Breaker Failure Relays (Code 50BF)

Breaker Failure Relays are three-phase, solid state relays designed to provide protection and security for the power system against failure of the monitored breaker. Breaker failure relaying is the use of a current monitoring relay to determine whether or not current continues to flow into a faulted circuit some time after a circuit breaker has been instructed to interrupt the circuit. In the event that current continues to flow into the faulted circuit (after a defined period of time has elapsed sufficient for the breaker to have interrupted the current), then the circuit breaker is considered to have failed, and steps must be initiated to trip the next set of breakers up stream in the power system. The back up scheme must be designed to isolate both the faulted circuit and the failed breaker. Following are several reasons why a breaker fails to clear a fault:

• Trip circuit can be open (broken wire, blown fuse, open trip coil).
• Interrupting mechanism can stick, leaving a single phase of a three-phase circuit connected. 
• Interrupter can flash-over due to the loss of dielectric strength through contamination or damage.
• Operating mechanism can fail to operate. 

Breaker failure relays detect these conditions and initiate contingency or back up procedures. Typically, breaker failure protection is applied to transmission and sub transmission systems. However, breaker failure protection may be applied to any portion of the power system where failure of a circuit breaker to operate properly could result in severe system damage or instability.

Trip Timing 

Within the relay are two timers that control operation of the device. Timer 1 defines the delay time between recognition of the breaker trip signal and the interrogation of the current monitoring circuits for the presence of current. Adjustment range for this timer is 18 to 500 milliseconds

At the same time that timer 1 is initiated, the control timer is also initiated. . The purpose of the control timer is to increase security by limiting the response time of the relay to a short period following any given attempt to interrupt the breaker. The control timer also terminates the closure of the BF output relays, and breaks the seal of the seal-in circuit (if used).