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

Date Code 20000421 Applications 5-9
SEL-251, -2, -3 Instruction Manual
Some applications may require that the permissive trip input supervise a low-set ground residual
instantaneous overcurrent element. In the following example, 79SH also supervises the low-set
ground/residual instantaneous overcurrent element 50NL in a fuse-saving scheme.
Using SELOGIC Control Equations:
A(12) = 51NT (ground/residual time-overcurrent element)
B(12) = 50NL (low-set ground residual instantaneous overcurrent element)
E(34) = 79SH
F(34) = IN5 (input IN5 will function as a permissive trip input)
V(56) = A*F
W(56) = B*E*F
TR(1246) = V+W+... (TR is the programmable trip condition)
effectively, TR(1246) = (51NT*IN5)+(50NL*79SH*IN5)+...
which reduces to: TR(1246) = [51NT+(50NL*79SH)]*IN5+...
close TRIP output contacts = TR+ ...
Disable Ground/Residual Overcurrent Elements with Block Trip Input
When a block trip input is energized, selected overcurrent elements are disabled for tripping.
When a block trip input is de-energized, selected overcurrent elements are enabled for tripping.
This function is the opposite of a permissive trip input.
Inputs IN5 and IN6 are available in the Relay Word and can be effectively used as block trip
inputs.
Using SELOGIC Control Equations:
A(12) = 51NT (ground/residual time-overcurrent element)
L(1234) = IN5 (input IN5 to function as a block trip input)
V(56) = A*!L (!L=NOT(L) = NOT(IN5))
TR(1246) = V+... (TR is the programmable trip condition)
effectively, TR(1246) = 51NT*NOT(IN5)+...
close TRIP output contacts = TR + ...
Some applications may require that the block trip input supervise a low-set ground residual
instantaneous overcurrent element. In the following example, 52AT also supervises the low-set
ground/residual instantaneous overcurrent element 50NL in a fuse-saving scheme.
Using SELOGIC Control Equations:
A(12) = 51NT (ground/residual time-overcurrent element)
B(12) = 50NL (low-set ground residual instantaneous overcurrent element)
E(34) = 52AT
L(1234) = IN5 (input IN5 will function as a block trip input)
V(56) = A*!L (!L=NOT(L) = NOT(IN5))
W(56) = B*E*!L
TR(1246) = V+W+... (TR is the programmable trip condition)