Ground 120v receptacle bonded to welding table?

[joshuabardwell]

I've installed a box for a 120v receptacle on my welding table. It's metal, so I just tack welded it to the table leg. Normally, when doing work like this, I would bond the box to ground, so that the box wouldn't become energized if a cable came loose or whatever. I'm not sure if there will be any unusual implications of the table being part of the welding circuit, though. I have done some web searching on this topic, and I can find a lot of discussions of whether a welding table needs to be grounded, but none of them seem to address the issue of whether the presence of 120v receptacles changes the equation.

In retrospect, maybe I should have gone with a GFCI receptacle... but... but... It would have been like, an extra $11!!! :hissyfit:

 

[CNC Dan]

Unless there is something wrong with your welder, there should be no problem grounding the bench.

The primary and secondary windings in the welder are not connected electrically, just magneticly.
So there shouldn't be any way for welding current to flow through the ground wire.

 

[Rock knocker]

A long conversation regarding the pro's and potential cons:
http://www.tractorbynet.com/forums/welding/251190-grounding-welding-table.html

 

[joshuabardwell]

A long conversation regarding the pro's and potential cons:
http://www.tractorbynet.com/forums/welding/251190-grounding-welding-table.html

Thanks for that. Not sure why I didn't turn that up in my original searching, but it is exactly what I'm looking for. To sum up:

1. The welder's "ground clamp" is really better referred to as a "work clamp". It is unrelated to earth ground, as used to protect against fault in household wiring.

2. If the welding table is not grounded, there is potential for a faulty 120v wire or tool to cause the table to become energized, creating a shock hazard.

3. If the welding table is grounded, and the welder's work clamp is not connected to the table, and the electrode comes into electrical contact with the table, and the table is in contact with a part of the building that is earth-grounded (such as a metal I-beam in a warehouse), there is potential for welding current to burn out wiring in the building.

Is all of this correct? If so, it seems that grounding the table is the right choice, since point 3 seems pretty far-fetched, whereas point 2 seems relatively likely (not very likely, but relative to point 3). But even better, it seems like not grounding the table and putting a GFI receptacle in would address both problems at the same time--is that right? The receptacle would derive earth ground from the house via the cord, and if there was a short, either in the receptacle wiring or in the tool, the receptacle would cut out. Meanwhile, the lack of bonding between the table and the receptacle would prevent any odd occurrences with the welding current.

Is that all right?

On a semi-related note, I still can't figure out why, if you were to bond the table to earth ground, and then touch the electrode to it, you wouldn't get an arc, or at the very least, burn out some ground wires. It seems like there must be current flow in that situation. This is why I'm not an electrician! But if anybody cares to explain it to me, I'm all ears.

 

[Rock knocker]

1. The welder's "ground clamp" is really better referred to as a "work clamp". It is unrelated to earth ground, as used to protect against fault in household wiring.
Very optional. I bet most professional refer to it as ground
2. If the welding table is not grounded, there is potential for a faulty 120v wire or tool to cause the table to become energized, creating a shock hazard.
Correct
3. If the welding table is grounded, and the welder's work clamp is not connected to the table, and the electrode comes into electrical contact with the table, and the table is in contact with a part of the building that is earth-grounded (such as a metal I-beam in a warehouse), there is potential for welding current to burn out wiring in the building.
Correct. Your scererio is a little wachy, but the basics are correct

since point 3 seems pretty far-fetched,

Slightly more unlikely than some other possible failure, but not really far fetched. You have to remember that electrical safety by and large is not based on the possibility of a single failure and it's results, but on the cascade effects of multiple failures in a system.

 

[joshuabardwell]

Do you agree with my conclusion that not bonding plus GFI is the right choice?

 

[Rock knocker]

Is the table mobile of stationary? Would the GFCI protect the recepticle at the table, or the circuit and cord supplying the table.

I would think the best would be an isolated box on the table, supplied from a GFCI receticle or circuit breaker in/on the house.

 

[joshuabardwell]

The table is mobile. Usually, it will be plugged into a GFI outlet in the outside of the house, but I wouldn't want to assume that. I would expect that a GFI outlet anywhere in the circuit would protect from a fault anywhere in the circuit. Is that not true?

 

[joshuabardwell]

Well, folks, I have to tell you that this whole question turned out to be moot. I installed the receptacles today and even though I didn't run a separate ground wire to the box, the table is bonded to the receptacles' ground. The bond seems to be occurring through the receptacles' mounting plate. When I wired up my 240v receptacle for my welder, the box had a separate grounding screw, but this one doesn't seem to have, or require such a thing.

I did go back and pick up a GFI receptacle for extra protection just in case the table is ever plugged into a non-GFI receptacle. And to answer the question about what the GFI receptacle protects, now that I have read the instructions for wiring one up, it appears that the receptacle protects everything on its own "load" side, but not its "line" side. So the cable feeding the table is not protected, if it were to fray or something, but anything plugged into the table's outlets is protected.