5-2 Applications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Fuse-Saving Scheme Using the 79SH Bit
The 79SH bit asserts (79SH = 1) for selected reclosing relay shot counter states (shot = 0, 1, 2, 3,
or 4). 79SH is controlled by the M79SH setting. To assert 79SH for shot = 0 and shot = 1,
M79SH is set as:
M79SH = 11000
The digits of the M79SH setting (from left to right) are for shots 0 through 4. For a reclosing
scheme with only three shots (three automatic reclosures) only the first four digits of the M79SH
setting are pertinent (shots 0 through 3). The shot counter increments when the reclosing relay
issues an automatic reclosure (CLOSE output contact assertion). Shot = 0 when the circuit
breaker is closed and the reclosing relay is reset.
With the M79SH setting shown above, 79SH remains set from the end of the preceding reset
interval (79RST), until the relay issues the second close (shot=2).
Figure 5.2: Reclosing Cycle for Permanent Fault Beyond Tap Fuse
If element 50NL is supervised by 79SH, then 50NL is enabled for shot = 0 and shot = 1 only.
Using SELOGIC
TM
Control Equations:
B(12) = 50NL
E(34) = 79SH
V(56) = B*E
TR(1246) = V+... (TR is the programmable trip condition)
effectively, TR(1246) = 50NL*79SH+...
close TRIP output contacts = TR + ...
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