Industrial Control Panel - Motor Grounding on DIN rail

[alsiuw928als93la]

A company we have contracted is in the process of building a 100 amp motor starter control panel for a heavy industrial plant. The motors are 3 phase 480 vac 5 HP with 1-2 higher HP (20-40 HP). The other components in the panel are 24 VDC PLC I/O for the motors to connect motor current transmitters, motor contactor coils and auxiliary contacts, overload relays, MCP/MPCB auxiliary contacts, and misc cabinet connections.

There are some questions on grounding the motors. In the current panel, the motor grounds are supposed to go to a ground terminal block (that connects to the DIN rail). The ground terminal block is on aluminum DIN rail and it is screwed into the white painted back panel. I do not believe any paint has been etched off on the DIN rail. There is no extra screw dedicated for earth on this DIN rail. There is no wire bonding of the DIN rail to the AC ground. The AC ground is connected at another point in the panel at a dedicated ground point.

The ground for the motors is therefore expected to travel through the ground terminal block on the DIN rail, through the screw on the DIN rail, through the back panel to the AC ground. Aesthetically it is a nice solution but I'm not sure on functionality or side effects. I have tried to dig up past articles, but a lot of grounding questions were tied to lower voltage/lower amperage panels. It was harder to find this scenario with motor panels.

a) Would you find this acceptable?
I have read a forum post that suggested aluminum wasn't acceptable, but steel was. I had a hard time trying to find documentation on this. I also read where an inspector required a separate screw for the ground on the DIN rail as well as requiring a separate bonding cable from the DIN rail to the primary AC ground screw.

b) If the DIN rail/screw/back panel ground is acceptable for 480 vac, can the 24 VDC components be ruined because of this? Let's say there is a fault, could I make a bad thing even worse by frying all of the 24 VDC? What if there wasn't a large fault but a small fault where there is a small amount of current.

c) Last of all, is there a common convention? For example, even if the above is acceptable, would you do something else and what would that be?

Thank you!

 

[tom baker]

This is probably not an NEC issue. If the control panel is UL508A industrial motor control listed, then it’s per UL508A and the mfg of the DIN rail and terminal blocks.
All of the bonding in the hundreds of control panels I have installed was via DIN rail grounding terminals, much as you describe.
If it’s a listed control panel the AHJ can not request additional grounding.

 

[alsiuw928als93la]

This is probably not an NEC issue. If the control panel is UL508A industrial motor control listed, then it’s per UL508A and the mfg of the DIN rail and terminal blocks.
All of the bonding in the hundreds of control panels I have installed was via DIN rail grounding terminals, much as you describe.
If it’s a listed control panel the AHJ can not request additional grounding.

Thanks for the note. Panel won't be listed but will be built by a UL panel builder. We are not in a location that is inspected (industrial). There are many ways to do things even within existing regulations. I'm trying to also find out what is convention/best practice. From your note, sounds like ground terminal blocks are very common for motors.

 

[tom baker]

I had a non UL motor control panel, we had it sent back to mfg and redone as a listed control panel.
And I have had issues with UL508 listed control panels, the quality depends upon the panel builder.
One option is to find control specs from a public project and use those.

 

[don_resqcapt19]

The ones I have worked on that used DIN rail grounding blocks, always had the feeder EGC directly connected to one of the grounding blocks and then the blocks were jumpered together using factory jumpers and all of the field EGCs connected to the other terminal on the grounding blocks.

 

[tom baker]

The ones I have worked on that used DIN rail grounding blocks, always had the feeder EGC directly connected to one of the grounding blocks and then the blocks were jumpered together using factory jumpers and all of the field EGCs connected to the other terminal on the grounding blocks.

Yes now that you mention that is how mine were for power. But 4-20 mA Shields’s just used a grounding din rail terminal as this is just for noise

 

[petersonra]

These things are listed for grounding use so I don't see any problem with having the green blocks snap on to the aluminum or metal rail that is screwed to the subpanel. There is a screw that you tighten on the ground blocks that makes it very secure to the rail.

If it makes you feel better you can improve the grounding as much as you want. Personally with bigger motors I tend to buy either the three or five pole connectors from lugs direct and bolt them directly to the panel and have them bring the motor ground wires directly there. But for small motors like this I think the din rail mounted blocks are just fine.

 

[SSDriver]

That is perfectly fine. That's what those grounding blocks are for. No paint needs to be removed from the back panel as long as the screws holding the din rail are machine threads and not self tappers. One of the great things about a control panel is the ability to use chasis grounding.

 

[alsiuw928als93la]

These things are listed for grounding use so I don't see any problem with having the green blocks snap on to the aluminum or metal rail that is screwed to the subpanel. There is a screw that you tighten on the ground blocks that makes it very secure to the rail.

If it makes you feel better you can improve the grounding as much as you want. Personally with bigger motors I tend to buy either the three or five pole connectors from lugs direct and bolt them directly to the panel and have them bring the motor ground wires directly there. But for small motors like this I think the din rail mounted blocks are just fine.

That is perfectly fine. That's what those grounding blocks are for. No paint needs to be removed from the back panel as long as the screws holding the din rail are machine threads and not self tappers. One of the great things about a control panel is the ability to use chasis grounding.

In a fault scenario at the motor, do you think the current going through the back panel could affect the low voltage components that also ground to the panel? Or is that a minimal risk?

 

[petersonra]

In a fault scenario at the motor, do you think the current going through the back panel could affect the low voltage components that also ground to the panel? Or is that a minimal risk?

I don't think it's much of a risk. How is it any different than if the motors are brought back to a copper ground bar that is bolted to the back plane. They are all connected to the back plane one way or the other.

 

[SSDriver]

I've worked on thousands of control panels over the years. Fair amount of them have the grounding through din rail TBs. When it's a motor failure they have been isolated to that motors equipent(motor starter, vfd, contactors, fuses , etc). I've worked and built many panels with multiple drives and the other drives never get damaged unless it's a fault to the incoming power (i.e. surge, etc).