Schweitzer Engineering Laboratories SEL-251 - Disable Ground;Residual Overcurrent Elements; Disable Ground;Residual Overcurrent Elements with Permissive Trip Input

5-8 Applications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
enough to warrant concern about cold load inrush. Thus, 50L continues to be disabled for
52APU time only after the circuit breaker closes from the lockout state.
To disable 50L only after the circuit breaker closes from the lockout state, make the following
settings.
Using SELOGIC Control Equations:
B(12) = 50L
E(34) = 79RS+79CY+52AT
V(56) = B*E
TR(1246) = V+... (TR is the programmable trip condition)
effectively, TR(1246) = 50L*(79RS+79CY+52AT)+...
close TRIP output contacts = TR + ...
79RS = 79CY = 0 when the relay is in the lockout state. 79RS + 79CY = 1 when the relay is not
in the lockout state. Thus, 52AT effectively supervises 50L only when the relay is in the lockout
state. 52ADO can be set to any value. 50L is enabled while the reclosing relay is cycling. The
79RST setting should be greater than the 52APU setting.
DISABLE GROUND/RESIDUAL OVERCURRENT ELEMENTS
Ground relays are often temporarily disabled during distribution circuit paralleling operations.
This is done to avoid ground relay trips resulting from residual current unbalance. This
unbalance results from line switches not closing or opening all phases in unison when making or
breaking parallel, respectively. Ground/residual overcurrent elements in the SEL-251 Relay can
be disabled by inputs functioning as permissive trip, block trip, or torque control inputs. You can
apply the principles in the following examples to other elements.
Disable Ground/Residual Overcurrent Elements with Permissive Trip Input
When a permissive trip input is energized, selected overcurrent elements are enabled for tripping.
When a permissive trip input is de-energized, selected overcurrent elements are disabled for
tripping.
Inputs IN5 and IN6 are available in the Relay Word and can be effectively used as permissive
trip inputs.
Using SELOGIC Control Equations:
A(12) = 51NT (ground/residual time-overcurrent element)
F(34) = IN5 (input IN5 to function as a permissive trip input)
V(56) = A*F
TR(1246) = V+... (TR is the programmable trip condition)
effectively, TR(1246) = 51NT*IN5+...
close TRIP output contacts = TR + ...