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Old October 8th, 2017, 04:33 PM   #1
ryangriggs
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Grounding and aluminum subpanel

After reading this thread, I'm questioning the propriety of using aluminum subpanels with steel DIN rails and clip-on grounding terminal blocks.

I'm assuming if all grounding terminal blocks are bonded together with a wire, eliminating dependence on steel-to-aluminum connections to the subpanel as a grounding backplane, the grounding issue should be mitigated.

Am I correct in this assumption, and also can you think of other issues or problems associated with an aluminum subpanel that would be mitigated by using mild steel?

I was favoring aluminum because it's lighter and thicker, and thus is conducive to drilling and tapping for component placement.

Thanks for any insight.
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Old October 8th, 2017, 08:46 PM   #2
sparkie
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Quote:
Originally Posted by ryangriggs View Post
After reading this thread, I'm questioning the propriety of using aluminum subpanels with steel DIN rails and clip-on grounding terminal blocks.

I'm assuming if all grounding terminal blocks are bonded together with a wire, eliminating dependence on steel-to-aluminum connections to the subpanel as a grounding backplane, the grounding issue should be mitigated.

Am I correct in this assumption, and also can you think of other issues or problems associated with an aluminum subpanel that would be mitigated by using mild steel?

I was favoring aluminum because it's lighter and thicker, and thus is conducive to drilling and tapping for component placement.

Thanks for any insight.
NEC 250.96:

Summary (I'm not typing it all out): Any metal raceways or enclosures that are intended to be a part of the equipment grounding system shall be connected where necessary to ensure electrical continuity and capacity to carry potential fault current. Threads and all contact surfaces must have any nonconductive paint or similar coating removed.

NEC 250.8:

Equipment grounding conductors shall be connected in one of the following means:

Essentially a listed busbar, weld, machine screws that engage not less than 2 threads or are secured with a nut, thread-forming screw or are manufacture or UL listed for this specific purpose.

250.28(B)
Where the bonding jumper is of screw type only, the screw shall have a green finish.

Lots of other running around in that section, but that is the gist. The screw to connect the DIN rail to the panel must be UL listed as an appropriate ground screw to be considered a bond. Now, there is a loophole here:

If your self-tapper or machine screw has at least two threads dug into the middle, AND you clean the area of the panel under the DIN rail of all non-conductive coatings AND you paint the head of it green AND the self-tapper is made of a corrosion resistant metal, then you would not need a specific grounding screw, which usually isn't the case because of the pitch on self-tapper threads and the thickness of most cabinet walls.
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Old October 8th, 2017, 11:45 PM   #3
James Mcquade
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If you are using an aluminum subpanel, is it ul listed for that purpose?

I cannot cite book chapter and verse, but any component in an electrical installation must be listed for the intended purpose.

simply putting an aluminum backpanel in an electrical enclosure and mounting components on it just because its lighter and easier to work with won't cut it
unless the aluminum subpanel is ul listed for the job.

regards,
james
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Old October 9th, 2017, 12:59 AM   #4
GaryS
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A couple of things.
Aluminum din rail is NOT UL listed as a grounding means, the steel ones are listed if installed properly.
The grounding (Bonding) of a din rail is not through the mounting screws. I don’t know where people are getting that idea bit its’ wrong. The grounding is the clamp of the grounding terminal block. That clamp is steel and cannot be used as an electrical connection to an aluminum din rail. At least one of the terminal blocks must be electrically connected to the system ground. (Green Wire). The steel din rail can be used to connect all the clamp type grounding terminals together to the ground. If you are not using the clamp type terminals then an external jumper must be used.
Steel to Aluminum cannot ever be used as an electrical connection without anticorrosion compound that’s not going to happen with a din rail.
As for the use of self tapping screws used for grounding. I don’t know of anybody that would use them for that or any inspector that would approve them. In all cases the screw itself is not a grounding mechanism. 30 years ago we would use self tapping screws to mount hardware but whenever we grounded something we had to use a machine screw. At that time we didn’t need to use a green screw but the code has changed since them now all grounding must be readily identified as a ground ( color Green) and that’s a good thing. Way to many unknowns with self tapping screws be considered for use as ground screw. They could strip out when installed, go through the panel and toughen up on something behind.
To be grounded the code requires that the point of contact be first the cleaned of all paint. That alone would require that you sand off the paint where you are mounting the din rail that’s never going to happen. Grounding the panel itself the preferred method is the welded stud that most panel builders install when they build it.
Tapping a stud in a panel may get you by if your are careful but a better way would be to install a bolt through a hole in the panel after properly cleaning the area.
The screw is just a means of binding the connection together you still must us lugs of some type to make the connection from the wire to the panel must be an approved type, a wire wrapped around a screw would never be excepted as grounded. The green color of the screw just tells future workers that that screw is used in the panel as a mechanical connection in the grounding system and removing or loosening that screw could weaken the integrality of the overall ground system.
Grounding in a panel is a big deal and needs to be taken seriously. Everything that is conductive ( That is not live) must be connected to the system ground. Panel doors must have a jumper around the hinge
( the hinge is considered not conductive) the mounting panel inside must be electrically connected to the main panel. On these new modular panels that are becoming popular each section must be electrically connect back to the single system ground point.
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