Fault inception at any part of the power system would lead to one or more main protection devices to operate and issue a trip output to the circuit breaker(s) associated with the faulted circuit. Operation of the circuit breaker is essential to isolate the fault and prevent damage / further damage to the power system. For transmission/sub-transmission systems, slow fault clearance can also threaten system stability. It is therefore common practice to install circuit breaker failure protection, which monitors that the circuit breaker has opened within a reasonable time.

A. Circuit Breaker Failure Logic

When fault occurs on a feeder, the protection function(s) in the relay(s) would issue a trip command and at the same time, Initiation / Start signal is released to the circuit breaker failure function. CBF initiation/start signal could be internally [through relay logic] if the function is equipped in the same relay or externally if the CBF is equipped in a separate relay.

After the CBF function is initiated, the logic will wait for a time called Retrip Time / CBF Timer_1/CBF 1st Stage Timer and this time equals to the CB Open Time + ARC Extinguishing Time + Safety Margin [CB aging and others] which is typically 120mSec, after that time the relay would ask a question, Did the CB open? And that could be recognized either by

• Currents Measurement [3phases or 3Io] that must be less than a setting value.

• CB status is open that is configured through relay binary input.

If the CB didn’t open, then the CBF would issue a Retrip command for the same CB but on the second trip coil, then again, the CBF will wait for the same period of time of the CBF 1st stage time [another 120mSec] this time is called Backup Trip Time / CBF Timer_2 / CBF 2nd Stage Timer and then asks the same question, Did the CB open?. If the CB didn’t respond to the retrip command, then it means the CB fail to trip and a Backup trip would be issued to isolate the fault.

• Adjacent CBs / Busbar Trip [Trip all CB connected with the CB on the same Bus]

• Remote Trip [Line Feeder]

• Upstream Trip [Transformer Feeder]

If the CB reported a defective, unhealthy, not ready, or faulty alarm to relay due to

• Trip Circuit Supervision Failure of both Trip Circuits

• CB SF6 Low stage 2 alarm operated

• Hydraulic Pressure Lockout [Hydraulic CBs]

• Spring Open Block Alarm [If applicable based on CB type and facilities]

Hence, the relay logic will directly issue the Backup trip command as of CBF 2nd stage and wouldn’t wait for the Retrip or Backup trip times.

B. Reset mechanisms for breaker fail timers

It is common practice to use low set undercurrent in protection relays to indicate that circuit breaker poles have interrupted the fault or load current. 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. Thus, reset of the element may not give a reliable indication that the circuit breaker has opened fully.

For Transformer Feeders, mechanical protections as Buchholz, Oil Low Level, Oil Temperature, Pressure Relief Protections could operate while the transformer not loaded [Only one side CB is closed], then the CBF reset mechanism shall recognize the CBF open through the status not through the current measurements that would be under the level in that case.