Selective Coordination

[necnotevenclose]

It has been my understanding that when you do a coordination study you are not allowed to overlap OCPD. I have a few cases where the CB does not overlap but the curves are right next to each other. Is that considered overlap?

 

[alehman]

Curves for low-voltage circuit breakers usually include tolerance bands, so right next to each other is usually acceptable.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[ScottyUK]

For any given fault those tolerances will combine in the most inopportune manner.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[marks1080]

You should allow sufficient margine for trip time. 400ms is generally a good margin.

 

[alehman]

Scotty - yes, very true. It's good to leave more margin if you can (keeping in mind that sometimes at the cost of increased arc flash hazard).

marks1080 - 400mS may be a comfortable margin for lower current ranges, but will probably be impractical many cases in the short time ranges.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.