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 Post subject: Depth of penetration
PostPosted: Fri May 01, 2015 7:53 am 
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Trying to wrap my head around a possible issue.

How long can the screws for mounting/changing equipment in an MCC bucket be? I figured that the spacing requirements would be a NEMA standard or published by the manufacturers but this seems to be some obscure information I can't find anywhere.

Thinking of the "blind reaching" problem. There clearly has to be some kind of standard established for this but I can't find it anywhere and it may be simply that I haven't come up with the right terminology to find the answer. NEMA ICS clearly implies that there needs to be a 1" bus clearance including fasteners but I can't find anything on the fasteners themselves.


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 Post subject: Re: Depth of penetration
PostPosted: Mon May 11, 2015 6:37 pm 
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PaulEngr wrote:
How long can the screws for mounting/changing equipment in an MCC bucket be?


They can be as long as whatever the local hardware store stocks in the right thread size.

In all seriousness, that level of prescriptive design detail is probably not going to be in the equipment standards. The reference to 1" clearance, or the ability to pass certain DC and AC high potential tests is probably all we're going to find.

If you're talking about the machine screws used to mount equipment to the back panel of the MCC bucket, we could speculate that manufacturers would want to use one size of screw to mount each of the common components, so the length would be selected for the thickest mounting base of any component that is in the MCC's UL file as a standard component.

This is is similar to questions about clearance between MV components in equipment. There are various tables and rules of thumb available, however the right answer for manufactured equipment is whatever distance the manufacturer can use to pass the BIL tests.


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 Post subject: Re: Depth of penetration
PostPosted: Wed May 13, 2015 10:16 am 
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With respect to MV clearances...despite the fact that creep and impulse requirements are performance based and not based on any kind of standard for dimensions, there is remarkable consistency among all manufacturers.

The same criteria seems to be applied to low voltage equipment so that for instance 1" seems to be the standard criteria for around 480 V. Im just surprised that despite the well recognized fact that there is often substantial field modification necessary for a given installation, that there are no criteria for doing so aside from again, performance testing. The attitude seems to be that manufacturers neither support nor condemn the practice, especially when most of them will sell you hardware specifically for field installations, add prepared spaces to equipment, etc. So the manufacturers are not supporting it with specific engineering guidelines but aren't condemning it either. The attitude seems to be pass the buck...if you do a field modification, you're on your own to do your own testing on the final product.

With MV and strangely enough with low voltage connectors, there are industry accepted typical distances that work. It just seems odd that we have neither a statement that you can't do field installations nor any kind of instructions on what is and is not acceptable.


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