Grounding in control panels

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sagman

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I am currently working for an engineering firm in which we have our own panel shop (UL508A), and there has been a couple issues concerning grounding in which I am trying to resolve:

1. When attaching DIN rail to the backplane do the screws used have to be drilled and tapped or can self tapping screws be used in order to maintain a proper ground for grounding terminals attached to the DIN rail.

2. The last inspection we had, the inspector told me that the new code requires the enclosure doors to be grounded (with a conductor) even if the doors use a continuous hinge. In the past the continuous piano hinge was an acceptable grounding method and no additional ground conductor was necessary.

3. Is there a minimum size wire for grounding inside the panel, or is it sized by load of the device.

Thanks in advance.

Steve
 
Re: Grounding in control panels

This probably isn't particularly useful because I'm not familiar with the requirements you have to satisfy but when I used to do enclosure designs we used "PEM"s rather than tapping. It's sort of a sirated nut that's pressed into the material and provides more elctrical and mechanical surface area than tapping.
 
Re: Grounding in control panels

You need to get a copy of UL508A.

Our local UL inspector allows self tapping screws as long as they have machine threads.

Don't know about the door hinge, but the standard says "...metal-metal contact or by means of an internal bonding conductor..."

The internal grounding conductors are sized based on the load. External field wiring terminals are sized similar to the NEC.

edited to correct missing space after word "contact".

[ March 25, 2005, 12:23 PM: Message edited by: jim dungar ]
 
Re: Grounding in control panels

409.60 Grounding
Multisection industrial control panels shall be bonded together with an equipment grounding conductor or an equivalent grounding bus sized in accordance with Table 250.122. Equipment grounding conductors shall terminate on this grounding bus or to a grounding termination point provided in a single-section industrial control panel.

The NEC has nothing specific about bonding/grounding the enclosure door to anything inside of the control panel.

IMO there are several parts of Article 250 that could apply, but I feel it would be a loose intrepretation.
 
Re: Grounding in control panels

Sheet metal screws for grounding were outlawed in 1993. Each DIN rail would need a minimum of 1 machine screw that is drilled and tapped. The others can be sheet metal screws. There is no prohibition agains self tapping machine screws - almost every ground bar on the market comes with self tapping machine screws.
 
Re: Grounding in control panels

I tried to get a fellow electrician to use a self tap screw but he is to set in his ways to see it as not being a sheet metal screw.In its own words it says TAPPING :mad:
 
Re: Grounding in control panels

You can get self drilling / tapping screws with 10-32 machine threads, these are fine for grounding. :)
 
Re: Grounding in control panels

hello

Quote
"The last inspection we had, the inspector told me that the new code requires the enclosure doors to be grounded (with a conductor) even if the doors use a continuous hinge. In the past the continuous piano hinge was an acceptable grounding method and no additional ground conductor was necessary."

Steve,

The boxes I use when building control panels are made by Hoffman. They actually weld a threaded post on the door and in the cabinet for easy install of a bonding jumper around the hinge. They started doing this a couple of years ago. I doubt that they would do this if it was not required. However I could not find a written requirement



Mike
 
Re: Grounding in control panels

Peter, I don't think you will find a listed grounding screw. There are some green ones and the are packaged as grounding screws but they are not listed. :D
 
Re: Grounding in control panels

Thanks Charlie, I was just trying to stir the pot a little. Hence the little winky guy. :D

Find me a listed grounding screw and I will sell you a gallon of Watts.
 
Re: Grounding in control panels

Also, please use a real ground bar in this panel, not a bunch of box lugs fastened with the same 1/4-20 screw.
 
Re: Grounding in control panels

This is what we do.
1. drill and tap holes to mount DIN rails
2. bonding jumpers from doors to panel
3. put a main grounding bar in
4. you can also purshase grounding type connectors that fasten into your DIN rails. These clip into the DIN rail.I use Wago brand.
5. I also make sure I use a bond bushing on the power wire coming into the panel or box.
 
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