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Old December 14th, 2015, 11:14 AM   #1
rpoet
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UL508A panel grounding question

All,
I was recently told by a colleague who works for a UL508A panel shop that DIN rail can't be considered bonded to the ground plane because the screws are "mounting screws" and not "bonding screws." His interpretation is that one must have at least (2) screws for physical mounting, and (1) screw, marked green, for bonding. The Bonding screw can be right next to the "mounting screw, but can't be used for mounting (WHAT???).

He also stated that the grounding-style terminal blocks that clamp to the DIN rail and form an electrical connection are not considered a bonding means, and that a jumper needs to be run from the block to a ground screw on the back plane.

Further, even when using star or split lock washers under the manufacturer-supplied nuts, the subpanel mounting studs can't be considered a bonding means because they're a "mechanical mounting means."

This all seams a bit ridiculous. Why would the electrons care? Is this a misinterpretation, or has UL really gone off the deep end? Admittedly, I don't have a copy of UL508A in front of me right now, but when reading over it previously, I remember nothing like this anywhere.


Thanks,

-rpoet
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Old December 14th, 2015, 12:46 PM   #2
keithkyll
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I think your colleague and/or his shop has gone off the deep end. Years ago, I had a panel certified onsite by UL. He was one of their top guys. He used common sense when evaluating our panel. Their intent is to make sure no one takes low cost shortcuts that compromise safety.

I use a white painted steel panel. I use a sanding disc on the corners for the mounting studs. The enclosure OEM supplies a star washer for one of the studs, and a ground sticker to indicate it.
I grind the paint off an area where the din rail screws bond the rail to the panel, and use external tooth stainless lockwashers with 8-32 stainless to bond them.
Use DIN rail grounding blocks.
Sand paint off 1 or all 4 legs of control transformer, and surface of panel where it bonds. Use at least one external tooth lockwasher to bond. This is in addition to always grounding the secondary.

Use common sense, and grind paint off the areas where you want to make it obvious that you are using bonding as a primary or backup ground.

Last edited by keithkyll; December 14th, 2015 at 12:48 PM. Reason: Clarification
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Old December 14th, 2015, 12:46 PM   #3
Bullzi
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I have never read anything that says anything like that either. The basic rule is that a Ground terminal must have continuity with all the metal parts of the enclosure (see the snip from the 508 manual). The operative word is "continuity" in that phrase. I remember when I took the 508 class they talked about the ability to "prove" that continuity. If it exist now can you guarantee that it will still exist when the panel gets to the job site?

To make sure that we don't run into any grounding issues, I always put a Ground Bar at the bottom of the panel. I will normally run one wire from one of the Ground Blocks to that bar. That way I make sure that the DIN rail the Ground Blocks are on is 100% grounded.

I will also run wire jumpers between the door, enclosure and back panel to the ground bar. In any situations where I have paint in the way I will ground off the paint were there is metal to metal contact.

As usual there are a bunch of marking requirements for grounding points. Green screws or green terminal blocks are a normal way to "mark" a grounding point.
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Old December 14th, 2015, 12:56 PM   #4
GaryS
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That's my understanding a grounding screw must only be used for grounding not mounting or anything else for that matter. You could use it if you grounded it through a separate ground wire to the panel ground. the Green color is only to identify that terminal as ground so you don't use it for anything else.

As for the DIN rail I found out a few years ago that the aluminum rail is not UL listed as a grounding buss while the steel is. I think it has to do with the clamping mechanism is steel and you can't connect steel to aluminum. but both are UL rated for mounting.
I never did find anybody that was able to explain it better.

Steel to aluminum is a very poor connection

Maybe others can explain it better

Last edited by GaryS; December 14th, 2015 at 12:59 PM.
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Old December 14th, 2015, 02:02 PM   #5
rpoet
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Quote:
Originally Posted by GaryS View Post
That's my understanding a grounding screw must only be used for grounding not mounting or anything else for that matter. You could use it if you grounded it through a separate ground wire to the panel ground. the Green color is only to identify that terminal as ground so you don't use it for anything else.

As for the DIN rail I found out a few years ago that the aluminum rail is not UL listed as a grounding buss while the steel is. I think it has to do with the clamping mechanism is steel and you can't connect steel to aluminum. but both are UL rated for mounting.
I never did find anybody that was able to explain it better.

Steel to aluminum is a very poor connection

Maybe others can explain it better
"... a grounding screw must only be used for grounding not mounting or anything else for that matter."

If I have three screws holding a piece of DIN rail to a backplane, why do I need a fourth for grounding? Can I arbitrarily state that one of the three is not there for mounting, only for grounding? Not to be pedantic, but which one? Can I pick? I see requirements like this as doing little to enhance safety.

What about the subpanel mounting studs? Can I use them for bonding the subpanel to the enclosure, presuming I can use the grounding terminal blocks to bond the DIN rail and subpanel? What if I'm using an enclosure in an environment where drilling holes in the cover and/or sides for ground screws violates the NEMA listing? I've seen a UL inspector fail a NEMA 4X panel because the 4-40 screws used to attach the data placard to the outside of the cover weren't listed for the purpose.

GaryS,
I only use steel DIN rail, mostly because it's cheaper. I don't really see a need for aluminum. Why would one use it over steel?


-rpoet
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Old December 14th, 2015, 03:37 PM   #6
seppoalanen
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Old December 14th, 2015, 04:15 PM   #7
Codenamecody
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I'm not sure about UL508A, but I've been told that it is good practice to not mix studs used for mechanical mounting and studs for grounding. The reason being is that if someone wants to remove a device from a panel, they shouldn't have to unhook the ground connection to do so.
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Old December 14th, 2015, 04:25 PM   #8
ASF
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Quote:
Originally Posted by rpoet
...DIN rail can't be considered bonded to the ground plane because the screws are "mounting screws" and not "bonding screws."...

...the grounding-style terminal blocks that clamp to the DIN rail and form an electrical connection are not considered a bonding means, and that a jumper needs to be run from the block to a ground screw on the back plane.
Here in Australia, that's a hard and fast requirement also. Something is either a mounting fixing, or an earthing fixing; and if it's an earthing fixing it must be identified as such. Standard practice for my panel shop is:
- Earth bar on each gear tray
- Earth wire from gear tray to panel frame
- Earth wire from panel frame to each door (whether there's equipment on it or not)
- Earth wire between each earth bar (in the case of multiple gear trays)

Then, if we have earth terminals on a DIN rail for a row of motor terminals, we run an earth wire sized to correspond to the largest motor directly from the earth bar to it's earth terminal. This now means that the whole DIN rail is earthed to the level required by the motors, so all the other earth terminals can just use their clamp mechanism onto the earthed DIN rail. We don't need to run a separate earth wire to each terminal.

Since it's always been this way, it really doesn't seem that strange to me, although I can see how it might if you're not used to IEC. But really, for the sake of full compliance - a handful of extra earth wires in a panel is about 0.1% of the effort that goes into building the panel.
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Old December 14th, 2015, 08:44 PM   #9
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As in all cases, the final arbitrator of what will pass or fail is with the inspectors themselves.

I was on a job last year and saw panel get rejected because they had 2 ground bars mounted on the same backplane right next to each other end for end and the panel guy forgot to install the copper jumper between them. Then I have seen others passed without any ground buss at all. It depends on their mood at the time.

As for grounding the DIN rail I don’t think you can directly. The screw used would have to connected only to the rail its self not tapped in the backplane. If you ran it through the DIN rail tapped into the backplane it would be mounting the DIN rail and not considered grounding. I believe you could use a clamping type ground terminal block clamped to the rail then use a green ground wire connected to the panel ground buss. Each grounding terminal block used should have a separate green wire connected to the panel ground buss. Each piece of equipment mounted in the panel is required to have a ground wire connected directly to the panel ground buss (No daisy chaining) We know that that is just not going to happen but that would be the best practice.

As for the use of aluminum DIN rail. I believe that most panel builders today are use the extruded aluminum rail so they can get the standoff and the angled mounting they want. I had at one time use the aluminum rail then switched back to steel when I became aware of the of the grounding issue.
Before that I started to have a local metal fabrication shop make me custom standoff brackets many years ago.
They became the standard for me. But if the end client wants aluminum rail that’s what they get I just don't use then for grounding.
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Old December 14th, 2015, 11:24 PM   #10
keithkyll
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DIN rail blog. Aluminum verses steel, and other info.
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Old December 15th, 2015, 03:32 AM   #11
BryanG
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Quote:
I grind the paint off an area where the din rail screws bond the rail to the panel, and use external tooth stainless lockwashers with 8-32 stainless to bond them.
Don't you have to be careful putting stainless steel and steel touching each other. I think the normal steel could allow the stainless steel to rust and cause a bad connection. I left a steel nail on my stainless steel sink and got a rust stain that I am still getting earache about.
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Old December 15th, 2015, 09:19 AM   #12
rpoet
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Quote:
Originally Posted by GaryS View Post
As for grounding the DIN rail I don’t think you can directly. The screw used would have to connected only to the rail its self not tapped in the backplane. If you ran it through the DIN rail tapped into the backplane it would be mounting the DIN rail and not considered grounding. I believe you could use a clamping type ground terminal block clamped to the rail then use a green ground wire connected to the panel ground buss. Each grounding terminal block used should have a separate green wire connected to the panel ground buss. Each piece of equipment mounted in the panel is required to have a ground wire connected directly to the panel ground buss (No daisy chaining) We know that that is just not going to happen but that would be the best practice.
To take this example to an extreme, say I have a 2' piece of steel DIN rail with slotted holes. I drill and tap a screw into the backplane at each and every slotted hole. I make one of those screws green. How could one possibly think the ground screw is physically mounting anything? I realize this is ridiculous, but how many "mounting" screws do I need before I can call one a "bonding" screw?

Also, how does one run a dedicated bonding wire to a piece of DIN rail, independent of either a mounting screw or grounding TB? Does this entire discussion pretty much invalidate the use of grounding TB's in enclosures that have to be UL508A listed?

A previous thread suggested that one shouldn't have to disconnect ground in order to remove a component. I completely agree when it comes to things like daisy chaining, etc. That said, with the grounding terminal block example I gave earlier, how could I possibly remove the DIN rail it's clamped to? The TB's are covering the screws.

As far as using the subpanel mounting studs for bonding, if I have to remove the subpanel, every wire in the panel is getting disconnected too, green wire or not.

What about my earlier question; does drilling a hole for a "required" bonding jumper invalidate an enclosure's NEMA rating, like many suggest it does? Not all enclosures have those handy threaded studs on the covers.

I agree that common sense should prevail, but we all know that UL and common sense don't always cooperate. I'll continue to use the grounding TB's and DIN rail for grounding / bonding. If someone says boo, I'll have to add that useless green wire too.


-rpoet

Last edited by rpoet; December 15th, 2015 at 09:30 AM.
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Old December 15th, 2015, 10:38 AM   #13
GaryS
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your example of a 2' DIN rail none of the tapped screws could be considered grounding or bonding screws. they would all be mounting screws. For grounding or bonding the screw can only be attached to the DIN rail not through it. just using a green screw doesn't make it a ground screw.

Subpanel mounting studs are not and can not be used for grounding or bonding. Most panels provide a welded grounding stud on the door and panels the backplane also have a separate bonding hole or stud usually they provide a bonding kit with the panels.
as I said before it's up to the inspector just what they will pass. I have seen panels fail just because one jumper was missed.
and do forget that ground or bond wires must be bare copper, green or clearly marked on both ends with green tape to clearly define it as earth potential. only recently have they allowed the green / yellow wire to comply with UE standards
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Old December 15th, 2015, 02:25 PM   #14
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UL508A defers the bonding connection standards to UL467. There is no mention of whether or not the bonding termination is also a mounting screw of not.

UL467 requires that the bonding resistance be less than 0.03 ohm on a circuit rated up to 30A. it might be that if you use a mounting screw on a small device or a DIN rail mounting screw, then the bonding ground terminal blocks, the combination of all of those connections, and the possibility that a mounting screw might become loose due to weight and vibration of the devices, RESULTS in that bonding resistance being too high.

Edit: not worded very well...
It might be that EXPERIENCE has taught people that using the mounting screws as the bonding point RESULTS in not meeting the low resistance requirements. UL doesn't dictate how you must do it or not do it, only that it ends up that way.

Last edited by jraef; December 15th, 2015 at 03:23 PM.
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Old December 15th, 2015, 03:27 PM   #15
rpoet
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Quote:
Originally Posted by jraef View Post
UL508A defers the bonding connection standards to UL467. There is no mention of whether or not the bonding termination is also a mounting screw of not.

UL467 requires that the bonding resistance be less than 0.03 ohm on a circuit rated up to 30A. it might be that if you use a mounting screw on a small device or a DIN rail mounting screw, then the bonding ground terminal blocks, the combination of all of those connections, and the possibility that a mounting screw might become loose due to weight and vibration of the devices, RESULTS in that bonding resistance being too high.
Now that makes more sense. In my case, my typical panels rarely exceed a 20A feed, so I'm good. I always meter the bonding from the power input tail to various points on/in the cabinet and components. I've never had my meter read anything but 'LOW' and my meter reliably reads down to 0.1 ohm.

DIN rail grounding blocks for large conductors do concern me a little. I've seen them offered for wires up to 1/O wire; I wouldn't want to rely on the DIN rail to carry those kinds of fault currents; DIN rail as a fusible link is probably a bad idea

Thanks for shedding some light on this, jaref.


-rpoet
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