grounding on cranes

We actually had a crane set up next to a radio transmission site once. This particular crane had like 250' of stick, and the line was, through some electrical phenomemon, receiving the radio energy.

Problem was, the block kept getting HOT, and would shock people that touched it.

We had a kevlar endless loop made and attached it to the hook. Kevlar dosen't conduct, and nobody had to touch the crane, isolating the circuit.

Sounds like a badass difference in ground potential. Mr. Coffman can probably elaborate on the hows and whys.

I was on a job once where one day the bridge crane was still rigged up on a truck bed and every time the welder would strike an arc the crain would go into hoist, and stop when he stopped welding. A few weeks later the control pendant was touching the side of a truck bed (these were 240 ton haul truck beds) and as this cat would air arc, the pendant would blow out sparks and swing away from the bed, when it swung back and hit it, the same thing would happen again. I was standing about 50' away and several people were yelling at the guy to stop when the doors of a big electrical panel started blowing open with sparks and smoke and a small fire. It was one of the weirdest things I've encountered. By the time they figured out the problem I was on another job. Before I left they replaced all the lead on the welding machines with 4 ought, but that sure didn't sound like a solution to me, but I wouldn't know.

JTMcC.

Last year I ran this problem by an electrician when I was working for an overhead crane outfit. In typical electrician attitude he told me that false grounds don't happen and welding machine grounds and electrical grounds are separate.... Yeh Right! Maybe in a perfect world. Has anyone seen swivel bearings for the hook or sheave bearings that are non-conductive?? Another thought I have is a UHMW sleeve on the hook. Right now we put an old welding glove inside the hook when we have to hang and weld. If we don't notice that the glove has slipped off or shifted we get arcing. With all the overhead cranes in use in welding shops someone must have thought up something. Fabric slings are Ok for some stuff but they don't tolerate heat very well. Randy

I was on a job once where one day the bridge crane was still rigged up on a truck bed and every time the welder would strike an arc the crain would go into hoist, and stop when he stopped welding. A few weeks later the control pendant was touching the side of a truck bed (these were 240 ton haul truck beds) and as this cat would air arc, the pendant would blow out sparks and swing away from the bed, when it swung back and hit it, the same thing would happen again. I was standing about 50' away and several people were yelling at the guy to stop when the doors of a big electrical panel started blowing open with sparks and smoke and a small fire. It was one of the weirdest things I've encountered. By the time they figured out the problem I was on another job. Before I left they replaced all the lead on the welding machines with 4 ought, but that sure didn't sound like a solution to me, but I wouldn't know.

JTMcC.

I think you do know.

Steve Smith

I really don't. At the time I thought the electricians working on the problem were dumfounded, and came up with the problem being caused by too small of welding lead (they had about 20 to 30 welding machines in the building, mostly large motor generator Lincolns, with a few inverters thrown in) because they couldn't find the real problem. Since then I've decided that I have no idea, and maybe they were right, maybe not. I know they replaced hundreds of feet of perfectly good lead. I'm still curious though, and it was interesting to hear that others have observed similar problems.

JTMcC.

>

Nylon lifting rope or loop. One that will hold the weight X n times for safety - which is easy on that type of rope.

One could have swivel hooks attached to one or both ends as well for a metal to metal touch - so your strap doesn't get cut.

Martin

I once had a similar incident on a smaller scale in my shop. I was TIG welding away one day on some 1/8" aluminum window frames, when I smells burning rubber/plastic. It took me a while to figure out that it was the extension cord to my air compressor, and it was a melted mess. This was a 10-3 extension cord only about 12 feet long. I was quite baffled as to how or why this happened. And then I realized the air compressor had vibrated itself until it shifted sideways enough so the handle on the front of teh compressor was resting against the water-block at the base of my TIG torch. So while I was welding the arc was grounding through the chassis of the compressor and therefore the ground wire of the extension cord.

Somehow the extension cord didn't like 250 amps passing through a 10 gauge wire.

It sure made a mess of that cord. I decided to wrap the water block in sheet rubber after that.

I've seen man-lifts with plastic gibs. I just thought they were for efficient wear. Maybe they insulate the lift chassis in case you run into something hot?

But you would lose too much headroom, which is many times at a premium. A lot of these bridge cranes are in the 200 + ton range.

JTMcC.

Hi, One way to avoid current passing through chains and other devices is to run an old welding return cable from the work to a stanchion. This is known as a safety earth. Stray currents will pass directly to ground avoiding an embarrassing situations. Just make sure that the safety cable is always attached.

Harry

By Code, the crane has to be safety grounded to an Earth ground. If it isn't, there can be an electrocution hazard if there is a wiring fault with its lift motors, or if it inadvertently comes in contact with hot wires.

The problem you're experiencing is due to either a welding machine fault, or improper secondary wiring practice.

If the welding machine has a secondary fault which allows one side of the secondary to connect to Earth via the machine chassis, then you can have sneak paths from the machine primary safety ground through any Earth grounded object to the workpiece. *Both* welding leads should be insulated from the welder's chassis. Neither should be deliberately Earth grounded at the welding machine.

Or, if you have a wiring fault such that the misnamed "ground clamp" of the welder is attached to something other than the workpiece, the same thing can happen. This lead is circuit return only, not actually a ground, and shouldn't be connected to ground (except incidently via a workpiece which happens to be grounded). That's why this lead is insulated.

With both leads properly floating, the only path for welding current is through the workpiece. You can't then have sneak paths to things like crane lift chains.

Gary

Thanks Gary, Could I do some checking with an ohmmeter and the welders turned off? I imagine I could check for problems by touching the ground cable then welder frame? Similarly between cable at wire feeder and junction box on wall? I already have a suspicion which welder is the culprit.. an old Canox. Would the code prevent putting a permanent insulating jacket over the crane hook to isolate the load? Randy

By Code, the crane has to be safety grounded to an Earth ground. If it isn't, there can be an electrocution hazard if there is a wiring fault with its lift motors, or if it inadvertently comes in contact with hot wires.

The problem you're experiencing is due to either a welding machine fault, or improper secondary wiring practice.

If the welding machine has a secondary fault which allows one side of the secondary to connect to Earth via the machine chassis, then you can have sneak paths from the machine primary safety ground through any Earth grounded object to the workpiece. *Both* welding leads should be insulated from the welder's chassis. Neither should be deliberately Earth grounded at the welding machine.

Or, if you have a wiring fault such that the misnamed "ground clamp" of the welder is attached to something other than the workpiece, the same thing can happen. This lead is circuit return only, not actually a ground, and shouldn't be connected to ground (except incidently via a workpiece which happens to be grounded). That's why this lead is insulated.

With both leads properly floating, the only path for welding current is through the workpiece. You can't then have sneak paths to things like crane lift chains.

Gary

Yes. If you read a short, the likely causes are a short to the core in the transformer or a reactor or breakdown of insulation on a heatsink. A bad return lead connector at the front panel is another likely possibility.

Note too that some well meaning, but ignorant, electrician may have deliberately grounded the "ground" cable. *IT IS NOT A GROUND*. It is a *HOT* secondary conductor, no different than the one that goes to the welding electrode (in fact you should be able to swap them when you want to reverse welding polarity).

The best option is to locate the short and repair it. A welder with failing insulation is a potential fire and shock hazard. OSHA probably wouldn't be amused if it came to their attention.

Gary

Well, *both* leads in the arc welding process are HOT LEADS. Calling one of them ground is a misnomer. Unfortunately, it is common welding terminology, but to a qualified electrician it is simply wrong.

By Code, ground leads are *never permitted* to carry operating currents. Ground is for safety only, and the circuit must be arranged so that only

*fault* currents can flow to or from ground. Since it should be obvious that the welding "ground" lead *does* carry operating currents, it can't actually be a grounding lead.

Any welding current that does flow through ground is *fault current*, and the cause of the fault must be found and repaired immediately. Typically this will be a fault in the welding machine or its installation which allows a sneak path to ground for the secondary circut. Such conditions are dangerous and should not be allowed to persist.

Gary

While this may "work" in some situations, it is just a bandaid, and isn't addressing the real problem.

For a current to flow, a completed circuit is required. Ground is *never* permitted to carry operating currents, so it can *NEVER* be allowed to be part of a complete circuit. If it is, either there is a welding machine fault, or the secondary circuit is wired incorrectly.

Ground has no mystical properties. It is just a big old sheet conductance with a resistance between any two points somewhat less than 25 ohms, think of it as a fat wire. That "wire" should never be allowed to be a path for operating currents.

Gary