Grounding Welding Table - ????

[oldyellr]

The "ground" of the welding side of the circuit is distinctly different that the ground of the line voltage circuit

Okay, I'm learning something here. I just went out to the garage and checked my cheapie AC stick welder and sure enough, there's no continuity between the ground prong of the plug and the ground clamp. That puzzles me. Is there a specific reason for this? I can see where would be mandatory for an AC/DC welder, but I can't see a reason for it for AC.

Also, as Steave mentions, the welding and electrical circuit grounds are connected in industrial settings. Is there anything anywhere that says an AC welding table should not be connected to the building electrical ground and why?

[Steave]

[QUOTE=oldyellr;3017003]Okay, I'm learning something here. I just went out to the garage and checked my cheapie AC stick welder and sure enough, there's no continuity between the ground prong of the plug and the ground clamp. That puzzles me. Is there a specific reason for this? I can see where would be mandatory for an AC/DC welder, but I can't see a reason for it for AC.


The welder has a transformer and the power used to strike an arc comes from that. The welding clamp /ground clamp" and the electrode/ "welding rod holder" are connected only to the secondary side of the transformer in the welder. If this makes any sense you got me beat but basically that's it. The cabinet of your welder should have continuity between it and the ground prong of the plug.

[Rock knocker]

NEC 2008
630.15 Grounding of Welder Secondary Circuit
The secondary circuit conductors of an arc welder, consisting of the electrode conductor and the work conductor, shall not be considered as premises wiring for the purposes of applying Article 250

FPN: Connecting the welder secondary circuits to grounded objects can create parallel paths and can cause objectionable current over equipment grounding conductors.

[greasemonkeyok]

Let me see if I can say it in layman's terms. If the ground wire of the 110v circuit is connected to the metal welding table, and you have a electric drill plugged into that 110v circuit, then the current from welding circuit (secondary circuit coming out of the transformer) could be conducted into the case of the electric drill and bite your hand if you pick it up while welding, and your foot is touching some other ground (as in standing in water barefooted)?

[Rock knocker]

That would be possible. But I would think that is not why the NEC rules are such. I think the NEC is trying to prevent a scenerio where the typically high welding currents could enter and destroy the smaller line voltage wires. But your scenerio is quite possible.

So not to ridicule oldyeller, but ground is not ground. And ground is not ground in line voltage circuits where you have the grounded (neutral) conductor and the equipment grounding conductor

[greasemonkeyok]

Thanks. I was just trying to visualize the problem. Now I can substitute other things for the hand-to-foot part of the circuit.

I know GFI does not like it when you try to substitute one ground for another, but it probably doesn't take much current to trip a GFI.

[pat32rf]

I think a lot of the confusion is caused by referring to the negative welding terminal as "ground". It has no real relationship to the electrical ground in your building system. In most welding shops that I have worked in (as an electrician, I'm an amateur weldor) the welding tables are NOT permanently connected to the negative side of the welder. They may be bolted to a cable that comes from the welder, but weldors use both negative "ground" (straight polarity) and positive "ground" (reverse polarity) depending on the rod (job).
The welding tables might be permanently grounded to the electrical system (like if they have an electrical 110v box bolted to them).

[oldyellr]

Okay, so back to what I asked previously, is it actually "wrong" to have 110v receptacles mounted directly to a steel welding table, thus sharing the building's electrical ground with the welding ground? If so, how come welding can be done on steel structures in a building, where all the steel is connected together and grounded and shares the same ground with the building's electrical circuit?

[Rock knocker]

I give up

[oldyellr]

I give up

Are you implying that theoretically it's "wrong", but in practice it works okay, but you hesitate to condone it?