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Originally Posted by
EricGoetz92
The question reads:
"In a zone-interlocked low voltage circuit breaker protection scheme, the trip unit responsible for clearing the fault is inoperative. Which statement is true?"
The choices were: All circuit breakers in the scheme will trip simultaneously, the circuit breaker closest to the power source will trip, all breakers upstream of the fault will trip, the downstream circuit breaker will trip, or any circuit breaker in the scheme could trip.
None of these seem right to me. In a zone interlocked scheme, if a fault happens, the breaker directly upstream of the fault, the breaker directly upstream of that breaker, and so on and so forth until you reach the breaker closest to the source of power will all see the fault, but if everything works the way it is supposed to, all of the breakers will send blocking signals upstream and the only breaker that will fast trip is the one directly upstream of the fault because the breaker downstream of it didn't see a fault and didn't send a blocking signal.
Now the question says the breaker responsible for clearing the fault doesn't operate, so that would mean that the next breaker upstream now is the last in the line and will trip at it's faster zone interlocking speed because it isn't getting a blocking signal from its down stream breaker, but it is still sending a block signal to it's upstream breaker preventing it from fast tripping.
So the correct answer from how I am understanding it should be the breaker directly upstream of breaker that didn't operate should then clear the fault. What is the correct answer and why?