Electric Arc Welding and burned-up lighting wiring?

[factoryrat]

Today, at work, the millwrights welded metal guard railing to a metal framed work deck. They came to me and said the overhead fluorescent lighting went off. This overhead lighting hangs off a metal unistrut grid that attaches to the metal framed deck below, the deck they were welding on.

I went to the lighting panel and tried to reset the circuit breaker but it tripped immediately. Upon further investigation I found the circuit?s ground wire, that I believe, had got hot enough to melt through its insulation and through the branch circuit?s hot conductor?s insulation and cause a short. I fixed the problem and turned lights back on.

My question is how does arc welding on a steel platform cause a lighting circuit?s ground wire to pass current and melt its insulation, if that is what actually happened? Does anyone have any thoughts on this? Is there something wrong with the way the 120 volt lighting circuit is wired? Is there a way to avoid this problem in the future?

 

[iwire]

Not unusual I have have seen similar happen on two jobs.


Basically they must have put their welding circuit was using the EGC for all or part of the welding current.

This can happen when what they are welding on and their ground clamp are not directly connected.

 

[jumper]

Not unusual I have have seen similar happen on two jobs.


Basically they must have put their welding circuit was using the EGC for all or part of the welding current.

This can happen when what they are welding on and their ground clamp are not directly connected.

I think I know what you said, but I ain't sure.

 

[John120/240]

Not unusual I have have seen similar happen on two jobs.


Basically they must have put their welding circuit was using the EGC for all or part of the welding current.

This can happen when what they are welding on and their ground clamp are not directly connected.

I think I know what you said, but I ain't sure.

iwire did you just wake up from a nap ? I'm with jumper

 

[gar]

131013-2006 EDT

When arc welding it is necessary that the arc welding ground welding electrode clamp be just as close to the point of the arc welding arc as possible. Generally you are dealing with hundreds of amps of current. There can be multiple shunt paths that that current can take. Depending upon the relative resistances of the paths the amount of unwanted current on some of these paths can be great enough to burn up the wire.

I had substantial damage on some gaging equipment at a Chrysler plant where the welders just connected the ground electrode to the heavy base of a large machine, and then welded to an arrachment on the machine. The arrachment was made from 1/4" thick 6" square steel tubing. Yet there was enough voltage drop on the steel frame to force enough current thru Belden 8723 cable and burn it out.

.

 

[junkhound]

+1 on what gar said.


A corollary event -- welding using a CFL trouble light. Light was about 15-20 inches from the welding cable. About 3 feet from arc, 280 A welder setting.

Immediately when an arc was struck the CFL failed.

First thought was a piece of slag hit the CFL, but no melt spot or mechanical damage. Tried again and 2nd CFL fried . Moved light 10 feet away, 3rd CFL, on other side of arc, then 3 ft from arc across from the cable and OK. Tore down the CFL and found the little transistor that drove the inverter tiny transformer was fried.

Apparently the magnetic field from the high welding current in the cable saturated the tiny transformer in the CFL and burnt out the circuit.

Tried bringing a permanent magnet near a CFL and that fried the CFL also, probably same reason.

PS: magic trick at party - small NeBFe magnet between fingers, say you know you have a high aversion to mercury and mercury to you - wave hand past CFL light, light goes out. Probably will zap a LED bulb also, but those are more $$ than CFL and not worth trying.

 

[factoryrat]

Is this the solution?

Is this the solution?

gar said, ?When arc welding it is necessary that the arc welding ground welding electrode clamp be just as close to the point of the arc welding arc as possible.?

So am I to understand that to avoid this situation in the future the welders need to put ground clamp as close as possible to the area they are going to weld? Does this help ensure the welding current does not find any alternate paths to travel?

 

[don_resqcapt19]

They need to clamp on to the actual piece that is being welded. The only path should be between the welding rod and the work clamp. There should not be any "alternate" paths. You get the "alternate" paths only when the work clamp is not connected to the piece that is being welded, unless the work clamp cable has missing insulation or other defects that result in additional connection points.

 

[gar]

131014-0958 EDT

It is not always possible to isolate the item being welded from a structure or other components. Thus, there can be parallel current paths for the welding current. The objective is to minimize the magnitude of current in these other parallel paths. If possible, and known, then open other paths.

.

 

[don_resqcapt19]

131014-0958 EDT

It is not always possible to isolate the item being welded from a structure or other components. Thus, there can be parallel current paths for the welding current. The objective is to minimize the magnitude of current in these other parallel paths. If possible, and known, then open other paths.

.

The piece being welded does not have to be isolated from the building structure to eliminate parallel paths. If you connect the work clamp to the piece being welded, there should be no parallel paths unless there is some type of problem with the welding equipment.

 

[factoryrat]

Thank you.

Thank you.

Thank you for the responses. I respect you guys and your answers. I feel more confident with the information you guys have provided. Thanks again.

 

[gar]

131014-1053 EDT

Don:

Defective welding equipment is not necessarily the cause of parallel paths. Parallel paths can exist with good welding equipment.

Consider a round ring. Something is to be welded to it at one spot. The ring has a circumference of 1 foot. The welding ground is connected 4" from the welding spot. The ratio of the two resistances around the ring is 8/4 or 4/8 as you choose. Thus, considerable current flows in the 8" segment, 1/2 of the 4" segment, or 1/3 of the total.

.

 

[don_resqcapt19]

Ok...I see those parallel paths. I just don't see paths that are a part of the structure that is being welded as a "problem" parallel path. The problem ones are where the current is on the EGC or passing through something like a load cell or flexible gas piping.

 

[gar]

131014-1144 EDT

Don:

If the intent of the EGC is to connect the chassis of a machine or light back to a main panel neutral or equivalent point and there may be multiple equipments connected to the EGC and these equipments are also connected to metal structure interconnecting the various pieces of equipment, then the EGCs can easily be parallel paths to the structure.

.

 

[GoldDigger]

Ok...I see those parallel paths. I just don't see paths that are a part of the structure that is being welded as a "problem" parallel path. The problem ones are where the current is on the EGC or passing through something like a load cell or flexible gas piping.

No, the problem would occur if the part of equipment where the clamp was placed was not solidly connected in the structure of the machine to the place where the arc was struck.
If one part of the machine has a very low resistance connection to the building structure and the other part has a very low resistance connection to the EGC, then as long as the internal path between the two parts has a high enough resistance, a substantial part of the welding current will still flow through the parallel path containing the EGC.
All very hypothetical, and all depending on there not being a sufficiently low resistance internal path.
Possibly a good thing to do would be to disconnect the equipment, including EGC, before welding if you are not going to be careful about where the clamp is placed.
That way you could end up welding the two pieces of a dovetail slide together instead of burning up the EGC.

 

[don_resqcapt19]

..
Possibly a good thing to do would be to disconnect the equipment, including EGC, before welding if you are not going to be careful about where the clamp is placed.
...

That would be almost impossible where they are making field welds on piping or structural steel in an industrial plant. I have repaired/replaced electrical equipment that was damaged by "stray" welding current a number of times, but never when the work clamp had been connected directly to one of the pieces that was being welded.
I can see where such damage is possible, even with the work clamp on the piece being welded, but I have not actually seen that happen.

 

[zbang]

I've found (surprise) that many welders don't want to haul a ground cable around when there's a convenient piece of metal to carry the return. So far, the worst I've seen was using the fire sprinkler system to go up to the roof of a 12 story building.

 

[GoldDigger]

I've found (surprise) that many welders don't want to haul a ground cable around when there's a convenient piece of metal to carry the return. So far, the worst I've seen was using the fire sprinkler system to go up to the roof of a 12 story building.

As long as they carry around a good very low frequency ohmmeter and a reference ground to connect it to instead to verify that they have a good connection. Wait a minute, that would be a ground cable.
:roll: