P74x/EN OP/N
(OP) 5-
1.3.3.2.3 Logic reset Criterion
This is for instances where circuits may carry a very low level of load, or even may operate
unloaded from time to time. Where 52a contact (CB closed) supervision is set, the relay
looks only for 52a resetting to stop the breaker fail timers.
This criterion is based on checking the state of the circuit breaker auxiliary contacts. i.e. to
see if the 52a contact is open for open circuit breaker conditions. In the P74x protection
system, this detection method is used with the '52a' threshold.
1.3.3.2.4 Logic AND Current reset Criterion
This is for instances where circuits may carry a very low level of load, or even may operate
unloaded from time to time. Where 52a contact (CB closed) supervision is set, the relay
looks for I< undercurrent, and 52a resetting to stop the breaker fail timers.
This criterion relies on verifying the disappearance of the current AND of the state of the CB
auxiliary contacts. In the P74x protection system, this detection method is used with the 'I<
AND 52a' threshold.
1.3.3.2.5 Processing a Circuit Breaker Failure Condition
Due to the nature of the busbar protection, the substation topology can manage the system
under circuit breaker failure conditions (50BF).
There are several options for circuit breaker failure protection installations. Generally these
depend on the substation construction and wiring:
• Internally initiated CBF i.e. Initiation from the differential element, 87BB trip issued by the
Central Unit
• Externally initiated, for example by the feeder protection, but using the busbar
protection’s integral 50BF protection to execute tripping procedure
• Separate 50BF protection to the busbar protection (such as a P821)
The breaker failure logic uses fast acting undercurrent elements to provide the required
current check. These elements reset within 15ms, thereby allowing the use of the P74x relay
at all voltage levels.
Since the Overcurrent element within the peripheral units may also be used in blocking
schemes to provide back-up protection, it is possible to reset the Overcurrent start signals
after the breaker fail time delay has elapsed. This ensures that the upstream back-up
protection can be maintained by removal of the blocking signal. This would also ensure that
the possible risk of re-trip on re-closure of the circuit breaker is minimised.
CB Trip 3 ph:
• Triphase Circuit Breaker Trip from CU (Init 50BF TBF1 / TBF2), Logical OR of 87BB,
50BF, Manual Trip Zone X
CB Trip phase A:
• Phase A Circuit Breaker Trip (Init 50BF TBF3 / TBF4), Logical OR of O/C Protection,
External Trip A, External Trip 3ph
CB Trip phase B:
• Phase B Circuit Breaker Trip (Init 50BF TBF3 / TBF4), Logical OR of O/C Protection,
External Trip A, External Trip 3ph
CB Trip phase C:
• Phase C Circuit Breaker Trip (Init 50BF TBF3 / TBF4), Logical OR of O/C Protection,
External Trip A, External Trip 3ph
Note: The CB fail alarm is raised as soon as tBF1 or tBF2 or tBF3 or tBF4
has been reached (Logical OR of the signals 8, 9, 10, 11, 12, 13 in the
following figure).
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