P54x/EN OP/La4 Operation
(OP) 5-94
MiCOM P543, P544, P545 & P546
OP
1.35.1 Reset mechanisms for breaker fail timers
It is common practice to use low set undercurrent elements in protection relays to indicate
that circuit breaker poles have interrupted the fault or load current, as required. This covers
the following situations:
Where circuit breaker auxiliary contacts are defective, or cannot be relied upon to
definitely indicate that the breaker has tripped.
Where a circuit breaker has started to open but has become jammed. This may result in
continued arcing at the primary contacts, with an additional arcing resistance in the fault
current path. Should this resistance severely limit fault current, the initiating protection
element may reset. Therefore, reset of the element may not give a reliable indication
that the circuit breaker has opened fully.
For any protection function requiring current to operate, the relay uses operation of
undercurrent elements (I<) to detect that the necessary circuit breaker poles have tripped
and reset the CB fail timers. However, the undercurrent elements may not be reliable
methods of resetting circuit breaker fail in all applications. For example:
Where non-current operated protection, such as under/overvoltage derives
measurements from a line connected voltage transformer. Here, I< only gives a reliable
reset method if the protected circuit would always have load current flowing. Detecting
drop-off of the initiating protection element might be a more reliable method.
Similarly, where the distance scheme includes Weak Infeed (“WI”) trip logic, the reset of
the WI trip condition should be used in addition to the undercurrent check. Set: ‘WI Prot
Reset’ = Enabled.
Where non-current operated protection, such as under/overvoltage derives
measurements from a busbar connected voltage transformer. Again using I< would rely
upon the feeder normally being loaded. Also, tripping the circuit breaker may not remove
the initiating condition from the busbar, and hence drop-off of the protection element may
not occur. In such cases, the position of the circuit breaker auxiliary contacts may give
the best reset method.
Resetting of the CBF is possible from a breaker open indication (from the relay’s pole dead
logic) or from a protection reset. In these cases resetting is only allowed provided the
undercurrent elements have also reset. The resetting options are summarized in the
following table.
Initiation (menu selectable) CB Fail timer reset mechanism
Current based protection
(e.g. 50/51/46/21/67)
The resetting mechanism is fixed
[A< operates] & [B< operates] & [C< operates] &
[N< operates]
Non-current based protection
(e.g. 27/59)
Three options are available. The user can Select
from the following options:
[All < and N< elements operate]
[Protection element reset] AND [All < and N<
elements operate]
CB open (all 3 poles) AND [All I< and N<
elements operate]
External protection
Three options are available:
The user can select any or all of the options.
[All < and N< elements operate]
[External trip reset] AND [All < and N<
elements operate]
CB open (all 3 poles) AND [All < and N<
elements operate]
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