrush currents”, Step 2 — Determine the required value of Nominal Melting
and “transients”. Electrical pulse conditions can vary con- I2t by referring to Chart II (page 6). A figure of 22% is
siderably from one application to another. Different fuse shown in Chart II for 100,000 occurrences of the Pulse I2t
constructions may not all react the same to a given pulse calculated in Step 1. This Pulse I2t is converted to its
condition. Electrical pulses produce thermal cycling and required value of Nominal Melting I2t as follows:
possible mechanical fatigue that could affect the life of the Nom. Melt I2t = Pulse I2t/.22
fuse. Initial or start-up pulses are normal for some applica- = 0.0512/.22 = 0.2327 A2 Sec.
tions and require the characteristic of a Slo-Blo® fuse.
Slo-Blo® fuses incorporate a thermal delay design to enable
them to survive normal start-up pulses and still provide
protection against prolonged overloads. The start-up pulse
should be defined and then compared to the time-current
curve and I2t rating for the fuse. Application testing is rec-
ommended to establish the ability of the fuse design to
withstand the pulse conditions.
Nominal melting I2t is a measure of the energy required to
melt the fusing element and is expressed as “Ampere
Squared Seconds” (A2 Sec.). This nominal melting I2t, and 10
the energy it represents (within a time duration of 8 milli-
seconds [0.008 second] or less and 1 millisecond [0.001
Current (Amperes)

8
second] or less for thin film fuses), is a value that is con-
stant for each different fusing element. Because every fuse
type and rating, as well as its corresponding part number, 6
has a different fusing element, it is necessary to determine
the I2t for each. This I2t value is a parameter of the fuse
itself and is controlled by the element material and the con- 4
figuration of the fuse element. In addition to selecting fuses
on the basis of “Normal Operating Currents”, “Derating”, Normal Operating Current
and “Ambient Temperature” as discussed earlier, it is also l2 t
2
necessary to apply the I2t design approach. This nominal Pulse
melting I2t is not only a constant value for each fuse ele- Energy
ment design, but it is also independent of temperature and
voltage. Most often, the nominal melting I2t method of fuse .001 .002 .003 .004 .005 .006
selection is applied to those applications in which the fuse Time (Seconds)
must sustain large current pulses of a short duration. These
high-energy currents are common in many applications and Figure 1
are described by a variety of terms, such as “surge cur-
rent”, “start-up current”, “inrush current”, and other similar
circuit “transients” that can be classified in the general

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FUSE SELECTION GUIDE

Step 3 — Examine the I2t rating data for the PICO® II, 125V,
CHART I very fast-acting fuse. The part number 251001, 1 ampere
design is rated at 0.256 A2 Sec., which is the minimum fuse
rating that will accommodate the 0.2327 A2 Sec. value
WAVESHAPES FORMULAS calculated in Step 2. This 1 ampere fuse will also accom-
modate the specified 0.75 ampere normal operating
current, when a 25% derating factor is applied to the
1 ampere rating, as previously described.
ip TESTING: The above factors should be considered in
A i=k selecting a fuse for a given application. The next step is to
I2t = ip2 t verify the selection by requesting samples for testing in the
t
actual circuit. Before evaluating the samples, make sure
ib the fuse is properly mounted with good electrical connec-
tions, using adequately sized wires or traces. The testing
ip should include life tests under normal conditions and over-
B i = ip -kt load tests under fault conditions, to ensure that the fuse will
I2t = (1/3)(ip2 + ipib + ib2) t operate properly in the circuit.
t
CHART II
PULSE CYCLE WITHSTAND CAPABILITY
100,000 Pulses Pulse I2t = 22% of Nominal Melting I2t
ip 10,000 Pulses Pulse I2t = 29% of Nominal Melting I2t
C i = ip sin t 1,000 Pulses Pulse I2t = 38% of Nominal Melting I2t
t I2t = (1/2) ip2 t 100 Pulses Pulse I2t = 48% of Nominal Melting I2t
100000

D ip
I2t = (1/3) ip2 t
t 10000
Number of Pulses

E ip OR
i = kt2 OR i = ip (1-kt)2
I2t = (1/5) ip2 t
t t 1000

F ip i = ipe–kt)
t1 I2t ≅ (1/2) ip2 t1 100
10% 100%
Pulse I2 t / Average Melting I2 t
Note: Adequate time (10 seconds) must exist between pulse
events to allow heat from the previous event to dissipate.

FUSEHOLDER SELECTION GUIDE

RERATING: For 25°C ambient temperatures, it is current is considered to be the rated current of the fuse-
recommended that fuseholders be operated at no more holder, expressed as 100% of rating. Some of the more
than 60% of the nominal current rating established using common, everyday applications may differ from these UL
the controlled test conditions specified by Underwriters test conditions as follows: fully enclosed fuseholders, high
Laboratories. The primary objective of these UL test contact resistance, air movement, transient spikes, and
conditions is to specify common test standards necessary changes in connecting cable size (diameter and length).
for the continued control of manufactured items intended Even small variations from the controlled test conditions
for protection against fire, etc. A copper dummy fuse is can greatly affect the ratings of the fuseholder. For this
inserted in the fuseholder by Underwriters Laboratories, reason, it is recommended that fuseholders be derated by
and then the current is increased until a certain tempera- 40% (operated at no more than 60% of the nominal current
ture rise occurs. The majority of the heat is produced by rating established using the Underwriter Laboratories test
the contact resistance of the fuseholder clips. This value of conditions, as stated above).

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CIRCUIT PROTECTION TECHNOLOGIES

Littelfuse is at your service to help solve your electrical protection problems. When contacting Littelfuse sales engineers,
please have all the requirements of your applications available. Requests for quotes or assistance in designing or selecting
special types of circuit protection components for your particular applications are also welcome.
In the absence of special requirements, Littelfuse reserves the right to make appropriate changes in design, process, and
manufacturing location without prior notice.

STANDARDS

Fuse ratings and other performance criteria are evaluated INTERNATIONAL ELECTROTECHNICAL
under laboratory conditions and acceptance criteria, as COMMISSION (IEC)
defined in one or more of the various fuse standards. It is Publication 60127, Sheet 1, 2, 3, 5, 6 (250 Volts)
important to understand these standards so that the fuse
can be properly applied to circuit protection applications. The IEC organization is different from UL and CSA, since
IEC only writes specifications and does not certify. UL and
UL/CSA/ANCE (Mexico) 248-14 FUSES FOR SUPPLE- CSA write the specifications, are responsible for testing,
MENTARY OVERCURRENT PROTECTION (600 Volts, and give certification.
Maximum) (Previously UL 198G and CSA C22.2, No. 59) Certification to IEC specifications are given by such
UL UL LISTED
®
organizations as SEMKO (Swedish Institute of Testing and
A UL Listed fuse meets all the requirements of the UL/CSA Approvals of Electrical Equipment) and BSI (British
248-14 Standard. Following are some of the requirements. Standards Institute , as well as UL and CSA.
UL ampere rating tests are conducted at 100%, 135%, and IEC Publication 60127 defines three breaking capacity
200% of rated current. The fuse must carry 110% of its levels (interrupting rating). Low breaking capacity fuses
ampere rating and must stabilize at a temperature that must pass a test of 35 amperes or ten times rated current,
does not exceed a 75°C rise at 100%. whichever is greater, while enhanced breaking capacity
fuses must pass a test of 150 amperes and finally high
The fuse must open at 135% of rated current within one breaking capacity fuses must pass a test of 1500 amperes.
hour. It also must open at 200% of rated current within 2
minutes for 0-30 ampere ratings and 4 minutes for 35-60 Sheet 1 – Type F Quick Acting, High Breaking Capacity
ampere ratings. Sheet 2 – Type F Quick Acting, Low Breaking Capacity
The interrupting rating of a UL Listed fuse is 10,000 Sheet 3 – Type T Time Lag, Low Breaking Capacity
amperes AC minimum at 125 volts. Fuses rated at 250 Sheet 5 – Type T Time Lag, High Breaking Capacity
volts may be listed as interrupting 10,000 amperes at 125 Sheet 6 – Type T Time Lag, Enhanced Breaking Capacity
volts and, at least, the minimum values shown below at
250 volts. The letters ‘F’ and ‘T’ represent the time-current characteris-
tic of the fast-acting and time delay fuses. One of these
Ampere Rating Interrupting Rating Voltage letters will be marked on the end cap of the fuse.
of Fuse In Amperes Rating
0 to 1 35 250 VAC UL/CSA/ANCE (Mexico) 248-14 vs. IEC 60127 FUSE
1.1 to 3.5 100 250 VAC OPENING TIMES (UL/CSA/ANCE (Mexico) 248-14 Was
3.6 to 10 200 250 VAC Previously UL 198G and CSA 22.2, No. 59) vs. MITI B
10.1 to 15 750 250 VAC
15.1 to 30 1500 250 VAC Percent UL & CSA IEC TYPE F IEC Type F IEC Type T IEC Type T MITI
of Rating STD 248-14 Sheet 1 (*) Sheet 2 (*) Sheet 3 & 4 (*) Sheet 5 (*) B

® Recognized Under the Component Program 110 4 Hr. Min. — — — —

of Underwriters Laboratories 130 — — — — — 1Hr. Min.

The Recognized Components Program of UL is different 60 Minutes

135 — — — —
Max.
from UL Listing. UL will test a fuse to a specification request-
60 Minutes 60 Minutes 60 Minutes 60 Minutes
ed by the manufacturer. The test points can be different from 150 —
Min. Min. Min. Min.
the UL Listed requirements if the fuse has been designed for
160 — — — — — 1 Hr. Max.
a specific application. Application approval is required by UL
2 Minutes 2 Minutes
for fuses recognized under the Component Program. 200
Max.
— — — —
Max.
UL 275 AUTOMOTIVE GLASS TUBE FUSES (32 Volts) 210 —
30 Minutes 30 Minutes 2 Minutes 30 Minutes
Max. Max. Max. Max.
UL Listed
UL ampere ratings tests are conducted at 110%, 135%, and (*) Note: The IEC Specification is only written up to
200%. Interrupting rating tests are not required. 6.3A, any components above these ratings are not
recognized by the IEC (although the fuses may have
CSA Certification
®
those opening characteristics).
CSA Certification in Canada is equivalent to UL Listing in
the United States. IEC also has requirements at 275%, 400% and 1000%;
The Component Acceptance Program of CSA is
®
however, the chart is used to show that fuses with the
equivalent to the Recognition Program at UL. This CSA same ampere rating made to different specifications are not
Program allows the manufacturer to declare a interchangeable. According to the IEC 60127 Standard, a
specification. CSA then verifies the test results. one ampere-rated fuse can be operated at one ampere. A
one ampere-rated fuse made to UL/CSA/ANCE 248-14
MITI APPROVAL should not be operated at more than .75 ampere (25% der-
MITI approval in Japan is similar to UL Recognition in the ated — See RERATING section of FUSEOLOGY).
United States. MITI B covers only one characteristic i.e. there are no ‘delay’
MITI B has its own design standard and characteristics. definitions on other performance variants.
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CIRCUIT PROTECTION TECHNOLOGIES

STANDARDS AND PACKAGING INFORMATION

Publication IEC 60127-4 MILITARY/FEDERAL STANDARDS

(Universal Modular Fuse-Links [UMF] ) See Table of Contents for Military Product Section.
Fuses and holders approved to the following Military
This part of IEC 60127 covers both PCB
specifications are on the Qualified Products List (QPL)
through-hole and surface mount fuses. This standard
for that specification.
covers fuses rated 32, 63, 125, and 250 volts. This
standard will be accepted by UL/CSA making it the first MIL-PRF-15160 and MIL-PRF-23419
global fuse standard. This specification uses different fusing These specifications govern the construction and
gates than IEC 60127-2; the gates used here are performance of fuses suitable primarily for military
125%, 200%, and 1000%. electronic applications.
The fuses must not open in less than one hour at 125% of MIL-PRF-19207
rated current and open within two minutes at 200% of rated
current. The 1000% overload is used to determine the fuse This specification governs the construction and perfor-
characteristic. The time for each rating is listed below. mance of fuseholders suitable for military applications.
Type FF: Less than 0.001 sec. DESC Drawing #87108
Type F: From 0.001 - 0.01 sec. This drawing governs the construction and performance
Type T: From 0.01 - 0.1 sec. of .177" x .570" (2AG size) cartridge fuses and axial lead
versions suitable for military applications. DESC #87108
Type TT: From 0.1 - 1.00 sec.
designation is included in the fuse end cap marking.
These characteristics correlate to the terminology used
in IEC 60127-1. FEDERAL SPECIFICATION W-F-1814
Breaking capacity (interrupting rating) varies based on volt This specification governs the construction and performance
age rating. Parts rated at 32 & 63 volts must pass a test of of fuses with high interrupting ratings that are approved for
35 amperes or ten times rated current, whichever is federal applications. Fuses approved to these specifications
greater.Parts rated at 125 volts must pass a test of 50 are on the Federal Qualified Products List.
amperes or ten times rated current, whichever is greater.
Parts rated at 250 volts are further defined as either low, Write to the following agencies for additional information
intermediate or high breaking. The low breaking capacity on standards, approvals, or copies of the specifications.
fuses must pass a test of 100 amperes or ten times rated Underwriters Laboratories Inc. (UL)
current, while intermediate breaking capacity fuses must 333 Pfingsten Road
pass a test of 500 amperes and, finally, high breaking Northbrook, IL 60062
capacity fuses must pass a test of 1500 amperes.
Att: Publications Stock
Canadian Standards Association (CSA)
Packaging Suffixes 178 Rexdale Boulevard
Rexdale, Ontario, Canada M9W 1R3
A/X = 1 unit per bag
V = 5 units per box Att: Standard Sales
T = 10 units per box International Electrotechnical Commission (IEC)
H = 100 units per box 3, Rue de Varembe
U = 500 units per box 1211 Geneva 20
M = 1000 units per box Switzerland
P = 2000 units per box
Att: Sales Department
W = 3000 units per box
N = 5000 units per box Naval Publications and Military Standards
R = Taped & reeled fuses Form Center (for Military and Federal Standards)
M1 = Taped & reeled. Spacing = 4 mm. 5801 Tabor Avenue
1000 pieces per reel Philadelphia, PA 19120
MT1 = Taped & reeled. Spacing = 2.062 inches (52.4 mm) Att: Commanding Officer
1000 pieces per reel
MT2 = Taped & reeled. Spacing = 2.50 inches (63.5 mm) Defense Supply Center Columbus (DSSC)
1000 pieces per reel 3990 East Broad Street
MT3 = Taped & reeled. Spacing = 2.874 inches (73 mm) Columbus, OH 43216-5000
1000 pieces per reel
Ministry of International Trade and Industry (MITI)
Kasumigaseki
NT1 = Taped & reeled. Spacing = 2.062 inches (52.4 mm)
Chi-Youda-Ku
5000 pieces per reel
Tokyo 100, Japan
NT2 = Taped & reeled. Spacing = 2.50 inches (63.5 mm)
5000 pieces per reel
NT3 = Taped & reeled. Spacing = 2.874 inches (73 mm)
5000 pieces per reel

Tx = Taped & reeled. Spacing to be determined.

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CIRCUIT PROTECTION TECHNOLOGIES

PTC FACTS

Overcurrent circuit protection can be accomplished with the TEMPERATURE RATING: The useful upper limit for a PTC
use of either a traditional fuse or the more recently devel- is generally 85°C while the maximum operating tempera-
oped resettable PTC. Both devices function by reacting to ture for fuses is 125°C. The following temperature derating
the heat generated by the excessive current flow in the cir- curves that compare PTCs to fuses illustrate that more der-
cuit. The fuse melts open, interrupting the current flow, and ating is required for a PTC at a given temperature.
the PTC changes from a low resistance to a high resistance
to limit current flow. Understanding the differences in perfor- Key to chart: Curve A: Thin-Film Fuses and 313 Series (.010 to .150A)
Curve B: Very Fast-Acting, and Spiral Wound Fuses
mance between the two types of devices will make the best Curve C: Resettable PTCs

circuit protection choice easier. 140

C

The most obvious difference is that the PTC is resettable.

120
The general procedure for resetting after an overload has A

PERCENT OF RATING*
B
occurred is to remove power and allow the device to cool 100
B
down. There are several other operating characteristics that
differentiate the two types of products. The terminology 80

used for PTCs is often similar but not the same as for 60
A

fuses. Two parameters that fall into this category are leak-
25°C
age current and interrupting rating. 40
C

LEAKAGE CURRENT: The PTC is said to have “tripped” 20

when it has transitioned from the low resistance state to the -60°C -40°C -20°C 0°C 20°C 40°C 60°C 80°C 100°C 120°C
high resistance state due to an overload. -76°F -40°F -4°F 32°F 68°F 104°F 140°F 176°F 212°F 248°F
AMBIENT TEMPERATURE
• Ambient temperature effects are in addition to the normal derating.

Additional operating characteristics can be reviewed by the

circuit designer in making the decision to choose a PTC or
a fuse for overcurrent protection.
AGENCY APPROVALS: PTCs are Recognized under the
Log resistance (ohms)

Component Program of Underwriters Laboratories to UL

Thermistor Standard 1434. The devices have also been
certified under the CSA Component Acceptance Program.
Approvals for fuses include Recognition under the
Component Program of Underwriters Laboratories and the
Trip Point CSA Component Acceptance Program. In addition, many
fuses are available with full “Listing” in accordance with the
new Supplementary Fuse Standard UL/CSA/ANCE
(Mexico) 248-14.
Temperature (°C)
RESISTANCE: Reviewing product specifications indicates
that similarly rated PTCs have about twice (sometimes
Protection is accomplished by limiting the current flow to more) the resistance of fuses.
some leakage level. Leakage current can range from less
than a hundred milliamps at rated voltage up to a few hun- TIME-CURRENT CHARACTERISTIC: Comparing the
dred milliamps at lower voltages. The fuse on the other time-current curves of PTCs to time-current curves of fuses
hand completely interrupts the current flow and this open show that the speed of response for a PTC is similar to the
circuit results in “0” leakage current when subjected to time delay of a Slo-Blo® fuse.
an overload. SUMMARY: Many of the issues discussed become a
INTERRUPTING RATING: The PTC is rated for a maxi- matter of preference, but there is an important area of
mum short circuit current at rated voltage. This fault current application where the use of resettable PTCs is becoming a
level is the maximum current that the device can withstand requirement. Much of the design work for personal comput-
but the PTC will not actually interrupt the current flow (see ers and peripheral devices is strongly influenced by
LEAKAGE CURRENT above). A typical PTC short circuit Microsoft and Intel System Design Guide which states that
rating is 40A. Fuses do in fact interrupt the current flow in “Using a fuse that must be replaced each time an overcur-
response to the overload and the range of interrupting rat- rent condition occurs is unacceptable.” And the Plug and
ings goes from hundreds of amperes up to 10,000 amperes Play SCSI (Small Computer Systems Interface)
at rated voltage. Specification for this large market includes a statement that
“. . . must provide a self-resetting device to limit the maxi-
The circuit parameters may dictate the component choice mum amount of current sourced”.
based on typical device rating differences.
The PTC / fuse discussion provides some insight as to
VOLTAGE RATING: General use PTCs are not rated above when PTCs may be the appropriate choice for providing
60V while fuses are rated up to 600V. overcurrent circuit protection. A selection guide worksheet
CURRENT RATING: The operating current rating for PTCs appears on the following page as an aid in choosing the
can be up to 11A while the maximum level for fuses can best circuit protection component.
exceed 20A.

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CIRCUIT PROTECTION TECHNOLOGIES

SELECTION GUIDE WORKSHEET

1. Define the circuit operating parameters.

Complete the following form:

Normal operating current in amperes:

Normal operating voltage in volts:

Maximum interrupt current: (see page 3)

Ambient Temperature/Rerating: (see page 4)

Typical overload current:

Required opening time at specified overload:

Transient pulses expected: (see page 5)

Resettable or one-time:

Agency Approvals:

Mounting type/form factor:

Typical resistance (in circuit):

2. Select the proper circuit protection component.

LITTELFUSE CIRCUIT PROTECTION COMPARISON TABLE:

Surface Mount 30V PTC 60V PTC ‘0603’ SMF ‘1206’ SMF
PTC (Pg. 22-25) Leaded (Pg. 26-27) Leaded (Pg. 28-29) (Pg. 34-35) (Pg. 33, 36)
Operating 0.300 - 0.900 - 0.100 - 0.250 - 0.125 -
Current Range 2.6A 9A 3.75A 5A 7A
Maximum Voltage (*) 60V 30V 60V 32V 125V
Maximum
40A 40A 40A 50A 50A
Interrupting Rating (**)
– 40°C to – 40°C to – 40°C to –55°C to –55°C to
Temperature Range
85°C 85°C 85°C 125°C 125°C
Thermal Rerating Medium Medium Medium Low Low
Opening time at Fast to
Slow Slow Slow Fast
200% IN (***) Medium
Transient Withstand Low Low Low Low Low
Low to
Resistance Medium Medium Low Low
Medium
UL, CSA, UL, CSA, UL CSA,
Agency Approvals UL, CSA UL, CSA
TUV TUV TUV
Operational Uses Multiple Multiple Multiple One Time One Time
Mounting/Form Surface Surface Surface
Leaded Leaded
Factor Mount Mount Mount
(*) Maximum operating voltage in the series, parts may be used at voltages equal to or less than this value.
(**) Maximum interrupting rating at specified voltage which may be less than maximum operating voltage.
(***) Opening time is in relation to other forms of protection. A fast device will typically operate within three seconds at 200% of rated current.

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CIRCUIT PROTECTION TECHNOLOGIES

SELECTION GUIDE WORKSHEET

3. Determine the opening time at fault.

Consult the Time-Current (T-C) Curve to determine if the selected part will operate within the constraints of your applica-
tion. If the device opens too soon, the application may experience nuisance operation. If the device does not open soon
enough, the overcurrent may damage downstream components.
To determine the opening time for the chosen device, locate the overload current on the X-axis of the appropriate T-C
Curve and follow its line up to its intersection with the curve. At this point read the time listed on the Y-axis. This is the
average opening time for that device. If your overload current falls to the right of the curve the device will open. If the
overload current is to the left of the curve the device will not operate.

4. Verify ambient operating parameters.

Ensure that the application voltage is less than or equal to the device’s rated voltage and that the operating temperature
limits are within those specified by the device.

5. Verify the device’s dimensions.

Using the information from the Designer’s Guide page, compare the maximum dimensions of the device to the space avail-
able in the application.

LITTELFUSE CIRCUIT PROTECTION COMPARISON TABLE:

Nano2® SMF Fuse PICO® II Fuse 2AGs 5x20mm 3AGs/3ABs Midgets
(Pg. 40-41) (Pg. 48-51) (Pg. 55-57) (Pg. 62-70) (Pg. 58-61,71) (Pg 76-84)
Operating 0.062 - 0.062 - 0.100 - 0.032 - 0.010 - 0.100 -
Current Range 15A 15A 10A 15A 35A 30A
Maximum Voltage (*) 125V 250V 250V 250V 250V 600V
Maximum
50A 50A 10,000A 10,000A 10,000A 200,000A
Interrupting Rating (**)
–55°C to –55°C to –55°C to –55°C to –55°C to –55°C to
Temperature Range
125°C 125°C 125°C 125°C 125°C 125°C
Thermal Rerating Low Low Low Low Low Low
Opening time at Fast to Fast to Fast to Fast to Fast to Fast to
200% IN (***) Medium Medium Medium Slow Slow Slow
Low to Low to Low to Low to Low to Low to
Transient Withstand
Medium Medium High High High High
Resistance Low Low Low Low Low Low
CSA, BSI,
UL, CSA, UL, CSA, UL, CSA, UL, CSA,
Agency Approvals VDE, MITI, UL, CSA
MITI MITI MITI MITI
SEMKO, UL
Operational Uses One Time One Time One Time One Time One Time One Time
Mounting/Form Surface Leaded or Leaded or Leaded or
Leaded Cartridge
Factor Mount Cartridge Cartridge Cartridge
(*) Maximum operating voltage in the series, parts may be used at voltages equal to or less than this value.
(**) Maximum interrupting rating at specified voltage which may be less than maximum operating voltage.
(***) Opening time is in relation to other forms of protection. A fast device will typically operate within three seconds at 200% of rated current.

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CIRCUIT PROTECTION TECHNOLOGIES

PulseGuard® Suppressors

ESD Suppressor FACTS

Electronic devices that rely on integrated circuitry are causing an associated explosion in the magnitude of data
becoming more sensitive to the threats of electrostatic that must be handled. Data transmission rates, by
discharge (ESD) transient overvoltage events. Using the necessity, have increased and will continue to increase.
input/output communication ports as entryways, ESD
pulses are able to pass from the outside of the electronic As the transmission rate of data increases, the inherent
equipment to the I/O pins of the integrated circuit (IC) capacitance of the ESD suppressor becomes an issue.
chips inside. The ESD transients are generated by Capacitance will cause degradation to the signals that are
people and transferred to the equipment during normal passing along the data line. PulseGuard suppressors have
operation and maintenance. less than 1 pF of capacitance and will not affect the signals.
Typical effects on the data waveshape can be seen below.
IC’s are typically manufactured to withstand ESD events
up to 2,000 volts; however, ESD events often occur at For those applications where the speed of the data streams
levels exceeding 15,000 volts. Because of this protection is approximately 100MHz or less, Littelfuse also offers
discrepancy, reliability of the electronic equipment is com- electroceramic and silicon products for ESD protection. The
promised. The solution to this problem is to supplement MultiLayer Varistor (MLV) devices should be used to protect
the on-chip protection against ESD events by installing ESD data lines where the speed of the signal is approximately
suppressing components in parallel with the input/ 100MHz or less. The SP series contains the SP720, SP721,
output communications lines as shown below. SP723, and SP724 devices. Both of these product families
also provide protection against Electrical Fast Transients
(EFT’s) and have limited surge (8x20 µs) capabilities.
As an example, the SP724 would be the ideal solution for
USB1.1 data lines, which transmit data up to speeds of 12
Mbps. The new USB2.0 serial bus will be able to transmit
data at speeds up to 480 Mbps. For that application, the
PulseGuard product would be the ideal solution.

Protection is provided by the PulseGuard suppressor as it

transitions from a high resistance state to a low resistance
state. In the “off” state, the high resistance causes the part
to be electrically transparent to the circuit. After being trig-
gered, the ESD protector shifts to the “on” state, becomes
conductive, and shunts the ESD pulse from the signal line
to ground. The amount of voltage that the system experi-
ences due to ESD is thus minimized. After the ESD energy Aside from reliability, the IEC 61000-4-2 test specification
is dissipated, the PulseGuard suppressor “resets” itself to is an important design consideration. Created by the
the high resistance “off” state. International Electrotechnical Commission (IEC), this
A factor that complicates the protection of data communica- specification provides the definition of the ESD waveform,
tion lines is that signal transmission rates are increasing severity levels, and the methodologies that are used to
continuously. The information age has mandated the need test the ability of electronic equipment to survive multiple
for more communication links between electronic systems, ESD events. The following chart includes the waveshape
and voltage level information relating to this specification.

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CIRCUIT PROTECTION TECHNOLOGIES

PulseGuard® Suppressors

ESD Suppressor FACTS

Currently, electronic equipment manufacturers are required RESISTANCE: While in the “off” state, the suppressors
to certify that their equipment can survive testing to the remain electrically transparent to the circuit. The measured
IEC standard if they are selling that equipment into the resistance of the suppressors is 10 MΩ, or greater.
European Union. Non-compliance is a prosecutable offense.
Compliance is voluntary in the United States. Use of Pulse-
Guard ESD suppressors will help our customers to meet TIME-VOLTAGE CHARACTERISTIC: Because the
this important specification. magnitude of the voltage and the time duration vary with
the individual ESD event, a general form of this curve is
shown below.
LEAKAGE CURRENT: Until the PulseGuard suppressor
transitions to the “on” state, it is electrically transparent to
the circuit. Leakage current passing through the device is
less than .1 µA.

INTERRUPTING RATING: ESD suppressors are not

rated as current-breaking devices; however, PulseGuard
suppressors are able to withstand the 45A that are present
during worst case ESD discharges.

VOLTAGE RATING: PulseGuard suppressors are rated

for use in operating environments up to 24 VDC.

TEMPERATURE RATING: The operating temperature

SUMMARY: The decision to use the surface mount
range is –65OC to +125OC. These devices do not operate
suppressor or the connector array suppressor is left to the
as a result of thermal action; therefore, there is no derating
individual application. The ideal location is at the connector
necessary.
site, so that the ESD pulse is shunted to ground before
the pulse enters the body of the electronic equipment.
However, protection against the ESD threat will also be
AGENCY APPROVALS: At this time, there are no
realized if the surface mount PulseGuard suppressors
applicable standards for ESD suppressor components.
are installed as close as possible to the source of ESD.
Nonetheless, PulseGuard suppressors have been
That is, on the PC board behind the connector so that
subjected to all levels of severity of the IEC 61000-4-2
the suppressor is the first device encountered by the
test specification using both the Contact Discharge and
ESD pulse after it passes through the connector.
Air Discharge injection methods. In all cases, clamping
of the ESD transient is provided.

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