what is SCCR if it doesn't prevent arc flash?

[Deltaforce]

Assuming you mean Professional Engineer, there are far more installations designed and installed by contractors and electricians than ones designed by PE. And even there the mechanical and electrical PEs often don't talk to each other so equipment with SCCR far lower than the available fault current is specified by the mechanical engineer.

But inspector of the install?
They also miss?

 

[don_resqcapt19]

But inspector of the install?
They also miss?

almost all the time

 

[petersonra]

Tangentially related to this subject... In article 440 (Air-Conditioning and Refrigerating Equipment), article 440.10 requires the SCCR to exceed the available fault current on motor controllers or industrial control panels of multimotor and combination-load equipment. My question is...what motors does this generally include and exclude? My niche is office / K-12 / government / healthcare...not so much industrial/manufacturing/etc.
* chillers, condensing units, cooling towers - obviously yes
* pumps related to such equipment - ??
* packaged air handlers / RTUs that include integral cooling - obviously yes
* packaged air handlers that push heated/cooled air, but do not include integral cooling - ??
* exhaust fans - assume no
* commercial kitchen freezer and coolers
* what else?

(I am aware of the similar requirements in 620.51 for elevator control panels. And of course switchboards, switchgear and panels. Mainly interested in what motors need to be remembered.)

Thank you!

SCCR does not apply to loads like motors at all. Only to the upstream control equipment and to the equipment containing protective devices such as circuit breakers, but not the actual protective equipment.

And to respond to another post, many times devices have limitations on wire sizes that restrict the SCCR. Usually larger wires have higher SCCR in these cases, while smaller wires may have just the default rating. You have to look it up and not rely on blurbs that say 100kA SCCR and #12-4/0 wire. Its quite possible the SCCR with #12 is 10 kA.

 

[kwired]

But inspector of the install?
They also miss?

Some do, some do not.

If there is reason to assume rather high available fault current and you end up with a rather short length feeder or branch circuit the knowledgeable ones will be asking questions at the very least. The ones that aren't so knowledgeable on this topic may have no clue what the possibilities might be in some short circuit or arcing incident.

With enough experience, one just kind of naturally knows most the time when there is little concern vs you at least need to be thinking about the SCCR.

 

[mayanees]

Hi all, really appreciate the insight. Bolted fault (typ during installation) vs arc fault (typ during maintenance and testing) was very illustrative.
Also the description of the Square D testing lab imagery. Really, just about every response here helped push my understanding forward.

I'm preparing some material to teach younger staff the basics of short circuit studies. Trying to teach something sure makes you realize how little you know, right? This will surely not be my final line of questions on the topic

malachi, arcing fault current on low-voltage systems (<1000V) is about 55% of the bolted fault current, whereas on MV-HV systems it's closer to 95%. That's good info to present to your younger staff.