Poor Grounding by Manufacturer

[petersonra]

I have never seen a ground connection like that in an MCC bucket from the manufacturer. But we rarely use Square D.

In any case, if UL tested it, I would buy into the idea that it works.

 

[kwired]

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.

There is also a difference between a #10 screw carrying 10kA for a few cycles to open an overcurrent device, and continuously carrying 10kA.

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...

When #12 or 1/4" screws are used, they usually have thicker panel wall at the point where the screw is installed. Besides, if they have it listed for use with a particular screw, you do not have to worry if proper number of threads are engaged, the listing says it has been determined adequate.

The Square D grounding bars do use 8-32 screws. they typically also use at least two mounting screws in many cases, which may be better than a single 1/4 inch screw.

 

[jap]

i dont have an issue with the bonding screw threading into the enclosure regardless of its size.

My issue is with the pressure connection that is made on the underside of the screw head to the neutral bar.

To me, thats where the weak link lies.

JMO.

Jap.

 

[pete m.]

250.4(A)(5) Effective Ground-Fault Current Path. Electrical equip-
ment and wiring and other electrically conductive material
likely to become energized shall be installed in a manner that
creates a low-impedance circuit facilitating the operation of
the overcurrent device or ground detector for high-impedance
grounded systems. It shall be capable of safely carrying the
maximum ground-fault current likely to be imposed on it from
any point on the wiring system where a ground fault may
occur to the electrical supply source. The earth shall not be
considered as an effective ground-fault current path.


The verbiage above seems to put the responsibility of an effective fault path onto the installer (not related to Listed equipment).

I would probably make an argument if I came upon a 6/32 being utilized to fasten a lug to a cabinet for the purposes of a bonding jumper on a larger feeder.

Pete

 

[iwire]

The verbiage above seems to put the responsibility of an effective fault path onto the installer (not related to Listed equipment).

I would probably make an argument if I came upon a 6/32 being utilized to fasten a lug to a cabinet for the purposes of a bonding jumper on a larger feeder.

Pete

Glad I live in MA where 90.4 has been amended to require inspectors to accept listed equipment instaled per the listing.

 

[pete m.]

Glad I live in MA where 90.4 has been amended to require inspectors to accept listed equipment instaled per the listing.

I was attempting to make an example of a "field installation".

I do not typically question Listed and Labeled equipment unless I see something obviously wrong.

Pete

 

[kwired]

i dont have an issue with the bonding screw threading into the enclosure regardless of its size.

My issue is with the pressure connection that is made on the underside of the screw head to the neutral bar.

To me, thats where the weak link lies.

JMO.

Jap.

That can be a problem.

I just had a call the other day, neutral bar in a panel was hot. Wasn't the box bonding screw but a screw that tied one segment of neutral bar to another segment apparently worked loose enough and of course we had both a hot neutral bus as well as voltage problems and damaged equipment here and there because of open neutral issues.