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# Practice Test Question: Zone Interlocked Scheme

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## Practice Test Question: Zone Interlocked Scheme

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?

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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?
As the question is written, I believe "any breaker in the scheme could trip" would be the best answer. You say that the next breaker directly upstream should trip, which would be true in an ideal scenario but that wasn't an option and we don't know anything else about the coordination of the system or conditions of the fault.

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Originally Posted by storm89
As the question is written, I believe "any breaker in the scheme could trip" would be the best answer. You say that the next breaker directly upstream should trip, which would be true in an ideal scenario but that wasn't an option and we don't know anything else about the coordination of the system or conditions of the fault.
It is true that they didn't mention the coordination of the system, so if all breakers did have equal delays when receiving a block signal, then it is true that any upstream breaker could trip, but not any breaker in the scheme because none of the breakers downstream of the fault in the scheme would trip because they don't see the fault. So that would only leave options "the circuit breaker closest to the power source will trip" and "all breakers upstream of the fault will trip". I picked the second one and it was counted wrong. Depending on the coordination of the breakers (which is information that isn't given), it could very well be the breaker closest to the power source that trips next, but that's assuming it has a faster trip time than all of it's downstream breakers, which would be a weird assumption to expect us to make. It still seems to me this question is missing valuable information (like a single line diagram that shows delays for the zone-interlocking and normal trip times) or the answers aren't worded correctly.

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Originally Posted by EricGoetz92
It is true that they didn't mention the coordination of the system, so if all breakers did have equal delays when receiving a block signal, then it is true that any upstream breaker could trip, but not any breaker in the scheme because none of the breakers downstream of the fault in the scheme would trip because they don't see the fault. So that would only leave options "the circuit breaker closest to the power source will trip" and "all breakers upstream of the fault will trip". I picked the second one and it was counted wrong. Depending on the coordination of the breakers (which is information that isn't given), it could very well be the breaker closest to the power source that trips next, but that's assuming it has a faster trip time than all of it's downstream breakers, which would be a weird assumption to expect us to make. It still seems to me this question is missing valuable information (like a single line diagram that shows delays for the zone-interlocking and normal trip times) or the answers aren't worded correctly.

To me, the breaker closest to the power source is incorrect because we don't know how close or far from the power source the breaker in question is, let alone what "the power source" even refer's to.

As for downstrem breakers, we can assume that the trip unit responsible for clearing the fault refers to the breaker closest to the fault, therefore no breakers are downstream.

Based purely on the information provided in the question I think any breaker could trip.

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