8-26
Breaker Monitor and Metering Functions
Date Code 20011205
SEL-311A Instruction Manual
If a ground fault occurs, the residual ground time-overcurrent element 51GT operates with the
sensitivity provided by pickup 51GP = 1.50 A secondary. The thermal demand meter, even with
setting DMTC = 5 minutes, does not respond fast enough to the ground fault to make a change to
the effective residual ground time-overcurrent element pickup—it remains at 1.50 A secondary.
Demand meters respond to more “slow moving” general trends.
Residual Ground Demand Current Goes Above Pickup GDEMP
When unbalance current I
G
increases, unbalance demand current I
G(DEM)
follows, going above
corresponding demand pickup GDEMP = 1.00 A secondary, and Relay Word bit GDEM asserts
to logical 1. This results in SEL
OGIC
control equation torque control setting 51GTC being in the
state:
51GTC = !GDEM + GDEM * 50G1 = NOT(GDEM) + GDEM * 50G1
= NOT(logical 1) + (logical 1) * 50G1 = logical 0 + 50G1 = 50G1
Thus, the residual ground time-overcurrent element 51GT operates with an effective, less-
sensitive pickup:
50G1P = 2.30 A secondary
The reduced sensitivity keeps the residual ground time-overcurrent element 51GT from tripping
on higher unbalance current I
G
.
Residual Ground Demand Current Goes Below Pickup GDEMP Again
When unbalance current I
G
decreases again, unbalance demand current I
G(DEM)
follows, going
below corresponding demand pickup GDEMP = 1.00 A secondary, and Relay Word bit GDEM
deasserts to logical 0. This results in SEL
OGIC
control equation torque control setting 51GTC
being in the state:
51GTC = !GDEM + GDEM * 50G1 = NOT(GDEM) + GDEM * 50G1
= NOT(logical 0) + (logical 0) * 50G1 = logical 1
Thus, the residual ground time-overcurrent element 51GT operates on its standard pickup again:
51GP
= 1.50 A secondary
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