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# Practice Test Question: Zone Interlocked Scheme

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1. ## Practice Test Question: Zone Interlocked Scheme

"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

"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.

3. 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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Glad to know that I'm not the only one questioning this answer. None of the choices make sense to me. Based on my understanding of ZSI, here's how I'd evaluate the choices:

A. All circuit breakers in the scheme will trip simultaneously - FALSE. Obviously the CB with a defective trip unit will not trip. Downstream breakers shouldn't trip either.

B. The circuit breaker closest to the power source will trip. - NOT NECESSARILY. If it is a sub-feeder, then the feeder should trip before the main.

C. All breakers upstream of the fault will trip. - FALSE. See above.

D. The downstream circuit breaker will trip. - FALSE. Duh.

E. Any circuit breaker in the scheme could trip. - FALSE. See above.

So I guess that B is the right answer because it's the only one that's even possibly true?

6. Originally Posted by storm89
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.
This. I think there is a lot of overthinking going on in this thread. Based on the information given (which isn't much) the best answer seems to be "any breaker could trip."

A. All circuit breakers in the scheme will trip simultaneously - False. The breaker with the next fastest settings will trip, we don't know which that is.

B. The circuit breaker closest to the power source will trip. - False. We can assume this is the breaker "responsible for clearing the fault" and its defective. Edit: Misread this but still false because we don't know what "the power source" is referring to, possibly the main.

C. All breakers upstream of the fault will trip. - False. See answer A.

D. The downstream circuit breaker will trip. - Obviously incorrect.

E. Any circuit breaker in the scheme could trip. - Most correct answer given because in reality any breaker upstream of the defective breaker can trip assuming the defective breaker is the closest to the fault. We don't know anything about the coordination in place.

7. Originally Posted by SecondGen
This. I think there is a lot of overthinking going on in this thread. Based on the information given (which isn't much) the best answer seems to be "any breaker could trip."

A. All circuit breakers in the scheme will trip simultaneously - False. The breaker with the next fastest settings will trip, we don't know which that is.

B. The circuit breaker closest to the power source will trip. - False. We can assume this is the breaker "responsible for clearing the fault" and its defective. Edit: Misread this but still false because we don't know what "the power source" is referring to, possibly the main.

C. All breakers upstream of the fault will trip. - False. See answer A.

D. The downstream circuit breaker will trip. - Obviously incorrect.

E. Any circuit breaker in the scheme could trip. - Most correct answer given because in reality any breaker upstream of the defective breaker can trip assuming the defective breaker is the closest to the fault. We don't know anything about the coordination in place.
I took and passed the NETA 3 last summer (shortly after I created my original post) and this question was on it. Or should I say the properly written version was on it. There was an option for "The next breaker upstream trips" (or something like that) on the actual exam, which is what makes sense. Testguy and their admins should go through and vet their practice questions and make sure the choices make sense.

8. Originally Posted by EricGoetz92
I took and passed the NETA 3 last summer (shortly after I created my original post) and this question was on it. Or should I say the properly written version was on it. There was an option for "The next breaker upstream trips" (or something like that) on the actual exam, which is what makes sense. Testguy and their admins should go through and vet their practice questions and make sure the choices make sense.
Thanks for the input everyone, option D was changed to read "The next circuit breaker upstream will trip" and has been marked correct. The question itself has also been changed to read "which answer is most true?"

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Originally Posted by EricGoetz92

"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?
The issue I have with this is the CB that failed has a defective trip unit. Since it is defective, you cannot say with certainty that a blocking signal was sent to the other breakers at all ! IOW, the question is trying to tell you that a defective trip unit sent the blocking signal, even though defective ! You can't assume that, I would assume the trip unit is totally DOA and could not have sent the blocking signal.

So where does that leave you? I believe, the next breaker upstream should take it out but, that does not seem to be a choice.

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Originally Posted by whiskers
The issue I have with this is the CB that failed has a defective trip unit. Since it is defective, you cannot say with certainty that a blocking signal was sent to the other breakers at all ! IOW, the question is trying to tell you that a defective trip unit sent the blocking signal, even though defective ! You can't assume that, I would assume the trip unit is totally DOA and could not have sent the blocking signal.

So where does that leave you? I believe, the next breaker upstream should take it out but, that does not seem to be a choice.
The one possible answer would be "Any Circuit Breaker", all depending on the settings of each circuit breaker programmer.

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