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Appendix B Functional Description of Operation
of squares algorithm for both determining trip level and for
calculating metered values of the relay current level. The input
waveform is sampled several times to determine instantaneous
values. These instantaneous values are processed to obtain the
true RMS value of the input current.
8.5.3 Time Overcurrent Calculations
If the magnitude of the current exceeds the pickup setting, the
microprocessor calculates the required time delay by evaluating
the magnitude of the overcurrent in the context of the desired
time overcurrent response curve. (All of the time overcurrent
functions that are graphically illustrated in Figures A.1 through
A.1 6 are in the computer's memory.)
Operating
Power
POWER
SUPPLY
Loss-Of-Power
Detector
8 . 1 Functional
Figure
The current inputs are continuously polled and the calculations
updated once for every 60 Hz (line) cycle. As long as the sensed
current is above the pickup point, the elapsed time is accumu
lated toward trip. However, once the sensed current falls below
pickup, the relay is reset and all accumulated time is cancelled.
When the value of a sensed current exceeds the pickup setting,
the TIMING LED for that phase will light. The LED will stay lit as
long as the sensed current exceeds the selected pickup level.
8.5.4 High/Low Range Register
It was noted earlier in this section that the input transformers
must be wired for one of two sensing ranges: HIGH or LOW. This
information is recorded (at the factory) into the HI/LO Range
Block
Diagram
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nst 2
or
Close
Relay
Fail
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