700.27 Coordination Study

[anbm]

NEC rule 700.27 requires circuit breakers on emergency electrical system need to be fully coordinated.... If we specified solid breakers with L.S.I features, it is hard to make instantaneous feature to be fully coordinated throughout the emergency system or they won't coordinate. How do we meet this requirement?

 

[jim dungar]

NEC rule 700.27 requires circuit breakers on emergency electrical system need to be fully coordinated.... If we specified solid breakers with L.S.I features, it is hard to make instantaneous feature to be fully coordinated throughout the emergency system or they won't coordinate. How do we meet this requirement?

While it does depend on the available fault current, I regularly coordinate modern 'molded-case' LSI breakers. These breakers almost all contain a selective override of the Inst trip function. Also many manufacturers publish tested combinations of thermal magnetic devices.

In the past year I have worked primarily with Square D breakers, but have also used Cutler Hammer and Siemens.

 

[anbm]

I used these breaker from Square-D, for example 400A CB solid type 'PG' and 250A solid type 'PG', they coordinate well with L,S features but not I feature (instantaneous), similar 800A and 400A breakers, instantaneous feature won't coordinate. I use SKM software to do the coordination study.

 

[jim dungar]

I used these breaker from Square-D, for example 400A CB solid type 'PG' and 250A solid type 'PG', they coordinate well with L,S features but not I feature (instantaneous), similar 800A and 400A breakers, instantaneous feature won't coordinate. I use SKM software to do the coordination study.

I do know that a Square D P frame can be coordinated with one of their R frame even in the Inst region.

I don't trust the just software. I definitely recommend the selection guides from the manufacturers. The Square D one I use is 0100DB0501, available at http://static.schneider-electric.us/docs/Circuit Protection/0100DB0501.pdf.

 

[MNWildcat]

...Also many manufacturers publish tested combinations of thermal magnetic devices.

Yes, but a lot of those tested combinations are only good for a few thousand AIC, so you have to have low fault levels at that point.

You may have to increase the frame size of an electronic trip and then set the trip to a lower point. The larger the difference between breaker ratings the "easier" it is to selectively coordinate.

They can be hard to coordinate. We have had to redesign some systems with a "flatter" tree. E.G. 3000A--> 400A--> 20A instead of 3000A--> 1600A--> 800A--> 400A--> 200A -->20A.

Also verify with the AHJ to what level they are requiring the selective coordination. Is being coordinated to .05s acceptable? .10s? .001s? There was a proposal that failed for the 09 NFPA 99 to only have to be coordinated to .10s.

 

[don_resqcapt19]

...
Also verify with the AHJ to what level they are requiring the selective coordination. Is being coordinated to .05s acceptable? .10s? .001s? There was a proposal that failed for the 09 NFPA 99 to only have to be coordinated to .10s.

CMP 13 has rejected proposal 13-195 for the 2011 code which would have specified that the selective coordination start at 0.1 seconds with the following panel comment. The vote was 11 to 3 to reject. There were some strong comments in support of the proposal by the 3 panel members who voted to accept

Panel Statement: The 0.1 second limit in this proposal could reduce the level of safety by limiting the types of overcurrents that would need to be isolated to the nearest upstream device. Requiring selective coordination down to only 0.1 seconds will cover only overloads and a few minor phase-to-phase and minor ground faults