It appears that circuit breakers will not be permitted for emergency and legally required standby systems. 700.27 and 701.18 require these circuits to have selective coordination. It is not possible to have selective coordination for small breakers under ground fault or short circuit conditions. Don
The short circuit trip point of most breakers 125 amps and smaller overlap. If you have a short circuit on a 15 amp branch circuit that is protected on the supply side by a 100 amp panel main breaker, there is no way to know for sure which breaker will trip. Either or both breakers may trip and that is not selective coordination. Don
#98407 - 10/28/0407:15 AMRe: breakers and EM circuits
Don, I wouldn't limit it to low ampacity breakers. Any two or more breakers with an instantaneous characteristic, when installed with relatively small amounts of impedances between them (maybe a main and feeder breaker), will have overlapping characteristics, and not technically be selective. The breaker manufacturers are not sure what to do about this. I would guess, although I have not checked, that this was a proposal initiated by a fuse manufacturer. I think that this requirement is a bad idea. It brings us back to the switch and fuse panelboard days.
#98408 - 10/28/0409:16 AMRe: breakers and EM circuits
This was proposed by a gentleman named Todd Lottman. Here is a little about Mr. Lottman:
Todd Lottmann is an electrical engineer employed by Cooper Bussmann, Inc. focusing on codes and standards. Todd is an alternate member of NEC Code-Making Panel 12 representing NEMA, a member of the UL 508A standards technical panel, member of NEMA 1IS Industrial Controls section, and involved with the NFPA 79 technical committee. Todd is the Bussmann IAEI representative participating in the national section meetings and various chapter meetings around the country.
No big surprise here.
Don: You were saying that breaekrs ranging from 125 and smaller often overlap in thier trip curves. Often times on a medium sized commercial projects the feeder OCPD feeding the panel that has the BC breakers for the unit equipment are 200, 225 or 400. Would this still create a problem.
I don't think that you could ever use series rated systems when selective cooridination is required. The design of series rating systems is such that at least 2 breakers normally open to clear a fault. Don
#98410 - 10/28/0409:25 AMRe: breakers and EM circuits
I got called into work last weekend because half the building had no power. Turns out an outside light shorted out and brought down the subpanel. It was a 480 subpanel so it killed a bunch of lights and a transformer supplying 120 loads. This was normal power not emergency.
I have actually had a 120,000 sq ft building trip the main ground fault protection due to a leaky roof shorting out a single fixture.
It is my belief that this is not good. It may be acceptable by code and engineers but a single short in a 20 amp circuit should not kill everything.
Perhaps this proposal is a step in the right direction.
#98412 - 10/31/0408:45 AMRe: breakers and EM circuits
I can recall times when a fault in a receptacle connected device has caused the building main to trip, not the branch circuit CB, nor the panel main, nor the distribution feeder CB. There is certainly a need for selective coordination. I'll bet the Circuit Breaker folks will invent somthing soon, though. Maybe an "emergency rated" small circuit breaker.
#98413 - 11/10/0409:32 PMRe: breakers and EM circuits
I was talking with a friend of mine today at our IAEI meeting. He is a very qualified individual who has written several books and works as an instructor for the NFPA. I asked him about this and he was saying that he has talked to a lot of breaker guys and fuse guys. What he is being told is that it is nearly impossible to coordinate a fuse-breaker combonation, but it is possible to coordinate fuse-fuse or breaker-breaker. There is no denying that it will take a great deal of engineering to accomplish it, but he is being told that it can be done.
Don: I still understand your position, but I wanted to throw this out there as well.