Type - 2 Coordination
Type - 2 Coordination
Type - 2 Coordination
F E A T U R E
Protecting motor circuits
Type 2 protection helps maintain productivity and prevent downtime
By Steven R. Goble contacts is acceptable • Wire damage can oc-
and complete burnout cur under two condi-
OTOR CIRCUITS form a of the overload relay is tions. First, motor cir-
M
Motor
substantial part of any allowed. Therefore, a Damage
cuit conductors have a
commercial or industrial user cannot be certain short circuit withstand
installation. With greater that the motor starter rating that must not be
emphasis being placed on reduced down- will not be damaged exceeded. If the branch
time and increased productivity, proper just because it has been Contactor Breaking circuit protective device
application and selection of motor branch U.L. Listed for use with Overload
Relay
Current
is not capable of limit-
circuit protective devices is essential. a specific branch circuit ing the short circuit
Standard industry practice is to use com- protective device. U.L. current to a value be-
TIME IN SECONDS
Crossover Range
ponents listed by Underwriters Laborato- tests are for safety and (Ic = 7 – 10 3 Ie)
low the wire withstand,
ries, or certified by the Canadian Standard do not define the level the wire may be dam-
Association, and applied according to the of damage allowed. aged, or destroyed.
#12 Wire
National Electrical Code. However, U.L., Damage Secondly, overheating
CSA, and NEC testing and application Selecting protective caused by overload
procedures are largely oriented toward devices Thermal
Withstand conditions which ex-
Limit
fire safety, with little, if any, concern ceed the conductor
about maintaining productivity or pre- In selecting the branch Motor ampacity can lead to
venting downtime. Simply selecting the circuit protective de-
Start
wire damage, and con-
branch circuit protective device to com- vice, the designer must ceivably, a fire.
ply with one of these codes or standards be aware of the limits
may not provide an acceptable level of of the motor circuit • Contractor and over-
Contactor
protection. components, as follows Withstand load relay short circuit
Underwriters Laboratories has devel- (see Fig. 1): CURRENT IN AMPERES withstand ratings are
oped standards to verify that an electrical FIG. 1. Motor and Motor Circuit Damage Protection, 10H.P. @ 460V, 3 ph.
generally not available
product will not cause fire or electrical • Excessive currents from manufacturers.
shock hazards. UL 508 is the standard from single phasing, overloading, and The withstand capacity of these devices
used to evaluate the short circuit protec- locked rotor conditions cause motors to varies as a function of physical size and
tion of industrial control equipment. overheat, leading to eventual failure. construction. Withstand ratings are
Section 58 of the Standard outlines the Overload relays and branch circuit pro- expressed in energy (given by the Joule
testing requirements and criteria used to tective devices should be selected to open integral I2t) and peak current for various
determine if the test results are accept- the motor circuit before current levels levels of fault current. NEMA designed
able. Short circuit test values given in reach the heating curve (motor damage) products often have higher withstand ca-
Table 58.2 of UL 508 are used to establish of the motor. The overload relay and pabilities than the newer IEC style motor
the short circuit rating for the contractor branch circuit protective devices must starters. Regardless of the style of con-
and overload relay of motor starters. The also be able to withstand motor starting trol device used, the characteristics of the
manufacturer must state whether this rat- current or, “inrush current,” which typi- branch circuit protective device must
ing is restricted by a maximum fuse or cally lasts for up to 10 sec, without trip- ensure that the let-through energy and
circuit breaker size in order to pass the ping prematurely (see Fig. 1). peak current do not rise above the levels
minimum short circuit test requirements.
Compliance to the standard allows 430-52.
deformation of the enclosure, but the door MOTOR BRANCH CIRCUIT PROTECTION
shall not be blown open and it shall be
possible to open the door after the test.
Maximum rating or setting of protective devices+
The contacts shall not disintegrate, but
welding of the contacts is considered
acceptable. When testing with fuses, FUSE CIRCUIT BREAKER*
damage to the overload relay is not al-
Non-time Dual-element Instantaneous Inverse
lowed, and it must perform in accordance
with the calibration requirements. Tests Delay Time-delay Type Only Time Type
with circuit breakers allow the overload
relay to be damaged with burnout of the 300% 175% 700% 250%
current element completely acceptable.
For short circuit ratings in excess of the +See Article 430, Section 430-52, exceptions “1 thru 3”.
levels listed in Table 58.2 of UL508, the *For latest information, check manufacturer’s data and/or Underwriters’
damage allowed is even more severe. Laboratories U.L. Standard 508 for damage and warning label requirements.
Welding or complete disintegration of
September 1991 CEE News
that the contractor and overload relay can • The time-current characteristics must starter under short circuit conditions:
withstand. be such that currents above the contactor
breaker capacity and the thermal with- Type 1. Considerable damage to the
• Breaking capacity (current) is also of stand limit of the overload relay are contactor and overload relay is accept-
significance. The branch circuit protec- cleared. able. Replacement of components or a
tive device must operate at all currents completely new starter may be needed.
above the contactor breaking capacity. • The peak let-through current (Ip) and There shall be no discharge of parts
Sustained current exceeding the break- the let-through energy (I2t) of the branch beyond the enclosure.
ing capacity will cause destruction of the circuit protective device must be low Type 2. No damage is allowed to either
contactor which could lead to a more enough to prevent damage to the contactor, the contactor or overload relay. Light
hazardous failure of the control equip- overload relay, and conductors. The let- contact welding is allowed, but must be
ment. Therefore, the intersection of the through characteristics must be sufficient easily separable.
branch circuit protective device and to protect the circuit components for all
overload relay characteristic should not levels of fault current. The maximum Comparing branch circuit protective
exceed the breaking capacity of the acceptable Ip and I2t for fuses is limited by devices
contactor. This intersection is known as U.L. and CSA for each fuse class. The
the crossover point (current). At cur- more current limiting the fuse, the lower
rents immediately below the crossover these values will be.
point, the overload relay trips and the Motor
Damage
contactor is required to break the circuit. U.L. does not specify maximum Ip or I2t
Consequently, the crossover point for circuit breakers. Circuit breakers that
should not exceed the breaking capacity take 1/2 cycle or greater to clear a fault MCP (700% FLA)
of the contactor. For currents immedi- may not limit the Ip or I2t values to levels
ately above the crossover point, the below the withstand capabilities of the Contactor Breaking
Current
Overload
branch circuit protective device should motor starter. To obtain this information Relay
be sufficiently fast enough to avoid ther- the circuit breaker manufacturer must be
mal damage to the overload relay. contacted.
TIME IN SECONDS
Crossover Point
(Ic = 5.5 3 Ie)
requires that the branch circuit protective also protects the circuit components from
device shall not exceed the values calcu- damage, the designer must look beyond Motor
Start
of protection. The values given in Table either the starter or the surrounding equip-
CURRENT IN AMPERES
430-52 are maximum values and do not ment. Unfortunately, the information FIG. 2. Motor Circuit Protector Sized @ 700% FLA.
preclude the application of lower sizes. needed to thoroughly evaluate the coordi-
When selecting the protective device, nation of the branch circuit components
the following conditions must be satis- is not readily available. There is, however, A comparison of the branch circuit de-
fied to prevent damage to the motor cir- an IEC (International Electrotechnical vices commonly used will give an idea of
cuit and minimize downtime in the event Commission) Standard that offers guid- the level of performance the user can
of an overcurrent condition: ance in evaluating the level of damage expect.
likely to occur during a short circuit with A motor circuit protector (MCP) is a
• The time current characteristic must various branch circuit protective devices. magnetic only (short circuit only) device
be able to withstand the motor inrush IEC Publication 947, “Low Voltage that will operate under short circuit con-
current. The branch circuit protective Switchgear and Control, Part 4-1: ditions in excess of its instantaneous trip
device should also be capable of provid- Contactors and Motor Starters,” addresses setting. To allow a motor to start, and
ing “backup” motor overload protection the coordination between the branch cir- prevent nuisance tripping, typical trip set-
for the overload relay. If contacts have cuit protective device and the motor starter. tings are 700% to 1300% of motor full
welded or the overload relay has been It also provides a method to measure the load current (FLA). This device typically
miscalibrated, a second level of protec- performance of these devices should a takes 1/2 cycle of short circuit current to
tion is provided for the motor and motor short circuit occur. IEC defines two levels clear the circuit. Unless otherwise noted,
circuit conductors. of protection (coordination) for the motor these are not considered to be current
CEE News September 1991
Let-through energy for fast acting fuses, cuit. If the relays are unable to operate in
although within Type 2 limits, will gener- an overload or single-phase condition,
Motor
ally by higher than properly sized, lower properly sized fuses will open before the
Damage
rated time delay fuses of the same class. motor damage curve is reached. This siz-
Fast acting fuses sized at these values ing of fuses and relays is based on the
Fast-Acting offer no back-up overload protection for actual running current of the motor, if the
Fuse (45A)
the motor circuit. If the relays are unable motor is not fully loaded, or the motor
Contactor Breaking
Overload Current to operate during an overload or single- nameplate current–whichever is lower.
Relay
phase condition, the starter and other Several other benefits are established
components may be subjected to exces- when utilizing 125% sizing:
TIME IN SECONDS
CURRENT IN AMPERES
FIG. 3. Fast-Acting Fuse, IEC Type Sized @ 300% FLA.
• The crossover point (Ic) is between the
Motor optimum coordination limits of 7 to 10
Damage
times contactor ratings. For overloads up
to this value, the relay should operate
limiting devices. An MCP affords Type 1
first. If the relays do not operate, for
protection under short circuit conditions Class J
Time-Delay
whatever reason, the backup dual-ele-
due to the 1/2 cycle or greater opening Fuse (17⁄Ω™A)