UFC 3 -520-01
June 10, 2002
9-2.2.2.3 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. The ground fault current for a solidly grounded system can range
from 25 percent to 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 usually applied to the system with separate coordination studies performed
for the GFP system.
9-2.3
Coordination Time Intervals .
9-2.3.1 Purpose. When plotting coordination curves, maintain certain time interva ls
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 minimum
speeds of operation. The coordination time interval is intended to allow the device
closest to the fault the time necessary to detect, respond, and clear the fault before
other upstream devices respond or suffer damage. The following provides the required
coordination time intervals.
9-2.3.2
Overcurrent Relays.
9-2.3.2.1 When coordinating inverse time overcurrent relays, the time interval or margin
should be set between 0.3 to 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 might consist of the following
components:
Circuit breaker opening time (5 cycles)--0.08 second.
Overtravel--0.10 second (electromechanical relays only).
Safety factor--0.12 to 0.22 second.
9-2.3.2.2 The 0.3 to 0.4 second margin can be decreased if field tests of relays and
circuit breakers indicate the system still coordinates with the decreased margins;
however, the facility maintenance program is then obligated to periodically confirm
performance to ensure that coordination is maintained.
9-3
Previous Page
