CHAPTER 5: SETTINGS GROUPED ELEMENTSC70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 5-2055Determination of a breaker failure conditionThe schemes determine a breaker failure condition via three paths. Each of these paths is equipped with a time delay, afterwhich a failed breaker is declared and trip signals are sent to all breakers required to clear the zone. The delayed paths areassociated with breaker failure timers 1, 2, and 3, which are intended to have delays increasing with increasing timernumbers. These delayed paths are individually enabled to allow for maximum flexibility.Timer 1 logic (early path) is supervised by a fast-operating breaker auxiliary contact. If the breaker is still closed (asindicated by the auxiliary contact) and fault current is detected after the delay interval, an output is issued. Operation ofthe breaker auxiliary switch indicates that the breaker has mechanically operated. The continued presence of currentindicates that the breaker has failed to interrupt the circuit.Timer 2 logic (main path) is not supervised by a breaker auxiliary contact. If fault current is detected after the delayinterval, an output is issued. This path is intended to detect a breaker that opens mechanically but fails to interrupt faultcurrent; the logic therefore does not use a breaker auxiliary contact.The timer 1 and 2 paths provide two levels of current supervision, high-set and low-set, that allow the supervision level tochange from a current which flows before a breaker inserts an opening resistor into the faulted circuit to a lower level afterresistor insertion. The high-set detector is enabled after timeout of timer 1 or 2, along with a timer that enables the low-setdetector after its delay interval. The delay interval between high-set and low-set is the expected breaker opening time.Both current detectors provide a fast operating time for currents at small multiples of the pickup value. The overcurrentdetectors are required to operate after the breaker failure delay interval to eliminate the need for very fast resettingovercurrent detectors.Timer 3 logic (slow path) is supervised by a breaker auxiliary contact and a control switch contact used to indicate that thebreaker is in or out-of-service, disabling this path when the breaker is out-of-service for maintenance. There is no currentlevel check in this logic as it is intended to detect low magnitude faults and it is therefore the slowest to operate.OutputThe outputs from the schemes are:• FlexLogic operands that report on the operation of portions of the scheme• FlexLogic operand used to re-trip the protected breaker• FlexLogic operands that initiate tripping required to clear the faulted zone. The trip output can be sealed-in for anadjustable period.• Target message indicating a failed breaker has been declared• Illumination of the faceplate Trip LED (and the Phase A, B, or C LED, if applicable)Main path sequenceFigure 5-104: Breaker failure main path sequenceThe current supervision elements reset in less than 0.7 of a power cycle for any multiple of pickup current as shown in thefollowing figure.
