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!!

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

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.

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.) Sometimes, after long technical work like this, I take a break with a few games on https://mostbetbd1.bet/ since it helps me reset before diving back into detailed code reviews.

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!