DC arc welder causes sparks when work is touched by chain hoist

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We are having a strange problem with an AC/DC welder. The welder is being used on dc+ and it is connected to 480V 3PH source. They are trying to weld some chains and using a 3PH trolley chain hoist. The chain is grounded to work terminal of welder. Whenever they try to hook chain hoist into their work it sparks/shocks.

The welder and chain hoist are both on same 480V source.

I have checked hoist and it appears to be ok, no v from hoist to ground, even turned power off to hoist. I tried reversing polarity from welder, since dc goes from - to +, but it still is sparking. Ground (work lead) from welder looks like it's making a good connection on workpiece.

I am not that familiar with dc welders and I hope someone here can shed some light on this phenomena.
 

Volta

Senior Member
Location
Columbus, Ohio
I'm having trouble picturing this.
I assume that you have verified the EGCs to both units, and they are well connected.

1. Is the hoist trolley moved by chain, manually?
2. Is the electrical component limited to the lifting hoist, or is this a more complex unit with travel motors?
3. Is the chain to be welded part of the hoist/trolley, or is the hoist simply being used as a lifting means?
4. Does the DC output have a reference to the building ground internally (through the EGC)?
5. If the welder DC work ground would be connected to the building steel, would you read current?
6. Have you measured the voltage fron the work ground to the building ground?
7. Both are on the same 480 source, are either fed through a trandformer, or are they both 480 volt loads?

My guess is a floating output design, with a fault on the secondary, in the welder.
 
Hoist has electric travel motors for back and forth, side to side, and hoist motor to lift/lower the load.

The chain needing to be welded is a conveyor chain, being supported/moved with the chain of the hoist.

Scenario happens even if breaker is off to hoist, so I have ruled out hoist having electrical problem.

Based on readings I get with Fluke, I would be reluctant to attach work/ground clamp to building steel.

Welding machine, from bldg gnd to work (ground) terminal is 35 VDC-, to electrode terminal 35 VDC+, between the two I get 68 VDC. Reference to ground on terminal is in kiloohms. Machine case is referenced to ground.

Both are on same 480 power, no xfmr in between. I just wonder if this is a normal problem or if I have a problem internal to welding machine. I would think the latter.
 

Volta

Senior Member
Location
Columbus, Ohio
With such similar but opposite DC voltages, it may be that the welder output is designed to be and is mid-point grounded. Is there any chance that the unit has a switch to lift the grounding of the output, such as older amplifiers often had? If the EGC to the welder were disconnected momentarily, for testing purposes only, you may find no effective potential between the outputs and the building ground.

It should be considered unsafe to operate that way, especially with a 480 volt source, if a factory switch does not exist, but would locate the source of the trouble.
 

wireguru

Senior Member
With such similar but opposite DC voltages, it may be that the welder output is designed to be and is mid-point grounded. Is there any chance that the unit has a switch to lift the grounding of the output, such as older amplifiers often had? If the EGC to the welder were disconnected momentarily, for testing purposes only, you may find no effective potential between the outputs and the building ground.

It should be considered unsafe to operate that way, especially with a 480 volt source, if a factory switch does not exist, but would locate the source of the trouble.

and if thats the case, can you stick a roundsling between the hoist chain and workpiece to keep them isolated?
 

neutral

Senior Member
Location
Missouri
Could be a dirty or worn shoe on the crane rail, How long has it been since the crane was serviced by a qualified Electrical Maintenance Electrician?
 
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