Poor Grounding by Manufacturer

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joeblurton

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I was just amazed at the ground provided in a Sq D MCC. This is 250 amps 480 v. supplying a VFD. The screw holding the lug is an 8-32! The connection surface is painted, and a galvanized screw with copper lug are dis-similar. This is brand new switchgear. I know a ground fault will "spit" on this connection. Also the gear itself had many sharp edges and bending the 250 kcmil was impossible, as the auxiliary gutter was about 3" wide.
I ream conduit, scrape paint on grounds, with diligence, Why do manufactures get away with this shabby product?
IMAG0115.jpg
 
I believe that some of the enclosures use conductive paint otherwise I am not sure how they would get away with it.

Just Thursday my guys replaced some 8-32 screws that held a 200 amp breaker to the bus bar. I was surprised myself but apparently it is okay. This panel was 40 or so years old and apparently the screws were loose.
 
joeblurton said:
I was just amazed at the ground provided in a Sq D MCC. This is 250 amps 480 v. supplying a VFD. The screw holding the lug is an 8-32! The connection surface is painted, and a galvanized screw with copper lug are dis-similar. This is brand new switchgear. I know a ground fault will "spit" on this connection. Also the gear itself had many sharp edges and bending the 250 kcmil was impossible, as the auxiliary gutter was about 3" wide.
I ream conduit, scrape paint on grounds, with diligence, Why do manufactures get away with this shabby product?
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'Cause they probably paid UL around a quarter million for testing and it is listed.:D

See 90.7
 
Because there is no rule for them that says a tec screw can't be used as the current carrying connection between the lug and the enclosure.


I have yet to find electrcal equipment painted with conductive paint.
 
iwire said:
Because there is no rule for them that says a tec screw can't be used as the current carrying connection between the lug and the enclosure.


I have yet to find electrcal equipment painted with conductive paint.
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They do make a metallic paint, but as you said, I don't think they paint electrical boxes with it. It is used on fiberglass satellite dishes. Fiberglass just doesn't reflect those microwaves very well! :)
 
hillbilly1 said:
They do make a metallic paint, but as you said, I don't think they paint electrical boxes with it. It is used on fiberglass satellite dishes. Fiberglass just doesn't reflect those microwaves very well! :)
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I know they make it, I just don't think they want, or need to spend the money for it.:)
 
joeblurton said:
I was just amazed at the ground provided in a Sq D MCC. This is 250 amps 480 v. supplying a VFD. The screw holding the lug is an 8-32! The connection surface is painted, and a galvanized screw with copper lug are dis-similar. This is brand new switchgear. I know a ground fault will "spit" on this connection. Also the gear itself had many sharp edges and bending the 250 kcmil was impossible, as the auxiliary gutter was about 3" wide.
I ream conduit, scrape paint on grounds, with diligence, Why do manufactures get away with this shabby product?
View attachment 7830
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They use self threading screws in punched holes, so paint is removed as its threaded in, but 8-32 is pretty small. I think the smallest I have seen them use is a 10-32. Price is a big driver, that's why Square D came out with the Homeline series and Eaton the BR series to compete with GE, Siemens, and others.
 
Consider that the bonding screw of many loadcenters is a 10/32 that cuts the paint as it is threaded in,it is about 1.5 inches long and all tha fault curent of a service could flow through it.
 
Besides a possibly too-high contact resistance, what determines if a grounding screw is too small?

What screw size is used for 10 kA? 100 kA?
 
G._S._Ohm said:
Besides a possibly too-high contact resistance, what determines if a grounding screw is too small?

What screw size is used for 10 kA? 100 kA?
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Well as I mentioned, about 1.25" of a 10/32 screw is used the the factory bonding jumper on load centers rated to at least 10kA.

In these loadcenters there is no direct 'terminal to enclosure' contact, the full fault current must go through the screw itself.
 
As it happens my own panel has that design.

This is a GE and the neutral bar is on plastic isolators holding it about .75 of an inch off of the enclosure. The green bonding screw links them via the screw itself.

Bonding Screw 12-16-2012 11-39-59 AM.jpg
 
iwire said:
Consider that the bonding screw of many loadcenters is a 10/32 that cuts the paint as it is threaded in,it is about 1.5 inches long and all tha fault curent of a service could flow through it.
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Not only does the screw cut the paint when it is threaded in, but the hole has been machined in a way that there is more thickness at that point of the cabinet also - making more contact area between the screw and cabinet than if it were done in just any random spot of the cabinet.

I mostly install Square D panels, and they usually have 1/4 inch diameter bonding screws. Some older panels had 10-32 - but generally was 125 amp and less models. I can recall only 10-32 screws on occasion for a bonding screw for other brands in 200 amp series panels though.
 
jumper said:
I cannot find anywhere where it states a minimum screw size for attachment and/or bonding.

It seems that a correctly installed 8/32 would be okay, but 10/32 is the most common I have seen.
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I don't believe there is anywhere in the nec that tells us what size other than the listing for the panel. If the panel is listed with 8-32 screws well I guess that's it. :D
 
Just with a pre-coffee spin through the NEC:

250.12 Clean Surfaces. Nonconductive coatings (such as
paint, lacquer, and enamel) on equipment to be grounded
shall be removed from threads and other contact surfaces
to ensure good electrical continuity or be connected by
means of fittings designed so as to make such removal
unnecessary.
[emphasis added]

I assume that the UL listing gets around sizing the MBJ by 250.28(D)(1), since a 10-32 steel screw can't be as conductive as #8 copper. (Didn't look up the cross sections, it's too early for that, probably doesn't matter, in reality.)
 
zbang said:
Just with a pre-coffee spin through the NEC:

250.12 Clean Surfaces. Nonconductive coatings (such as
paint, lacquer, and enamel) on equipment to be grounded
shall be removed from threads and other contact surfaces
to ensure good electrical continuity or be connected by
means of fittings designed so as to make such removal
unnecessary.
[emphasis added]

I assume that the UL listing gets around sizing the MBJ by 250.28(D)(1), since a 10-32 steel screw can't be as conductive as #8 copper. (Didn't look up the cross sections, it's too early for that, probably doesn't matter, in reality.)
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When you are attaching something other than what has been tested and listed (which is what you have with manufacturers bonding screw they send with the panel) you do have to comply with this section. You remove non conductive coatings and install a lug, terminal bar, or other device in direct contact with the surface you cleaned. It doesn't necessarily take very large screw to maintain contact between these surfaces and the screw is not carrying all the current.

If you are installing a bonding screw, even the screws that hold equipment grounding bars in place in a panel, into a hole the manufacturer designed and had tested for use with a specific screw, then you do not have to worry about 250.12 for that particular instance, you are using equipment tested and listed for the purpose.
 
joeblurton said:
I was just amazed at the ground provided in a Sq D MCC. This is 250 amps 480 v. supplying a VFD. The screw holding the lug is an 8-32! The connection surface is painted, and a galvanized screw with copper lug are dis-similar. This is brand new switchgear. I know a ground fault will "spit" on this connection. Also the gear itself had many sharp edges and bending the 250 kcmil was impossible, as the auxiliary gutter was about 3" wide.
I ream conduit, scrape paint on grounds, with diligence, Why do manufactures get away with this shabby product?
View attachment 7830
Click to expand...

Do you see the UL sticker just to the right? Better ask the question from them. They require test data for ground fault. (Installation under controlled, manufacturing conditions versus field will produce more consistent results.)
 
We always take our EGC to the supplied ground bar at the bottom of the enclosure, not to the bucket. It is a more secure connection, although I'm sure the bucket EGC location is rated.:angel:
 
With high current and relatively low voltages (like the cranking circuit in a car or 50w/12vac cabinet lights) the important thing is low contact impedance which means high contact pressure which means larger bolts.

If a #10 screw works for 10kA, anybody have screw sizes for 20, 50 and 100 kA? It probably has to do with the tensile strength of the screw material. And they probably want fine threads so it's less likely to back out with temp. changes.
 
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NEC 250.8 allows threads into sheet metal for bonding. It requires at least 2 threads into the metal if no nut is used. If the metal is 1/16th of an inch, then an 8-32 or 10-32 screw would work (or #6 or #4 or smaller). To go larger would require either a nut or thicker metal since both #12 and 1/4" fine screws are 28 thread per inch.

Square D seems to have standardized on 8-32 screws for its residential grounding bars. I think most others are 10-32. I tried to put a small square D grounding bar in a 4 square box and its screw didn't fit the 10-32 hole...
 
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