4–368 869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUALCONTROL CHAPTER 4: SETPOINTSBreaker Failure (50BF)The 869 relay provides one Breaker Failure element.The Breaker Failure element determines that a breaker signaled to Trip has not cleared afault within a definite time. The Breaker Failure scheme must Trip all breakers that cansupply current to the faulted zone. Operation of a breaker Failure element causes clearingof a larger section of the power system than the initial Trip. Because Breaker Failure canresult in tripping a large number of breakers and this can affect system safety andstability, a very high level of security is required.The Breaker Failure function monitors phase and neutral currents and/or status of thebreaker while the protection trip or external initiation command exists. If Breaker Failure isdeclared, the function operates the selected output relays, forces the autoreclose schemeto lockout and raises FlexLogic operands.The operation of a Breaker Failure element consists of three stages: initiation,determination of a Breaker Failure condition, and outputs.Initiation of a Breaker FailureThe protection signals initially sent to the breaker or external initiation (FlexLogic operandthat initiates Breaker Failure) initiates the Breaker Failure scheme.When the scheme is initiated, it immediately sends a Trip signal to the breaker initiallysignaled to Trip (this feature is usually described as re-trip). This reduces the possibility ofwidespread tripping that can result from a declaration of a failed breaker.Determination of a Breaker Failure conditionThe schemes determine a Breaker Failure condition supervised by one of the following:Current supervision onlyBreaker status onlyBoth (current and breaker status)Each type of supervision is equipped with a time delay, after which a failed breaker isdeclared and Trip signals are sent to all breakers required to clear the zone. The delays areassociated with breaker failure timers 1, 2, and 3.Timer 1 logic is supervised by current level only. If fault current is detected after the delayinterval, an output is issued. The continued presence of current indicates that the breakerhas failed to interrupt the circuit. This logic detects a breaker that opens mechanically butfails to interrupt fault current.Timer 2 logic is supervised by both current supervision and breaker status. If the breaker isstill closed (as indicated by the auxiliary contact) and fault current is detected after thedelay interval, an output is issued.Timer 3 logic is supervised by a breaker auxiliary contact only. There is no current levelcheck in this logic as it is intended to detect low magnitude faults. External logic may becreated to include control switch contact used to indicate that the breaker is in out-of-service mode, disabling this logic when the breaker is out-of-service for maintenance.Timer 1 and 2 logic provide two levels of current supervision - high-set and low-set - thatallow the supervision level to change (for example: from a current which flows before abreaker inserts an opening resistor into the faulted circuit to a lower level after resistorinsertion). The high-set detector is enabled after the timeout of timer 1 or 2, along with atimer low-set delay that enables the low-set detector after its delay interval. The delayinterval between high-set and low-set is the expected breaker opening time. Both currentdetectors provide a fast operating time for currents at small multiples of the Pickup value.The overcurrent detectors are required to operate after the Breaker Failure delay intervalto eliminate the need for very fast resetting overcurrent detectors.OutputsThe outputs from the schemes are:• Re-trip of the protected breaker
