Selective Coordination: 700.27

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steve66

steve66

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700.27: Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. (The handbook says this is new for 2005.)

Is it just me, or is this impossible? How does one acheive this? I've been comparing some circuit breaker curves with SKM software, and above a certain current, it seems to be anyone's guess which breaker will trip first.

For example, the following curves show a standard Square D 20A QO breaker, a 100A LE breaker, and a 800A GE TKMA breaker. (Imagine a 800A main in a switchboard, feeding a 100A panel with some 20A branch circuits.)

BreakerCurves.jpg


It looks to me like above about 800A, the 100A feeder breaker may trip instead of the 20A branch breaker. And above about 2000A, the 800A breaker may trip before the 20A. Is that right? Does "selective coordination" also include the instantaneous region also?

So what does one do? Do I have to replace all the NQOD branch panels (that fit nicely inside a wall) with a room full of fusible switches?
 
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That is why that code change was a huge win for the Bussmann folks that proposed it.
It is almost always impossible to get coordination in the instantaneous region with CB's.
The definition is:
Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.
Many interpret that definition to mean selectivity is required in the instantaneous region, but AHJ's handle it differently in different jurisdictions.
SquareD has a white paper regarding certain combinations of devices that ensure selectivity, even in the instantaneous region. I'll see if I can find it on their website.
 
The concept behind the Square D white papers is similar to that behind the familiar "fuse ratios" that has been used for many years - a manufacturer has tested their products in specific combinations. As long as you stay with one manufacturer it appears selective coordination is possible.

I have seen breaker time current curves that show full coordination from a 120V 20A branch all the way up to the 3000A 480V breaker on the utility feed.
 
Thanks Ron. It will take a while to digest all those papers, but I intend to cover them in detail. I can't tell you what a relief it is just to know that Square D believes selective coordination can be acheived with circuit breakers.

I really don't think my AHJ will care one iota about the coordination. But then, inspectors are pretty much imune to lawsuits. On the other hand, this new paragraph could make myself (and my professional liability insurance company) easy targets for a lawsuit.

Article 517 dictates this same coordination be applied to the entire essential electrical system in a hospital. (Even the equipment system.) This really makes it harder to use several smaller transfer switches (which 517 strongly suggests) instead of one large transfer switch.

On top of that, NFPA 99 has a phrase that goes something like "consideration shall be given to the time required to reset a overcurrent device after a fault occurs....." So I think it is basically saying breakers are better than fuses because you can reset a breaker. Replacing a fuse may require shutting off power to other critical areas.

Steve
 
When the 2002 NEC introduced article 620 for elevators, it also incorporated ANSI/ASME 17.1.1 which is the Elevator code. This includes selective coordination, which also includes within the overload region & shortcircuit region of the overcurrent protection device. How can you selectively coodinate 1 region and not another? This also addresses "shunt_trip" , but that is left better for another discussion.
Steve: [QUOTE ]Article 517 dictates this same coordination be applied to the entire essential electrical system in a hospital. (Even the equipment system.) [/QUOTE] you are correct. Our local hospital has experienced 2 "black-out" situations within the last 4 months, including the Cancer center & it's patients. That problem is now fixed. However the nurses and doctors are Trained in black-out situations.
However OSHA 1910.334 states it is illegle to randomly reset a circuitbreaker. Remember that per UL489 that breaker is intended to clear a fault 1 time, not 2 times, or 3. You must clear that fault 1st, before resetting. You could be "smacking" that breaker right into the fault again.
The AHJ doesn't even get a chance to visually inspect the Elevator room, because the Elevator inspector "Locks" the Elevator room, prior to the Electrical Inspector arrival. It's a "truf" battle here.
Just my $.02
 
ryan_618 said:
There will be a new exception in 700 that will help a little bit...not much, but a little.
Click to expand...

Can you provide any more info. Ryan?

Regarding the elevators, it seens pretty easy to put a couple of fusible disconnects in the elevator room, and call them coordinated.

But coordinating a major portion of an entrie hospital, all the way from the MW generators to the individual receptacles sounds almost impossible.

Steve
 
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