Wrong to use screw threads as a ground?

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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.
 
Another factor may be how well the fastener secures to the metal to assured that it can be drawn down tight without stripping the threads.
I believe it may me a !minimum of 3-4 threads for a property drilled and tapped hole. As such that would limit the size of the screw and threads per inch based upon the thickness of the metal. Screws are probably either 24 or 32 threads per inch. As such at 32tpi the minimum thickness would be 3/32 to 1/8" thick which should be more than sufficient for the screw to be tightened securely without stripping.
 
templdl said:
Another factor may be how well the fastener secures to the metal to assured that it can be drawn down tight without stripping the threads.
I believe it may me a !minimum of 3-4 threads for a property drilled and tapped hole. As such that would limit the size of the screw and threads per inch based upon the thickness of the metal. Screws are probably either 24 or 32 threads per inch. As such at 32tpi the minimum thickness would be 3/32 to 1/8" thick which should be more than sufficient for the screw to be tightened securely without stripping.
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NEC only requires two threads.
 
luckylerado said:
Self taping screws are not machine screws and are not one of the approved methods regardless of how many threads.
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Some have a self tapping tip and the rest of the body is essentially a machine screw.

As mentioned in the past few posts the thing of most importance is not the screw pitch itself but rather how many threads are engaged. Most common self tapping screws do not have high enough number of threads per inch to meet the requirements to engage at least two threads in your average enclosure these days.

Two threads with 32 threads per inch means you must have 1/16 inch thickness of enclosure wall to fully engage two threads. So with a lot of things these days going with thinner walled enclosures to cut costs - you really need to pay closer attention then you ever did before if you want to be in compliance with that rule.

Enclosures with a raised bump where the bonding screw is intended to go have more thickness in which to engage threads at that bump.
 
ActionDave said:
NEC only requires two threads.
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Thanks, I was just going by what the industry standards that I ran across refered to. As such knowing what the NEC minimum is and also knowing that the purpose of the screw's use as a fastener is to hold the grounding conductor securely in place against a grounded surface that is free of paint etc. Thus it is not the threads of screw itself that is the ground connection.
From what I can tell when a 32tpi scew is used the metal must be at least ,0625" thick to be acceptable by the NEC which is about 14ga.
 
Not that it applies to this, but, not all self tapping screws are created equal. As we all know a lot of the enclosures nowadays are not 1/8" thick and I don't generally keep self tapping machine screws on my truck other than the short 8/32 green grounding screws.

Most will find that 10x3/4 self tapping tek screws with the 5/16 head that most use to self tap minies or straps to Z-perlins are useless when trying to secure something to the thickness of a standard Hoffman box or panel enclosure. The Tapping head drills a hole just samller than the threads and they strip out very easily. However if you use Wafer head needle point sheet metal screws, that aren't self tapping, that feel like they're almost going to break when they go through the metal, once they go in, you could hardly pull them out with a truck, yet they probably don't catch 2 threads.

Which makes me wonder, if you use more than one screw to fasten a ground bar to a panel can, instead of a machine screw, does each screw have to catch 2 threads ? or can the total of the (2) or more threads suffice?


JAP>
 
templdl said:
Thanks, I was just going by what the industry standards that I ran across refered to. As such knowing what the NEC minimum is and also knowing that the purpose of the screw's use as a fastener is to hold the grounding conductor securely in place against a grounded surface that is free of paint etc. Thus it is not the threads of screw itself that is the ground connection.
From what I can tell when a 32tpi scew is used the metal must be at least ,0625" thick to be acceptable by the NEC which is about 14ga.
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I have never in my career seen the white paint ground off of a Hoffman Backplate, or inside sidewall for that matter to install a ground lug.

JAP>
 
I take that back,,,, I've seen it,,,, but wondered who would do such a thing.

JAP>
 
jap said:
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.
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You would hate me, I will grind the paint to bare metal when bonding an SDS, connecting a GEC to building steel.

You don't think that it is required by 250.12?
 
iwire said:
I am going to have to pull you guys over and ask for the exception you guys are using. :)
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Officer, I have been doing it this way for fifteen years.

250.8 Connection of Grounding and Bonding
Equipment
(A) Permitted Methods. Equipment grounding conduc-
tors, grounding electrode conductors, and bonding jumpers
shall be connected by one of the following means:......................

(5) Machine screw-type fasteners that engage not less than
two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less
than two threads in the enclosure
(7) Connections that are part of a listed assembly


The threads are the contact surface. I have never ground off the paint behind a EGC buss.
 
The difference between hoffman cans grounding point and method has been sent, tested and passed the UL tests. A guy in the field adding a lug has not. Scrape the paint it takes less than 10 minutes even for a 3/0 lug. I use a razor knife and a flat head screw driver. Code or not its the professional thing to do, not to mention it may not even be in 250 it could be in the places listed by 250.3 and it could even be a building code thing.

Sent from my SCH-I545 using Tapatalk
 
ActionDave said:
Officer, I have been doing it this way for fifteen years.

250.8 Connection of Grounding and Bonding
Equipment
(A) Permitted Methods. Equipment grounding conduc-
tors, grounding electrode conductors, and bonding jumpers
shall be connected by one of the following means:......................

(5) Machine screw-type fasteners that engage not less than
two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less
than two threads in the enclosure
(7) Connections that are part of a listed assembly


The threads are the contact surface. I have never ground off the paint behind a EGC buss.
Click to expand...
"Thread-forming" machine screw equals "self-tapping" machine screw, not to be confused with "sheet metal" screw.
 
ActionDave said:
Officer, I have been doing it this way for fifteen years.

250.8 Connection of Grounding and Bonding
Equipment
(A) Permitted Methods. Equipment grounding conduc-
tors, grounding electrode conductors, and bonding jumpers
shall be connected by one of the following means:......................

(5) Machine screw-type fasteners that engage not less than
two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less
than two threads in the enclosure
(7) Connections that are part of a listed assembly


The threads are the contact surface. I have never ground off the paint behind a EGC buss.
Click to expand...

Yes. And if field installing a lug with nut and bolt through a drilled hole the paint gets scrapped off.
 
cloudibew said:
The difference between hoffman cans grounding point and method has been sent, tested and passed the UL tests. A guy in the field adding a lug has not. Scrape the paint it takes less than 10 minutes even for a 3/0 lug. I use a razor knife and a flat head screw driver. Code or not its the professional thing to do, not to mention it may not even be in 250 it could be in the places listed by 250.3 and it could even be a building code thing.

Sent from my SCH-I545 using Tapatalk
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All are saying "Scrape the Paint" prior to installing a lug, but no one is expressing to what extent?
As far as professional, do you take a resistance reading between your newly installed lug and the enclosure each time to be sure there is absolute zero resistance?
I guess I could take my knife point and scrape a single line behind the lug before I bolt it on to satisfy the requirement if that's what's intended, and although no one could see it I'd be good to go correct ?


JAP>
 
ActionDave said:
Yes. In cases like this I agree, scrape the paint.
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I'll agree to if I-wire will agree to grind the paint or laquer off of his Green bonding screw before installing it since 250.12 specifies "Surfaces" and "Threads" .:)
I think we're all guilty of breaking this rule to some extent.


JAP>
 
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