Withstand Capability

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4.2.2.4 Short-Circuit Current Concepts. From the foregoing, it can be seen that the
short-circuit current behaves differently in the first few cycles than it does later (if allowed to
persist). Former practice was to determine an asymmetrical value of short-circuit current by
applying simple multipliers to the calculated symmetrical value of short-circuit current. The
trend in recent years is to rate protective equipment on a basic symmetrical value. Asymmetry is
then accounted for by various application formulas depending on the class of equipment.
Recently, the concept of I 2 t has been introduced to supplement the symmetrical current
concept, because it represents the actual thermal and magnetic stresses imposed on equipment
carrying short-circuit current in the first few cycles. The quantity I 2 t represents the time
integral of the current squared for the time under consideration. An I 2 t rating is being applied
increasingly to electrical apparatus. Conceivably, all future protective equipment may be
coordinated on an I 2 t basis rather than a maximum current basis.
4.2.3 Analysis. The maximum magnitude of short-circuit current, as well as adequate
interrupting ratings, must be known in order to coordinate protective devices. For coordination,
minimum as well as maximum values may be required. Furthermore, it is often necessary to
know the maximum let-through current to verify the withstand capability of circuit elements in
series with the fault.
4.2.3.1 Withstand Capability. The fault current varies with time after the fault. A
protector, that does not interrupt until several cycles after initiation of the fault, usually allows
the fault current to decay from its maximum asymmetrical value. The protector and all series
devices, however, must withstand the maximum current as well as the total dissipated energy. A
protective device which interrupts in a fraction of a cycle (before maximum fault current is
attained) reduces the withstand requirements of series devices.
4.2.3.2 Calculations. Short-circuit currents may be calculated at the following
recommended times:
(a) First-cycle maximum symmetrical values are always required. They are often the
only values needed for low-voltage breakers with instantaneous trip devices and for fuses in
general.
(b) Maximum values (1.5 to 4 cycles) are required for medium-and high-voltage circuit
breaker application.
(c) Reduced fault current values (about 30 cycles) are needed for estimating the
performance of time-delay relays and fuses and for low-voltage power circuit breakers without
instantaneous trip devices. They must often be calculated after the fault has initiated so that the
proper current is known for setting time delayed protective Often, minimum values must
also be calculated to determine whether sufficient current is available to open the protective
device within a satisfactory time.
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