Selective Cooridnation Compliance

[bphgravity]

When considering selective coordination compliance for emergency systems, do you find it necessary and/or typical for the engineering to figure the time-current curves to be coordinated all the ay into the instantaneous range (<100 mSec/6 cycles) or is the short time and long time domains sufficient.

There is some conflict between the NEC and NFPA 110 on this issue. The Florida Agency for Healthcare Administration (AHCA) standards appear to NOT consider the instantaneous region.

Any thoughts on this?

 

[don_resqcapt19]

Bryan,
The code rules use the words "overcurrent devices". Article 100 says that "overcurrent" includes short circuit, ground fault and overload conditions. You have to use the instantaneous range to provide short circuit and ground fault protection. If you have fault current at the load side of a downstream breaker that is in the instantaneous range of the feeder breaker, it will not be possible to provide selective coordination using standard breakers. Remember that this code rule was written by an employee of Bussmann!!
Don

 

[jim dungar]

Bryan,
... it will not be possible to provide selective coordination using standard breakers. Remember that this code rule was written by an employee of Bussmann!!
Don

Don,

You are correct only in the use of your word "standard". It is possible to use a mix of standard, electronic trip, and powercircuit breakers to achieve 100% coordination.

It is not always possible to achieve 100% coordination when using only standard fuses either. In most cases fuses don't coordinate well with the two levels of ground fault required in a hospital system.

 

[steve66]

Square D has a couple of papers on this issue. For some reason, I can't get on their website right now to post the link, but Roger posted them here a couple of weeks ago. (Or just search their site for coordination or selective, or look under circuit breakers in the products).

Square D makes a list of their breakers they say are coordinated to either .1 sec or .01 sec. They specifically mention the AHCA, and says their requirements have been varying depending on which area of florida you are in.

Steve

 

[steve66]

I have several thoughts on this issue because I am up against it right now. I have a client who has some very critical loads served by a single transfer switch. I would like to add another system so 2 failures would actually have to occur for a full loss of power. However, I think the new "selective coordination" rules may prevent me from doing that.

Selective coordination is usually a trade off with system protection. You increase coordination while decreasing the protection of the cable, and other system components. So "total coordination" is not always possible, or desirable.

NFPA 99 says designers should consider the amount of time it takes to return a circuit to service after a fault. Circuit breakers that adequately protect the system reduces that time. Fuses increase that time. And are fuses realistic replacement for a panelboard full of breakers in an existing facility?

What about additions to exisitng systems? Does eveything have to be coordinated now, or just the new stuff?

Doesn't this also discourage the subdividing of the emergency systems into smaller systems with multiple transfer switches (something NFPA 70 even says should be done.)

Finally, how many faults are really bolted faults that happen in the instantenous region? My guess is not very many.

I hope this rule doesn't last long.

Steve

 

[don_resqcapt19]

Finally, how many faults are really bolted faults that happen in the instantenous region? My guess is not very many.
I hope this rule doesn't last long.

Just another example of where the NEC is going...full of rules that have a lot more to do with market position and product sales than with electrical safety.
Don

 

[davidr43229]

Just another example of where the NEC is going...full of rules that have a lot more to do with market position and product sales than with electrical safety

There is a new panel 20 (COP) Critical Operations & Power Systems, made up of firemen & poilcemen. Their primary concern was in a 911 Senario which includes Hospitals, Mass Transit, Large data collection centers and some gas stations. COP has endorsed the use of Selctive Coordination and believe it to be an important issue in another 911 incident.
Consideration should be given in Emergency systems, such as Hotels, theaters, sports arenas healthcare facilities and similar institutions where panic control is needed.
This also would apply to new construction by means of the NFPA Life Safety Code 2003 1.3.1 and in Chapter 4 , 4.5.3 means of Egress and Chapter 7 7.2.3.12 under Power & COntrol wiring for Elevator equipment and communications.

What about additions to exisitng systems? Does eveything have to be coordinated now, or just the new stuff

The code shall apply to New construction and existing buildings and existing structures.
Just my $.02

 

[davidr43229]

NFPA 99 says designers should consider the amount of time it takes to return a circuit to service after a fault. Circuit breakers that adequately protect the system reduces that time. Fuses increase that time

I am curious. Are you talking about going up to a breaker and relatching it?

 

[steve66]

I am curious. Are you talking about going up to a breaker and relatching it?

Yep, that's exactly what I'm talking about. I'll bet you are going to quote some OSHA rule saying I can't do that.

 

[davidr43229]

Yes sir,
1910.334
After a circuit is deenergized by a circuit protective device, the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual reclosing of circuit breakers or reenergizing circuits through replaced fuses is prohibited.
Note: When it can be determined from the design of the circuit and the overcurrent devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.