For instances where you are not the Designer of the electrical system, just doing the Arc Flash Study, how do you approach reporting/enforcing Selective Coordination?

As we all know, Selective Coordination is something we can ALMOST NEVER achieve with recommending different settings; although many PE's seem to think that is the case. To put it plainly: IF THE INSTANTANEOUS REGIONS OF TWO CURVES OVERLAP, THEY DO NOT SELECTIVELY COORDINATE!!

Selective Coordination is only required for Sections 700, 701, 702, 708 (Emergency & Life Safety) and a handful of other items like elevators, escalators, moving walkways, etc. Selective Coordination requirements do not apply to normal loads, or even Optional Standby loads (which people still often call "emergency" but what they mean is "generator supplied" or "backed-up").

How do you explain this to the designer?
Do you build a full model at the SD or DD level of design, even though we may have no idea what manufacturer of OCPDs we will get?
What do you do if the building is fully constructed and S.C. is not achieved?
Do you escalate the issue or just let it slide knowing full well it is a code violation?
What kind of recommendations do you make? (Personally I like recommending isolation transformers to limit fault current, and changing breakers out to a larger one so the INST regions will coordinate. But it makes me feel uneasy, like I am doing the design of the system, which we are not being paid to do.)

I recently learned there was a NEMA document issued in 2016 all about Selective Coordination. I've been trying to alert people to that fact. If you want to read it, it can be found here. [url]https://www.nema.org/standards/view/selective-coordination-of-low-voltage-circuit-breakers
[/url] The document is named NEMA ABP-1.
Cheers!

True



Not entirely true. They can/might coordinate up through a range of fault current (i.e. some amount of overlap) but not above. Refer to manufacturers published selective coordination tables.

To answer your question, in my opinion, the designer pretty much has o pick a basis of design manufacturer during design. Make sure his selections get the job done, make sure it's clear in his spec or drawings where selective coordination is required and through what range of time, and that alternate manufacturers must do the same.

If you're doing the study later and find a problem, all you can do is point it out and offer up what potential solutions there may be if it's in your scope to provide recommendations. How far you take that depends on your agreement.

I would add one more thing to Brent's comment: dynamic impedance is the reason some breakers can selectively coordinate in certain overlapping regions of their TCCs. As Brent indicates, these are tested combinations and published by the various manufacturers.

Mike

Thanks Mike. Can be very difficult to do otherwise.

For arc flash/coordination studies on existing facilities we report what is in the distribution system and make whatever recommended setting changes we can to improve the selectivity. If it does not meet code, we write that up in the report. If the client needs design services to correct the issue that is an add service under another scope of work.

My experience has been that very few design engineers understand or do their own coordination studies. If it's a new design, they mostly rely on an OEM to do the coordination study for them.

Thank you for this. Selective Coordination is challenging in general, so it makes me feel better about coming into an existing facility and being limited in what I can do/suggest.