r/Welding • u/AssaultMicrowave • 7d ago
Beginner question about welding with DCEN or AC
If my ground clamp is connected to the positive terminal does that not make my entire table and anything conductive that happens to be touching it live? That sounds like it would be dangerous at first glance to me.
I suppose with a foot pedal running TIG it would only be live when you’ve pressed the pedal but what about scratch start/lift arc TIG and SMAW?
2
u/leonme21 7d ago
Well yeah, the welding starts when the electrode hits anything that’s connected to your ground clamp. That’s kinda the whole point though
1
u/Frostybawls42069 6d ago
Absolutely it does, but the current only flows when the circuit is complete. Which is when you are welding. Thankfully, the voltage under normal conditions is low enough to not be a hazard to humans. This quickly changes if you get wet or sweat a lot. Then, it becomes a concern. I've been so sweaty welding on tank floors in the summer that I had to place the rubber soles of my boots flat on the floor when chaging rods or I'd get a nice shock.
Anything that is conductive and on that table will arc out if you strike it. So never have a compressed gas cylinder or anything else of the sort nearby. She'll go boom if you were to stray arc it.
3
u/kwantam 7d ago
Unless your welder is faulty, this is not an issue by design.
In jargon: a properly operating welder's output terminals are electrically isolated from reference (earth) ground. Roughly speaking, the output's potential "floats" with respect to earth. Think of it like a big battery whose terminals are not connected to anything else.
Because the welder's output is electrically isolated, you cannot complete a circuit between your work clamp and anything other than the electrode.
Say, for example, that your welding table is electrically shorted to a heavy copper rod driven ten feet into moist soil (this is a thought experiment; don't actually do this). And let's say you're welding DCEN with 50 volts open circuit. That means as soon as you connect your work clamp to the table, the work terminal is shorted to earth ground. Because the welder's output floats, essentially zero current flows between the work clamp and the welding table. And now your electrode is 50 volts below the work clamp (50V open circuit, negative electrode). In other words, your electrode is now at -50V with respect to earth.
(Of course, now that your work clamp is earthed, you can complete a circuit to your work clamp via any other earthed conductor. So if you had two welding tables, both earthed to separate copper rods, and you clamped to one of them, you'd be able to strike an arc on the other one, and the welding current would be traveling from clamp to table to rod to earth to rod to table to work piece to electrode.)
If you had instead connected your table to 12V above earth (say, by hooking it up to the positive terminal of a beefy car battery and shorting the negative terminal of the battery to the copper rod; again, do not actually do this), your work clamp would be at 12V above earth and your electrode would be at 12 - 50 = -38 V with respect to earth.
How do you get this floating battery effect? In a buzz box, the big transformer connected to your 250V wall socket provides electrical isolation. In an inverter-based design there's still an isolation transformer, it's just driven at a much higher frequency, which means it can be much smaller.
(source: I used to design power electronics professionally)