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

Don,
I think I see where you are going with this but could you expand on your thoughts?

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

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.

This was proposed by a gentleman named Todd Lottman. Here is a little about Mr. Lottman:

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.

And what about series rated systems?

BTW: The proposal can be found here: http://www.nfpa.org/PDF/NECPart4.pdf?src=necdigest

Ryan,
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

Ron,
I'm not sure that it is a bad idea. A single fault on a branch circuit should not be permitted to take a large part of or the complete emergency system down.
Don

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.

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.

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.

Ryan,
I agree that you can have breakers selectively coordinated for overloads, but not for short circuits or ground faults. Any time the fault current is in the instantaneous range more then one breaker may trip. Ask your sources if their comments applied to both overload and short circuit conditions.
Don

Another question on this topic: Does this requirement for selective coordination include branch circuit breakers for unit equipment? I'm trying to determine if unit equipment on a general lighting branch circuit is really considered a component of an "emergancy system" or of a "legally required standby system", or if perhaps it is just a plain old branch circuit with some unit equipment installed on it, and therefore the coordination is not required. What do you guys think?

Ryan and Don :


i can see where it going but the issue is the proper sizing of ocpd like breaker or fuse but also add on this one is the length of conductors to the device or equmpiment as need.

I deal with pretty big system too but really for emergecy system i really feel sticky with this one due slection coration and i end up putting a main fuse for breaker box because most breaker do trip much longer than fuse will trip per short circuit but overload the breaker useally take care of themself but i know Bussman and few other fuse manfacters do have listing for the fuse rating with time chart but breaker time chart most small and med size breakers [fixed trip types] the curives and timedelay almost fall the same

but i dont know what about the iec or " european breakers' they have very good tripping rating it is near to fuse charcatics but i dont have the chart with me yet but i will try to find it and try to compare both north american and european breakers but i am not sure if that will work out with this one or not .

merci, marc

Ryan,
The circuit supplying the unit equipment is not installed per the rules in 700 or 701, so coorination is not required.
Don

I'll have to ask what is "unit equipment"? If unit equipment includes emergency lighting branch circuits, I would think yes.
Fuses do not necessarily give selectivity in the instantaneous region either. Their curve extends out to the right (higher current) as you approach 0.01 seconds. You just don't see it, as the Time Current Coordination curves (TCC's) are generally drawn only down to 0.01

If you think it is helpfull, I can draw a couple of TCC's for example.

[This message has been edited by Ron (edited 11-12-2004).]

Unit Equipment is a emergency battery unit.

As Don has mentioned the branch circuit supplying an EBU is a 'normal' circuit not an emergency circuit. If DC leaves that EBU that would be an article 700 emergency circuit.

Bob

Ok, so an emergency light that is soley served by the gen, and not batteries, is an Article 700/701 circuit from is origin, right?

One way to solve coordination problems with a light fixture is to fuse the fixture with a fast blow fuse.

The wallpack lights where I work have fuses which are class CC fast blow 5 amp. Those are 150 watt HPS. When a ballast burns up and shorts the fuse blows and does not take anything else out.

You can get fuse kits for troffer lights like this.