GE P24DM Technical Manual

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9 APPLICATION NOTES9.1 RESET MECHANISMS FOR CB FAIL TIMERSIt is common practise to use low set undercurrent elements to indicate that circuit breaker poles have interruptedthe fault or load current. This covers the following situations:● Where circuit breaker auxiliary contacts are defective, or cannot be relied on to definitely indicate that thebreaker has tripped.● Where a circuit breaker has started to open but has become jammed. This may result in continued arcing atthe primary contacts, with an additional arcing resistance in the fault current path. Should this resistanceseverely limit fault current, the initiating protection element may reset. Therefore, reset of the element maynot give a reliable indication that the circuit breaker has opened fully.For any protection function requiring current to operate, the device uses operation of undercurrent elements todetect that the necessary circuit breaker poles have tripped and reset the CB fail timers. However, theundercurrent elements may not be reliable methods of resetting CBF in all applications. For example:● Where non-current operated protection, such as under/overvoltage or under/overfrequency, derivesmeasurements from a line connected voltage transformer. Here, I< only gives a reliable reset method if theprotected circuit would always have load current flowing. In this case, detecting drop-off of the initiatingprotection element might be a more reliable method.● Where non-current operated protection, such as under/overvoltage or under/overfrequency, derivesmeasurements from a busbar connected voltage transformer. Again using I< would rely on the feedernormally being loaded. Also, tripping the circuit breaker may not remove the initiating condition from thebusbar, and so drop-off of the protection element may not occur. In such cases, the position of the circuitbreaker auxiliary contacts may give the best reset method.9.2 SETTING GUIDELINES (CB FAIL TIMER)The following timing chart shows the CB Fail timing during normal and CB Fail operation. The maximum clearingtime should be less than the critical clearing time which is determined by a stability study. The CB Fail back-up triptime delay considers the maximum CB clearing time, the CB Fail reset time plus a safety margin. Typical CBclearing times are 1.5 or 3 cycles. The CB Fail reset time should be short enough to avoid CB Fail back-trip duringnormal operation. Phase and ground undercurrent elements must be asserted for the CB Fail to reset. Theassertion of the undercurrent elements might be delayed due to the subsidence current that might be flowingthrough the secondary AC circuit.P24xM Chapter 8 - CB Fail ProtectionP24xM-TM-EN-2.1 189