Schweitzer Engineering Laboratories SEL-351 - Residual Ground Demand Current below Pickup GDEMP; Residual Ground Demand Current Goes above Pickup GDEMP; Residual Ground Demand Current Goes below Pickup GDEMP Again

Date Code 20001006 Breaker Monitor, Metering, and Load Profile Functions 8-27
SEL-351 Instruction Manual
Residual Ground Demand Current Below Pickup GDEMP
When unbalance current I
G
is low, unbalance demand current I
G(DEM)
is below corresponding
demand pickup GDEMP = 1.00 A secondary, and Relay Word bit GDEM is deasserted to
logical 0. This results in SELOGIC Control Equation torque control setting 51GTC being in the
state:
51GTC = !GDEM + GDEM*50G5 = NOT(GDEM) + GDEM*50G5
= NOT(logical 0) + (logical 0)*50G5 = logical 1
Thus, the residual ground time-overcurrent element 51GT operates on its standard pickup:
51GP = 1.50 A secondary
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 SELOGIC Control Equation torque control setting 51GTC being in the
state:
51GTC = !GDEM + GDEM*50G5 = NOT(GDEM) + GDEM*50G5
= NOT(logical 1) + (logical 1)*50G5 = logical 0 + 50G5 = 50G5
Thus, the residual ground time-overcurrent element 51GT operates with an effective, less-sensitive
pickup:
50G5P = 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 SELOGIC Control Equation torque control setting 51GTC
being in the state:
51GTC = !GDEM + GDEM*50G5 = NOT(GDEM) + GDEM*50G5
= NOT(logical 0) + (logical 0)*50G5 = logical 1
Thus, the residual ground time-overcurrent element 51GT operates on its standard pickup again:
51GP = 1.50 A secondary
Courtesy of NationalSwitchgear.com