In any electrical system, an electrical fault must be localized and tripped by the breaker closest to the fault. But what happens in the event of a stuck breaker? A stuck breaker condition occurs when a circuit breaker fails to operate upon receiving a trip signal. In a medium or high voltage system, the failure of a breaker to trip during a fault condition will seriously undermine the overall protection system, harm personnel, and damage equipment. A breaker failure can occur for a myriad of reasons including mechanical component stress, loss of control power, failure of dielectric insulating medium, or an improper inhibit block signal.
Inadequate Protection with Time-Overcurrent Coordination
Time-overcurrent coordination-based protection systems are designed using time-delay principles so that the breaker closest to a fault (i.e. the intended breaker) trips first. Intentional time delays are established on upstream breakers to delay tripping in order to allow the intended breaker to trip and limit the electrical outage to the faulted area. If the intended breaker is stuck and has failed to trip, the upstream breaker will then be called upon to trip using time-overcurrent protection following the intentional delay. With typical coordination, the time delay to trip the upstream breaker can be as much as 600 milliseconds. However, such time delays are not acceptable since a prolonged fault duration can unnecessarily compromise personnel safety, cause equipment damage, and intensify an arc flash event.
Better Solution Using Breaker Failure Monitoring
A breaker failure protection scheme consists of monitoring the ancillary system conditions, within the intended breaker, to ensure that a successful trip has occurred. When a breaker trips properly, the current across the circuit breaker becomes zero and a mechanically-driven pushrod operates to physically denote an open circuit condition. Using ANSI-50BF breaker failure monitoring, when a breaker trip is initiated, the protection system monitors the current across the circuit breaker and the physical open-status of the circuit breaker. If both conditions are not satisfied by the ANSI-50BF protection scheme within a very short interval time window, a pilot transfer trip signal is immediately sent to the upstream breaker to initiate a backup trip. Using this specialized protection scheme, the total time delay to trip the upstream breaker can be reduced to less than 150 milliseconds. ANSI-50BF protection must be carefully designed because if a struck breaker condition is sensed incorrectly, and the upstream breaker is “nuisance-tripped”, the system outage will be larger than required during a local fault condition.
When designed properly, faster tripping will reduce hazards to personnel and minimize the damage to electrical equipment during a fault condition. Less equipment damage means reduced repair costs and reduced equipment downtime. For further information on how to improve your electrical system protection, feel free to call me at (240) 582-3900 — Priyan De Silva, Helios Electric LLC.