# Zone of protection



## kduff70 (Jan 20, 2015)

Hi I was working a practice problem from the engineer guide by Graffeo problem 35 and I don’t quiet understand how his answer for back up protection are cb 2,7,6. I understand how cb 7 and cb 6 can be back up but not cb 2 I would think cb 2 would trip also I have attached the problem if some would care to explain the steps to me I’m not all the way sure what I missing.



relay.pdf


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## Kovz (Jan 21, 2015)

Kduff,

I purchased the Graffeo book back in August 2014. He sent me an email with some corrections for the book back in October 2014. #35 is explained more in the file I have attached. Hope that helps explain why breakers 2, 7, 6 are the backup protection.

Updated errata Explanations and Errata 10-10 p.pdf


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## kduff70 (Jan 21, 2015)

[SIZE=10.5pt]Thank you, I think I got now after looking a Graffeo study reference on protective relays The Art &amp; Science of Protective Relayinghttp://www.gedigitalenergy.com/multilin/notes/artsci/artsci.pdf by C. Russell Mason on page 5 . He explain the field of protection for each breaker and where a fault will cause CB to open. So if look at the problem CB 4 zone of protection covers CB5 which is the line protection and it cover CB3 which is bus protection .Since CB7 and CB6 out of the zone protection they are the back up as well a CB2 I think that what he was saying . Your thoughts?[/SIZE]


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## Kovz (Jan 21, 2015)

Yes I agree, CB3 would be considered primary protection because it's on the same bus. CB5 would also be considered primary protection because it's on the same line. So CB2,CB6 and CB7 would all be considered backup protection.


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## iwire (Jan 21, 2015)

Kovz said:


> Yes I agree, CB3 would be considered primary protection because it's on the same bus. CB5 would also be considered primary protection because it's on the same line. So CB2,CB6 and CB7 would all be considered backup protection.


This...


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## kduff70 (Jan 21, 2015)

thank you


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