Heres one for you to ponder. Had a call from a friend who owns a welding shop. He was working on a free standing metal table, ( on concrete ) ...and had the work clamp attached to the table. His overhead crane base, was approx 4 to 5 feet away. The tip of the rod came into contact with the chain hanging from the crane, and he got a good spark. Kind of questioning how this would happen, as there is no direct metal contact between the table and the crane. So next time i went over there, and brought my megger. Attached one lead to the table and the other to the bolt on the base of the crane....pressed test, and had a dead short. Just as if the table was connected to the crane with a piece of metal. Like WTH?? I walked around the table several times to be sure we didnt miss anything. The megger is working fine, no issues there. Anyone seen anything like this??? He said that has never happened in the years before this. After discussing with some fellow sparkies, the only thing that makes sense, is that possibly the concrete may have a thin film of metal shavings across the surface and into the small cracks, etc....that it actually provides continuity across the floor. The rebar in the floor is about 8 inches down.....so that shouldnt be a factor. Next time im in the area, im going to bring a magnetic sweep and see if this makes any sort of a difference. I thought it may have some sort of a reading....but at 1kv it was 0..... and 500...the same. Any feedback would be welcome......cheers!!!
Welding shop - grounding??
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Current went thru the welder and to ground. Crane is grounded...
A previous employer I worked for had to replacing wiring after fighting with the GC and welder over who was responsible for a NM cable that got burnt up when the welder came into weld a structural steel beam in a new home... octagon box was touching the beam, and was grounded... wire caught fire...
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Perhaps a grinder or other grounded case tool was plugged in and sitting on the welding table, therefore making continuity with the system ground and welders workpiece ground?
Nope, nothing on the table except some pieces of metal....thats it.
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My understanding is the concrete is a good conductor, it's just not an easy material to bond to, you need a lot of surface area in contact. This is why ufers work so well.
I certainly don't know, but I am skeptical the metal shavings make a conductive skin on the surface of the floor. But I could see the metal shavings make a good connection between the feet of the table and the concrete. They'd conform to the surfaces and give you a lot of surface area in contact.
I wonder what you'd measure with the megger set to a low voltage (50V is probably the lowest setting) and cleaned a spot on the concrete by the table leg. Measure from the crane to the table, and compare to a reading from the crane to the cleaned spot with just the tip of the lead touching. I bet it's a lot more resistance.
I certainly don't know, but I am skeptical the metal shavings make a conductive skin on the surface of the floor. But I could see the metal shavings make a good connection between the feet of the table and the concrete. They'd conform to the surfaces and give you a lot of surface area in contact.
I wonder what you'd measure with the megger set to a low voltage (50V is probably the lowest setting) and cleaned a spot on the concrete by the table leg. Measure from the crane to the table, and compare to a reading from the crane to the cleaned spot with just the tip of the lead touching. I bet it's a lot more resistance.
Check the welding machine itself to see if the output (secondary) of the xformer is grounded. It really isn’t different at all than other work we do with step down xformers. An accidental ground of the output to the case, shouldn’t cause a problem until you run into something like this.
All welders are not the same. Concrete will conduct especially as there is rebar and the lift is probably connect to it somewhere underground. Now add snap start, Hot start or arc assist and there would be no surprises that the stick would arc on the chain. (wouldn't be able to weld that way but it would spark)
Older welders have isolated transformers. Im not sure if the modern inverter ones are or not. A simple ohm test should answer the question.
Older welders have isolated transformers. Im not sure if the modern inverter ones are or not. A simple ohm test should answer the question.
If you go back, slide the table over 6" and try the meggar again.
I will be heading back this week.....cleaning the area, megger & check again. Also at a lower voltage, see what comes up. And also move it back a few feet and try again. Its a Lincoln Invertec 350 - PRO by the way. Right off the top of my head, im thinking concrete would be more of an insulator, than a conductor.......interesting stuff.
If you work industrial you will be use to standing on mcc mats because concrete is not a good insulator.
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I would be interested if you laid a piece of steel on the concrete ground, could you lay a bead on it. If it is really steel dust making continuity back to the table that would make an interesting modern art pattern across the floor as it burnt up.
Why would you not get a reading from table to crane? As said concrete encased ground rods would be useless if it did not conduct.
I have seen welding tables on insulating pads to prevent this. That way if you don't have a good grounded table you don't weld, it helps with bad welds.
Cowboy
I have seen welding tables on insulating pads to prevent this. That way if you don't have a good grounded table you don't weld, it helps with bad welds.
Cowboy
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I'm sure concrete is conducive, but I would think it's a really poor conductor. An arc welder puts out something like 30v, I don't see there being any significant current flow there.
I wonder how much of that dead short on the megger is metallic dust and how much might be damp concrete under the table legs. That's why I suggested moving it over and trying again.
Open circuit voltage (OCV) is significantly higher than that on a stick welder. Modern industrial units will be in the high 70’s. Old pre-safety welding machines I’ve seen in the 90’s, nearly 100 volt.
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