Complex Imaginary Volume 2 Problem 5

If there's a fault at point D and breaker 5 fails to trip, which breakers should trip next? (Ans: says 1 & 3 would trip if 5 failed to trip.)

Can somebody explain this to me?



Source: 

From what I understand, you have to look at the zones of protection. Breakers 2,4,5 are primary protection for bus C, 6&7 for bus E. If there's a fault at D and 5 fails to open, you dont want to have breakers 2&4 as your backup because they could stick as well, there could be something on bus C causing those breakers to not open, so thats why want 1&3 to be your backup.

I just did the 2nd test and I don't remember getting stuck on this one. I'm going to look at it when I get home.

I looked at the 2011 post you attached and the question stated if breakers 1, 3 and 6 tripped what breakers most likely failed if there's a fault at pt D?

The way I understand it is the primary means of protection for the line at pt D is breakers 5 and 6. For 1 and 3 to have tripped Breakers 2, 4 and 5 failed.

eatsleep: my point is that if 2&4 breaker opens, load C will be disconnected. While if 1&3 breaker opens, load A,B &C will be disconnected. Isn't it confusing?

Sdhabik said:

eatsleep: my point is that if 2&4 breaker opens, load C will be disconnected. While if 1&3 breaker opens, load A,B &C will be disconnected. Isn't it confusing?

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Right but its about isolating the fault. Breakers 2&4 cannot be the backup protection for breaker 5 because breakers 2,4, &5 are the primary protection for that bus, then you go one breaker away, which is breaker 1&3.



Hopefully, this figure shows the point I am trying to make. See the Bus Protection zone? It may be easier to think about what happens if breaker 6 fails. Since breaker 6&7 are the primary protection for bus E, one breaker away would be the backup protection which is breaker 8. This question is asking you about zones of protection. Does that make sense? 

Power flows from Gen to Bus A to B to C. If there is fault in CB 6 then CB 8 can not be back up. If power flew in both directions then 8 could be back. I think in above case, if 6 fails to open then 4 should be next back up. Since 5& 6 are in the same zone. 

roy167 said:

Power flows from Gen to Bus A to B to C. If there is fault in CB 6 then CB 8 can not be back up. If power flew in both directions then 8 could be back. I think in above case, if 6 fails to open then 4 should be next back up. Since 5& 6 are in the same zone. 

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I see that logic, thanks for correcting me