ECN Forum Forums Code Related Discussion NEC & other Code issues breakers and EM circuits

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.