Multiple welders getting shocked between two different surfaces TIG/Stick

[SpencerFitch]

One welder using tig and one welder using mig, there’s typically 70v difference between whatever metals the machines are grounded to.. they’re 480 and I understand tig is reversed polarity, but don’t understand why there would be a potential difference between two seperate machines. Is there any fix to this?

 

[sblk55]

welders

welders

It is hard to say. but the first thing that comes to my mind would be to see where they are putting their grounds. I would like to see both grounds as close to the area that they are welding, and to have them be between where they are each welding.

 

[GoldDigger]

One welder using tig and one welder using mig, there’s typically 70v difference between whatever metals the machines are grounded to.. they’re 480 and I understand tig is reversed polarity, but don’t understand why there would be a potential difference between two seperate machines. Is there any fix to this?

Be sure to distinguish between the current return cable from the welder (typically clamped to the workpiece) and the electrical ground (usually made through the EGC to the welder chassis and possibly NOT connected to the workpiece in any way).

The welders must not connect their "ground" (actually return) clamp to a local ground reference (building steel or whatever is handy) and assume that current can flow at a low voltage drop through whatever (if anything) is grounding the workpiece.

Tell us more about how the welders are connected and what the "whatever the machines are grounded to" is. If the workpiece is in fact solidly grounded and the current return clamp is connected to the workpiece, what you describe should not be happening.

If for some reason the welding current is flowing through the welder EGC or through a casual metal to metal connection, what you are seeing is likely to happen, and that is why that configuration is not allowed.

 

[SpencerFitch]

It is hard to say. but the first thing that comes to my mind would be to see where they are putting their grounds. I would like to see both grounds as close to the area that they are welding, and to have them be between where they are each welding.

Their grounds are on two seperate pipe rotators. At one point they were trying to both tig and stick weld on the same pipe rotator but when attaching the mig ground there would be sparks (the 70 dc voltage difference I was refrencing to). Now they’re on two seperate units of metal but get the 70v dc shock if they touch both machines (they’re very close) the obvious answer is don’t touch them? Or weld both mig or stick at the same time. But realistically this makes no sense to me as each lead should only have potential voltage back to its own machine. I haven’t checked grounds of the machines themselves. I honestly haven’t had much time to investigate but it just doesn’t make any sense to me.

I cant understand how two seperate dc systems have voltage difference between them. Though their on the same 480 power source.

My knowledge of welding machines is very limited but it seems the diodes in the rectifier that change the ac to dc should isolate the two energy sources from
eachother, though my electrical theory isn’t the greatest.

 

[StarCat]

Very Well Stated

Very Well Stated

Be sure to distinguish between the current return cable from the welder (typically clamped to the workpiece) and the electrical ground (usually made through the EGC to the welder chassis and possibly NOT connected to the workpiece in any way).

The welders must not connect their "ground" (actually return) clamp to a local ground reference (building steel or whatever is handy) and assume that current can flow at a low voltage drop through whatever (if anything) is grounding the workpiece.

Tell us more about how the welders are connected and what the "whatever the machines are grounded to" is. If the workpiece is in fact solidly grounded and the current return clamp is connected to the workpiece, what you describe should not be happening.

If for some reason the welding current is flowing through the welder EGC or through a casual metal to metal connection, what you are seeing is likely to happen, and that is why that configuration is not allowed.

This is very well stated and it should be covered fully in the scope of " correct " arc welding practices and things to avoid.
I am going to add another scenario that I encountered when I was very green in the field.
Its common practice in HVAC service to use what is known as a " Pigtail " connector on a grounded 240V service to run vacuum pumps in the field, where 115V power is not easily accessible. Eastman used to sell a 3 WIRE connector with alligator clips for said job. One day I got on a job where there was in fact no ground path back to the service panel. With that EXTRA green wire clipped on to metal, guess what happened to the case on my pump?
The other thing that rookies learn quickly is what happens when you connect to a high leg......

 

[hbiss]

... but get the 70v dc shock if they touch both machines (they’re very close) the obvious answer is don’t touch them?

The obvious answer is to check the EGC on each machine to see that both gets back to the receptacle and the receptacle is wired properly with the EGC going back to the panel.

The fact that there is voltage on the housing of one or both machines screams no EGC. If there were, both machines would be connected together so there could be no potential between them.

You are correct that each machine should isolate the line input from the welding ground and electrode. Not necessarily because of the rectifiers, don't forger the transformer. But apparently the work grounds are bonded to the EGC and not left floating.

-Hal

 

[GoldDigger]

The obvious answer is to check the EGC on each machine to see that both gets back to the receptacle and the receptacle is wired properly with the EGC going back to the panel.

The fact that there is voltage on the housing of one or both machines screams no EGC. If there were, both machines would be connected together so there could be no potential between them.

You are correct that each machine should isolate the line input from the welding ground and electrode. Not necessarily because of the rectifiers, don't forger the transformer. But apparently the work grounds are bonded to the EGC and not left floating.

-Hal

:thumbsup:
Unless you use series capacitors (like in a voltage multiplier circuit) simple diodes will not and cannot remove any reference to earth ground that is present on the AC feed to the equipment. Depending on the configuration one or neither of the DC outputs may actually be at ground potential, but they will be at most at a constant AC and/or DC offset from that potential.
That isolation, which allows a welding machine to choose to ground one of its outputs, is provided only by a transformer.