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 direct-acting trip devices respond. They also verify the capability of the applied apparatus, such
as circuit breakers, fuses, switches, and reactor and bus brislings, to withstand the maximum
electromechanical stresses to which they could be subjected.
4.7.2.2 Interrupting Duty. The maximum 3-8 cycle interrupting duty current, at maximum
generation, will verify the ratings of circuit breakers, fuses, and cables. This is also the value of
current at which the circuit protection coordination interval is established. The maximum 3-8
cycle interrupting duty current, at minimum generation, is needed to determine whether the
circuit protection is sensitive enough to protect against damage that could result from low level
faults.
4.7.2.3 Ground Fault Currents. The most common faults in electrical systems are ground
faults. The magnitudes of ground fault currents are calculated using the method of symmetrical
components, using the impedance values for both the momentary duty and interrupting duty as
outlined above. The ground fault current for a solidly grounded system can range from 25 - 125
percent of the bolted three-phase fault current values, but for most systems does not exceed the
calculated three-phase fault current value. For low and high resistance grounded systems, the
ground fault current is limited by the impedance of the grounding device and is substantially less
than the three-phase fault current. The maximum and minimum generation cases need to
be determined, just as for three-phase faults, to determine whether the circuit protection is
sensitive enough to protect against damage that could result from low level faults. Separate
ground fault relays are generally applied to the system with separate coordination studies
performed for the ground fault protection system.
4.7.3 Coordination Time Intervals. When plotting coordination curves, certain time intervals
must be maintained between the curves of various protective devices in order to ensure correct
sequential operation of the devices. These intervals are required because relays have overtravel;
fuses have damage and tolerance characteristics; and circuit breakers have certain speeds of
operation. Sometimes these intervals are called margins.
4.7.3.1 Overcurrent Relays. When coordinating inverse time overcurrent relays, the time
interval or margin is usually 0.3-0.4 second. Time margin is measured between relay curves
either at the instantaneous setting of the load side feeder circuit breaker relay or the maximum
short-circuit current (which can flow through both devices simultaneously) whichever is the
lower value of current. The interval consists of the following components:
Circuit breaker opening time (5 cycles)
0.08 second
Overtravel
0.10 second
Safety factor
0.12-0.22 second
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