Common Ground (for welding)

90% of my welding work takes place on a 4'x8' (probably 500#) table. a tig, mig and two stick machines are up against a wall, with about 5'
between them and the table.
i'm considering running a common ground lead between the 4 machines and the table. anyone forsee problems with this? safety issue? i'm concerned about two welders running at the same time, sharing a ground.
two machines running simultaneously doesn't happen often, but i would install a ground line capable of handling the current for all 4 machines.
they wouldn't be hardwired.. since the welders move sometimes. maybe a plate on the wall with 4 lugs, all with ground clamps hanging.
i imagine using a sort of 'jumper cable' to ground work that isn't welded on the table.. making a ground connection from machine, through the table, to workpiece.
believe it or not i'd be doing this to make life easier, hoping to avoid the nest of cables that sometimes appear on the floor.
-tony
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What you propose is comonly done but personally I would not do it. I have had some expereinces with current interacting between a DC machine and an AC machine.. Unless you are going to regularly use more than one machine on the table at once I would be inclined to avoid a common ground. Randy

concerned
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I thought that someone more knowledgeable and more pedantic than me would have brought this up by now...
Those aren't grounds you are trying to tie together. They are the work side of the welder's output. There is nothing common about them between the various machines.
On a transformer based machine you might be able to get away with tieing the work connections together. Even here there is a possibility of current leakage through the transformer.
On inverters it seems like a really bad idea. The output of inverters, AFAIK, is not isolated from the rest of the circuitry. Some machines may have protection built in for situations where the work connection can be back fed but others may not.
Cheers,
Kelley
On Mon, 24 Nov 2003 04:53:14 GMT, "Randy Zimmerman"

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machines.
welded
table,
avoid
Not that it pertains to the discussion, but it is interesting to note that I (and thousands of others) have welded for over 20 years in situations like: A powerhouse where several 8 packs are all sitting in one area, all grounded to a column, new construction at a car plant, where 50 to 60 welding machines are again, all grounded to the structural, same in refineries, ect. A pipeline where 30 + welders are all grounded to and welding on the same piece of steel. No problem there.
JTMcC.

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I think your comment does pertain to this discussion. A lot of people, upon seeing a large ganged set of welding power supplies, would think that it's ok to do this with any random collection of welders. Most power supply manufacturers make specific series of welders for this type of use.
Cheers,
Kelley
On Mon, 24 Nov 2003 21:04:00 GMT, "JTMcC"

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tony wrote:

Rod: Take a LARGE MIG and a small mig. The Large MIG has STRONGER _diodes_ than the small mig. So, you don't want these two to go MIG to mig welding at the same time. ;) I know this isn't your case, or is it wrt the different machines? So, if one machine output, output's another, DON'T DO IT as a rule of thumb! My MAIN CONCERN HERE is a machine with HIGH FREQUENCY wrt other NON HIGH FREQUENCY machines. THEREFORE, I STRONGLY DO _NOT_ RECOMMEND YOUR IDEA IF HIGH FREQUENCY IS USED whether simultaneously with other machines, welding, or turned off! SO THE ANSWER IS _NO_ HERE - PERIOD!
Rod Ryker... It is reasoning and faith that binds truth.
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I was also thinking about doing what the original poster asked about... I think...
I'm confused why it would be a problem to clamp however many machines to a common clamp and run one grounding cable to the work table. Is there a difference between doing that and having 2 or 3 or 4 machines all clamped to the worktable itself and welding?
Does this issue ONLY arise when more than one machine is being used at the SAME time?
Thanks,
James, Seattle
www.jameskelseystudios.com
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RainLover wrote:

Rod: First, let the ONE cable be large enough in diameter to carry all of the required current of all machines.
Second, Use only machines that are the same - rule of thumb. I explained in my first post above.

Rod: James, study what I wrote above in my first post, and your questions I answered. Rule of thumb is, if the machines are identical and using the same polarity or all using AC, then this is fine. Take this in conjunction with what I said about High Freq.
Rod Ryker... It is reasoning and faith that bind truth.
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I looked through a few welder manuals and found something I hadn't expected. Both Miller and Lincoln use boiler plate language to discuss work leads and grounding (this I expected). Miller says to ground the work table and work if required by code. Lincoln recommends it in all the cases. I looked at manuals both for inverter and transformer machines. The text below is from a Lincoln manual:
"The work piece must be connected to an earth ground close to the work clamp, using one of the following methods:
a) A metal underground water pipe in direct contact with the earth for ten feet or more.
b) A 3/4 (19mm) galvanized pipe or a 5/8 (16mm) solid galvanized iron, steel or copper rod driven at least eight feet into the ground.
The ground should be securely made and the grounding cable should be as short as possible using cable of the same size as the work cable, or larger. Grounding to the building frame electrical conduit or a long pipe system can result in re-radiation, effectively making these members radiating antennas."
From Lincoln's recommendation it would seem that if you coupled several different welder's work connections together AND grounded them then you shouldn't have any problems since all of the work connections will be at ground potential. The ground connection is very important and should be kept in good condition. It's also important to make sure that the power input ground on each machine is in good shape.
Don't take my word for any of this, I don't know what I'm doing.Check it for yourself.
Going back to your original post I thought you might like this quote from the Lincoln manual.
"Connect work cable to the work as close to the welding area as practical to prevent welding current from traveling long, possibly unknown paths and causing electric shock and fire hazards."
Cheers,
Kelley On Tue, 25 Nov 2003 13:01:14 -0800, RainLover

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The ground they are talking about is in reference to primary power not welding voltage. The output of a welder is transformer isolated from the primary side. This is the same type of saftey ground any other appliance would have. This ground is needed to protect you in the event that primary power in some way gets connected to the output of the welder. Usually this would be caused by component failure.
wrote:

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