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S&C Instruction Sheet 1041-540 19
Switch Control Operation
Phase Overcurrent Detection
Phase overcurrent conditions are sensed using a combination of analog and digital
techniques. The switch control provides fault detection with a resolution of 1/3-cycle
and peak values of approximately 4000 amps RMS. Overcurrent measurements are
accurate to 0.5% of full scale, excluding sensors. (For information about scaling beyond
4000 amps, contact S&C.)
Note:
In the 5802/5803 controls, the control monitors each feeder (or branch) indepen-
dently and responds to changes on that feeder regardless of the condition of the other
pad-mounted switch and feeder.
To detect phase overcurrent faults:
STEP 1.
The switch control monitors the current on all three phases and compares it to
the
Phase Fault Detection Current Level
setpoint.
STEP 2.
When at least one peak overcurrent sample is above the setpoint every
18.75 milliseconds (a window of time slightly longer than one cycle), the switch
control registers an overcurrent condition (a potential, or pending, “fault”)
on that phase.
STEP 3.
Once the overcurrent condition is registered, the switch control starts the
Phase Fault Duration Time Threshold
timer.
STEP 4.
If the overcurrent condition is present continuously for the duration of the
timer, the switch control labels it a “phase overcurrent fault” and responds
accordingly. If during any 18.75-millisecond window overcurrent is not
detected, the switch control considers the “fault” (or the overcurrent condition)
to be no longer present and takes appropriate action.
STEP 5.
When a recognized “phase overcurrent fault” ends (after the timer has expired),
the software records the maximum RMS current measured during the fault
and the fault duration. Any fault lasting longer than 6.82 minutes is recorded as
6.82 minutes (409.6 seconds).
The switch control hardware measures ground current as an analog vector sum of the
three individually sensed phase currents. This analog signal is presented to a true RMS-
detecting circuit, yielding a very accurate, harmonic-independent measure of the true
RMS current integrated over several cycles. Because of this multi-cycle integration,
there is some delay in the response time through the hardware. This delay is inversely
proportional to the magnitude of the change in ground current. The larger the change
in ground current, the faster the circuit responds. The net result is very similar to the
time-current characteristics of a protective relay.
The control software samples the true RMS detection hardware on 50-millisecond
intervals. On each interval, the current is compared to the
Ground Fault Detection
Current Level
setpoint. If the current exceeds the setpoint and this condition persists
continuously for a period of time specified by the
Ground Fault Duration Time Thresh-
old
setpoint, an
Overcurrent Fault
condition is indicated and appropriate action taken.
Because the registration of
Overcurrent Fault
conditions is affected by the rela-
tionship between the minimum fault current detection level and RMS detector rise and
fall times, a family of time-current characteristic curves is generated. See the graph
under the “Ground Fault Detection Current Level (RMS Amps)” section in Instruction
Sheet 1041-530.” Each of these curves corresponds to a single
Ground Fault Detection
Current Level
setting.
The points on each curve represent the minimum amount of time the ground current
must be present to register a fault. For example, when the
Ground Fault Detection
Current Level
setting is set to 150 amps, a 500-amp ground current must be present for
approximately 42 milliseconds before the switch control registers a fault.
Ground Overcurrent
Detection