Graffeo Exam 35: Backup Protection

[Sthabik PE]

Question states "Which breaker(s) would be most likely to provide back up protection for breaker #4?". Ans in the book is option b. 2, 7 & 6



I understand the other options in the book were not valid so the correct will be option b.

But lets say we don't have any options then i think the correct one shall only be breaker #2 because the power flow is only in one direction. Tripping 7 & 6 will have no effect.

Who else agrees with me?

Is this the real way of setting backup protection for radial system?

 

[roy167]

We  need to know the zones to answer the question. 

 

[Rafis]

OP my understanding is that in a power system you have to clear the fault, you don't know what feeds are going out to BKR 7 and 6; they might be connected to another station for all that you know which is why you need to trip a breaker on the right of BKR 4.

Per the solution of 2,7,6. It means that it is considering that the backup protection should be outside of any zone that has BKR 4 inside, which is why it skipped BKR 5. Can somebody confirm if that problem solutions is correct for backup protection?

My confusion is; why wouldn't the backup protection be 3 and 5? Tripping 2,7,6 is essentially providing backup to the backup of BKR4.... What am I missing?

 

[Gab PE]

Question states "Which breaker(s) would be most likely to provide back up protection for breaker #4?". Ans in the book is option b. 2, 7 & 6

View attachment 12616

I understand the other options in the book were not valid so the correct will be option b.

But lets say we don't have any options then i think the correct one shall only be breaker #2 because the power flow is only in one direction. Tripping 7 & 6 will have no effect.

Who else agrees with me?

Is this the real way of setting backup protection for radial system?

Bkr 3 is a local backup and Bkr is a remote backup which must act between 12 to 30 cycles after a fault at bkr 4 primary zone.

 

[Gab PE]

Question states "Which breaker(s) would be most likely to provide back up protection for breaker #4?". Ans in the book is option b. 2, 7 & 6

View attachment 12616

I understand the other options in the book were not valid so the correct will be option b.

But lets say we don't have any options then i think the correct one shall only be breaker #2 because the power flow is only in one direction. Tripping 7 & 6 will have no effect.

Who else agrees with me?

Is this the real way of setting backup protection for radial system?

You can conclude that 6 and 7 may not be necessary IF you know for sure that this is a radial system ending at loads that do not feed the fault at bkr 4.

Now, if we don't know what is connected to feeders 6 and 7 then you should see those breakers as a remote backup for bkr 4. The same is for bkr 2, it is a remote backup for bkr 4, at a remote station and should act between 12-30 cycles after a fault at bkr 4. Bkr 3 is what is called a LOCAL backup in the same station. I presume the author was only talking about remote backup protection in this case.