P44x/EN AP/Hb
MiCOM P40 Agile P442, P444
(AP) 5-
circuit breaker trip coils, and is known as re-tripping. Should re-tripping fail to open the
circuit breaker, a back-trip may be issued following an additional time delay. The back-
trip uses ‘CB Fail 2 Timer’, which is also started at the instant of the initial protection
element trip.
CBF elements ‘CB Fail 1 Timer’ and ‘CB Fail 2 Timer’ can be configured to operate for trips
triggered by protection elements within the relay or via an external protection trip. The latter
is achieved by allocating one of the relay opto-isolated inputs to ‘External Trip’ using the
programmable scheme logic.
4.14.3 Reset Mechanisms for Breaker Fail Time-Delays
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 or
under/overfrequency, 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. (in that case setting will be: "Prot. Reset or I<")
• Where non-current operated protection, such as under/overvoltage or
under/overfrequency, 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.
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