Switch Control Setup
Figure 12. The Setup>General>Fault Detection screen.
This screen allows entering setpoint values the switch control will use to detect phase
and ground faults. See Figure 12. It includes the following elds:
Phase and Ground Overcurrent Detection Parameters
Phase fault current is measured by a high-speed detecting circuit that has a full scale
of approximately 4200 amps RMS and samples every 6.25 ms. Phase faults greater than
full scale are recorded as full scale. The Phase Fault Detection Current Level and
Phase Fault Duration Time Threshold setpoints control the phase fault detection
characteristics. Currents on all three phases are monitored and compared with these
setpoints. The Phase Fault Duration Time Threshold timer starts when an overcurrent
condition is registered. The switch control records a phase overcurrent fault when the
overcurrent condition is present for the duration of the timer. If the current drops below
the Phase Fault Detection Current Level setpoint before the timer expires, the switch
control will ignore the overcurrent condition and reset the timer.
Ground fault current is measured separately by a true-RMS detecting circuit as the
vector sum of the three individually sensed phase currents. The detecting circuit has a
full scale of approximately 800 amps and is slower and more accurate than the phase-
detecting circuit. The analog signal is continuously integrated over several cycles and
sampled every 50 ms. Because the circuit continuously integrates the current signal,
faults of a shorter duration than the sampling rate will be detected. The net response of
this circuit is similar to the Time Current Characteristic (TCC) curves of a protective
relay. See Figure 13 on page 30. The curves show that, with a Ground Fault Detection
Current Level setting of 150 amps, the switch control would detect a 500-amp fault in
1.42 ms. With a Ground Fault Detection Current Level setting of 231 amps, the switch
control would detect an 800-amp fault in 42 ms. The Ground Fault Detection Current
Level and Ground Fault Duration Time Threshold setpoints control the ground fault
detection characteristics.
Fault Detection
Configuration
S&C Instruction Sheet 1045-530 29
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