PowerTig 255EXT welding power problem

That totally explains it. That was one side of the output bonded right to the case ground. So when your table was grounded via the ground wire on your grinder or other power tool that has a grounded case, you had a nice big ground loop. I am somewhat surprised that the machine could not detect that it had an internal short to ground on one of the outputs. Out of curiosity, was the short on the bus that fed the torch lead or the work clamp? There really should be more of an air gap between something like that and the case. I know it's pretty tight in there, but is there a chance that something got bent, and there is room to bend it back away from the case? Relying on a thin piece of tape or even heatshrink in direct contact with the case is not good. Especially if there can be HF on that.

Does the picture here include a GFCI on either the grinder's or the welder's outlet? I would have expected them to trip. (I know GFCI detects an imbalance between hot-hot or hot-neutral currents; do they also trip if current is detected on ground?)

Considering how the current smoked the 120v receptacle, it'd be wise to inspect the ground wiring from the 120v receptacle back to the panel's bus, and do the same for the 240v circuit, particularly the ground wires inside the welder.

I'm trying to grasp why the electrical path would prefer the building ground over the heavy ground clamp wire. It'd guess it's because the short was actually on the torch lead, so a path existed from the ground clamp, to the building ground via the grinder, through the case, and to the torch lead. This would have been a much lower resistance path than through a spark gap to the torch electrode, which would explain why it wouldn't strike an arc, yet current was flowing through the grinder as soon as you hit the pedal.

My $.02,
Richard

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Originally Posted by RichardH

Does the picture here include a GFCI on either the grinder's or the welder's outlet? I would have expected them to trip. (I know GFCI detects an imbalance between hot-hot or hot-neutral currents; do they also trip if current is detected on ground?)

Considering how the current smoked the 120v receptacle, it'd be wise to inspect the ground wiring from the 120v receptacle back to the panel's bus, and do the same for the 240v circuit, particularly the ground wires inside the welder.

I'm trying to grasp why the electrical path would prefer the building ground over the heavy ground clamp wire. It'd guess it's because the short was actually on the torch lead, so a path existed from the ground clamp, to the building ground via the grinder, through the case, and to the torch lead. This would have been a much lower resistance path than through a spark gap to the torch electrode, which would explain why it wouldn't strike an arc, yet current was flowing through the grinder as soon as you hit the pedal.

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A GFCI on the grinder wouldn't have helped, as they do not break the ground line even if it tripped. A GFCI on the welder circuit might not have tripped either. Hard to say as the balance between hot lines should have still been ok. I'm not really sure as I am not sure of the ground current detection system used in a modern one. Last time I took one apart, they were still just called a GFI and didn't do anything but watch for imbalance between conductors via a small transformer. In theory the welding output is completely isolated from the input, so the GFCI might not even detect it. The ground wire to the grinder was no doubt a terminal bolted tightly to the chassis, and the grinder was bolted to the table so that ground path was pretty good. The point of highest resistance was probably the oxide layer on the plug and outlet, hence where it went poof. Combine that with the ground clamp of the welder being on a dirty part of the table or part, and a layer of oxides here and there, and the resistance could be much higher. You can see why ground loops can do the damnedest things. The torch lead bonded to the case does seem to be the best fit with what happened, though.

I would say if we don’t have a schematics of the welder it is hard to tell what the welder is thinking when something goes wrong it might have diodes that will not let it feedback and like Rambozo said it will take the easiest path. By the looks of the plug the black wire shorted to the ground because of grinding dust and with HF to help the jump it said a short cut here I come. I am not an electrician but I understand most of the theory of AC and DC and some of the parts that go with it if I have a schematics of what is going on.
As I said earlier in the forum the factory will figure it out and fix it as Duncan told me all the testing is done with the cover off and then it is put back on at the end and test again on a cart without a grinder or power tools plug in on it. Sh_t happens but the factory is aware of it now and will fix it and so far it is an isolated problem.

did you resolve your problem?
I have a 255ext on order and soon to arrive and I have a steel bench that I weld on
I also have my grinder and drill press on the same bench

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Originally Posted by kafer

did you resolve your problem?
I have a 255ext on order and soon to arrive and I have a steel bench that I weld on
I also have my grinder and drill press on the same bench

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See post 17, just a wire had rubbed against the cover...when it was on the final assembly after testing.

thanks appreciate the quick responce

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Originally Posted by performance

See post 17, just a wire had rubbed against the cover...when it was on the final assembly after testing.

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Mark
Just saw you post and for Kafer to look at post #17 just finished welding a snowmobile ski's and a titanium exhaust system will put pic's later. The weld is working just great.