Subplate or Back Panel Material? - NFPA79

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bachrachh

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Greetings Folks. I do custom machine control wiring. We inherited a project from another division where they have mounted all their electrical components (up to and including 120VAC) in an enclosure, but on a PLASTIC back panel (or subplate). I don't think this allowable, but I cannot find an NFPA requirement specifying the material subplates are to be made of. Can anybody steer me to the provision? TIA, Hunter
 
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UL508A 7.1.b "Emply materials and components that are determined to be usable in the application." Can't locate a provision in NFPA79-2007 that forbids plastic subplate. UL508, UL50 , NEMA250 may have additional provisions or requirements for plastic use in this application.
 
Here's a tidbit I found in NFPA 79 that has implications: Chapter 3, Definitions, 3.3.97 states that a Subplate is: "An internal metal surface separate from the walls of an enclosure or controller on which various component parts of the controller are mounted and wired". Hmmmm. H.
 
Objections to Plastic Back Panels

Objections to Plastic Back Panels

Well, here's a few that come to mind:

There's grounding issues. All devices needing grounding would require a jumper. Then there's flammability of the material. Outgassing from heat. Thread integrity. Rigidity or lack thereof.
I once saw a control panel (back panel, not the enclosure) on an oven made of plywood. Not kidding. Guy had used drywall screws to attach the devices. Well, that's one way to do it. I think people use plastic only because its easier than tapping threads in metal panels.

NFPA 79 does refer to "approved" or "acceptable" materials.

I guess I'm just an old school guy who sees plastic as an inferior material and the easy way out. I always adhere to the old adage: Do it right. Do it once.

JMO, Hunter
 
Would I choose to use a plastic backpanel? No. Is it forbidden? I suppose one could argue that the definition excludes plastic, but it doesn't go out of it's way to do so.
 
bachrachh said:
Greetings Folks. I do custom machine control wiring. We inherited a project from another division where they have mounted all their electrical components (up to and including 120VAC) in an enclosure, but on a PLASTIC back panel (or subplate). I don't think this allowable, but I cannot find an NFPA requirement specifying the material subplates are to be made of. Can anybody steer me to the provision? TIA, Hunter
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Check UL508A for safety standards for industrial control panels.

There are often live parts in such components as relay sockets and switches that must be safely separated from grounded parts such as the enclosure. Appropriate insulator materials may be used to separate live parts from the enclosure.

NEC Table 408.56 specifies a minimum distance of 1/2" for live parts to ground for voltages not exceeding 250 Volts nominal.
 
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