Wrong to use screw threads as a ground?

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wireman

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I'm trying to find in the NEC where it says grounding must be achieved by metal-to-metal surface contact and not simply by screw threads.
Similar to a receptacle's metal yoke being grounded to a metal box by the screw threads that hold it in place.

My situation is not with receptacle wiring but it is a similar situation.

Thanks.
 
I don't recall that the code specifies just how the connection is made.

I would not have a problem with using the threads to make the connection, especially for smaller lugs.

take a look at 250.12.

some people seem to feel that 250.8 applies to the means by which the connection is attached and not the actual connection, which may be why you see so much attention to scraping paint.

however, 250.12 clearly allows things like lock nuts that scrap just a tiny bit of paint away to be used to bond to an enclosure, and there is probably less surface area connected there then with a threaded connection.
 
Metal din-rail is mounted inside a control panel but it they are using 1" long/high PVC to raise the din rail off of the backpanel. Machine screws are used to secure everything.
Grounding terminals are mounted to the dinrail.

I thought there was some kind of a NEC clause that required metal-to-metal contact and not just depending on the screw threads, wouldn't be the first time I've been wrong.

Please don't derail this thread talking about UL.
 
petersonra said:
I don't recall that the code specifies just how the connection is made.

I would not have a problem with using the threads to make the connection, especially for smaller lugs.

take a look at 250.12.

some people seem to feel that 250.8 applies to the means by which the connection is attached and not the actual connection, which may be why you see so much attention to scraping paint.

however, 250.12 clearly allows things like lock nuts that scrap just a tiny bit of paint away to be used to bond to an enclosure, and there is probably less surface area connected there then with a threaded connection.
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I agree, although I don't scrape paint. Scraping paint causes rust and I hate rust and don't like people grinding off perfectly good paint. If I ever had any grounding issues from the threads and the underside of the screw making contact with whatever grounding device I'm installing, I'd probably never know about it anyway.
JAP>
 
wireman said:
Metal din-rail is mounted inside a control panel but it they are using 1" long/high PVC to raise the din rail off of the backpanel. Machine screws are used to secure everything.
Grounding terminals are mounted to the dinrail.

I thought there was some kind of a NEC clause that required metal-to-metal contact and not just depending on the screw threads, wouldn't be the first time I've been wrong.

Please don't derail this thread talking about UL.
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I'd bite the bullet and install a Din Rail Grounding terminal with a jumper back to the backplate.

JAP>
 
To the lug where the EGC that more than likely is going to be pulled in with the Feeder conductors is landed.

JAP>
 
wireman said:
Metal din-rail is mounted inside a control panel but it they are using 1" long/high PVC to raise the din rail off of the backpanel. Machine screws are used to secure everything.
Grounding terminals are mounted to the dinrail.

I thought there was some kind of a NEC clause that required metal-to-metal contact and not just depending on the screw threads, wouldn't be the first time I've been wrong.

Please don't derail this thread talking about UL.
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the problem is that it is really a UL issue and not a NEC issue.

note how 250.8 is worded. the word "listed" is used several times.

UL allows the bonding connection between the grounding terminals and the DIN rail to be made by the clamp in the grounding terminal that clamps onto the DIN rail. The bonding connection from the DIN rail to the metal of some kind of enclosure is made by the mounting screws that hold the DIN rail in place. These are all things that UL allows.

I am not sure just what spacer arrangement you have but it is not unusual to cut a short piece of tubing and use that as a spacer between the panel/enclosure and the DIN rail and run a long screw through the slots in the DIN rail and into the enclosure/panel. I don't have an issue with that.

if it makes you feel better, you can add a bonding conductor to a grounding terminal on the DIN rail, or to the DIN rail itself. I would point out that it is unlikely that UL has listed the DIN rail to be bonded running a conductor direct to the DIN rail, so that might be a violation if that is the sole bonding means.
 
luckylerado said:
Generally this is not optional.
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I'll continue to not grind paint until someone tells me I have to.
I'll probably start grinding paint about the time I start using a torch to torch holes in Hoffman boxes and welding them to the beams.
Factory finishes deteriorate quickly once you start grinding the paint off.


JAP>
 
Probably comes from my being an old Dodge Truck owner...... Great Truck....... Terrible paint job life spans...:)

JAP>
 
luckylerado said:
In some applications, machine screw threads are an acceptable means of connection. 250.8(A). What is the specific situation?
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I would say in a lot of applications this is the case.

The OP's scenario is not much different than a service disconnect or Service Panel where you install the green bonding screw thru a hole in the neutral bar and are relying on tension to maintain the connection.

Actually his scenario is even a little better since on some service panels the green bonding screw goes thru the neutral bar but doesn't actually have a spacer behind it that takes out the space behind the bar and the back panel.


JAP>
 
luckylerado said:
The factory does not need to comply with 250.12. Strictly speaking.
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Strictly speaking that may be true.

But that just goes to show that the factory could install a Ground Bar in the panel, and, even if there's a large resistance between the ground bar and the panel can, your probably not ever going to know the Panel can is not properly bonded anyway.


JAP>
 
wireman said:
I'm trying to find in the NEC where it says grounding must be achieved by metal-to-metal surface contact and not simply by screw threads.
Similar to a receptacle's metal yoke being grounded to a metal box by the screw threads that hold it in place.

My situation is not with reptacle wiring but it is a similar situation.

Thanks.
Click to expand...

This is an interesting thought since a fastener with threads usually has a head where the threads when turned pull the head againt as surface pulling/securing a device/conductor between the head of the screw and the surface the screw is threading into.
As such are the threads actually making the electrical connections? Or is it the fact that the head of the fastener being pulled against a surface by the action of the threads that secures the electrical connection. As stated previously the surfaces to be joined and secured together must be clean. And free of paint to assure that a good electrical connection is made.
 
Sierrasparky said:
]

Really I have yet to see factory installed ground bars with the paint scraped off.
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I can't remember where I know this from. I think it has to do with the factory holes being punched instead of drilled, so there is more surface area that the bolt/screw is in contact with.

They are bonded by the screw threads, just like in a "residential" panel when the Green bonding screw is installed in the main panel.

The reason specific grounding bars are sold to fit specific panels is because there are factory holes that line up for the mounting screws (which bond it to the enclosure).

Self tappers are not sufficient to bond (again, I don't know why I know this).
 
templdl said:
As stated previously the surfaces to be joined and secured together must be clean. And free of paint to assure that a good electrical connection is made.
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I'd agree with that if your depending on surface to surface contact much like installing a lug in a panel where you drill the hole and put a bolt through the lug with a nut on the back, since there's a possibility the bolt would not touch the outside of the drilled hole at all, but, I feel a self threading machine screw that cuts its own threads and catches enough of the metal required does make a good electrical connection without having to scratch any paint off. Much like most bonding screws we install.

JAP>
 
jap said:
I'd agree with that if your depending on surface to surface contact much like installing a lug in a panel where you drill the hole and put a bolt through the lug with a nut on the back, since there's a possibility the bolt would not touch the outside of the drilled hole at all, but, I feel a self threading machine screw that cuts its own threads and catches enough of the metal required does make a good electrical connection without having to scratch any paint off. Much like most bonding screws we install.

JAP>
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I agree. I don't scrape paint off for a threaded hole either.
 
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