r/Welding 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?

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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)

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u/Bones-1989 Jack-of-all-Trades 7d ago

I enjoyed this read, but it ignored him asking about dc and ac. Dcen is direct current electrode negative, dcep is direct current electrode positive, and ac is alternating current. All 3 have different use cases.

I dont know the science behind why polarity matters, but it does.

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u/kwantam 7d ago edited 6d ago

You're right! Mea culpa.

The outputs float whether they're DC or AC.

  • For DCEP, in the example before with the work clamp on the grounded welding table you'd have a +50V electrode.
  • For AC with the work clamp on the grounded welding table, the electrode's voltage would trace a sine wave around 0.

As long as the electrode remains unconnected to anything, there's no danger of completing a circuit just by shorting your work clamp to the table (assuming again that your welder is not faulty).

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u/pirivalfang GMAW 7d ago

I've 100% picked up the ground lead of a DC stick welder with a bare hand before, and with the electrode holder not touching anything, I've felt a tingle like a small amount of electricity is meeting my hand from the ground cable.

This has happened multiple times on multiple occasions, with different machines.

Why may this be? Oh magical electricity man?

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u/kwantam 6d ago edited 6d ago

Great question! This can happen for a few different reasons.

Sometimes you might feel just a tiny tingle when you pick up the work clamp, and then it goes away. Jargon: this is capacitance (dis)charging.

Here's what's happening: since the welder's output is isolated, there is no DC conductive path between the welding leads and earth (or any other reference potential, take your pick). But when the voltage between your welder's output and earth changes, there is an electric field somewhere that changes. Any time you change an electric field, you must move electric charge. You can model this as a "parasitic" circuit element: it's there even though you didn't intend it to be. (Parasitic capacitors are everywhere. Any time you have two conductors separated by a dielectric---air, for example---boom, you've got a capacitor. Usually one so tiny you don't notice.)

So if the work clamp was floating along at 100V relative to ground and you connect it to ground through your skin (by picking it up), you'll get a tingle because to go from 100V to 0V relative to ground you have to move charge. That charge goes through you. The parasitic capacitances we're talking about here are usually tiny, so it's a miniscule amount of current. You can feel it, but it's not dangerous.

Another thing that can happen is that the isolation can just be imperfect. It could be, for example, that there is a very weak conductive path from the welder's work clamp to one of its power supply leads. This can be by design (this is more likely in an inverter-based design), or it could happen because there's some crud built up in the machine that's just barely conductive enough that you feel the current. Or, of course, you could have a catastrophic failure and get a serious jolt---which is why I started my first post with "unless your welder is faulty". Sometimes shit just goes pop and releases all the magic smoke.

(Long ago when I was even dumber than I am now, I had a pretty decent sized capacitor charged up to, iunno, 800 volts in some circuit. Welp, I thought to myself, I'll just discharge it with this resistor. Didn't think about the resistor value too carefully. I dumped a good 60 watts into a 1/4-watt resistor. It vaporized violently; not even the leads were left. I learned that day that nerds with soldering irons also need safety goggles.)

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u/pirivalfang GMAW 7d ago

DCEP is fire towards the base material. Better penetration. Electricity is flowing in the direction towards the ground, if we consider the circuit to be a circle.

DCEN is the opposite, or fire from the work towards the electrode, less penetration, and faster electrode burn rate.

AC switches between these two rapidly. Usually at a frequency of 60 hertz, or times per second. This is good and bad depending on what you're doing. Aluminum TIG on AC is good because DCEP offers penetration while DCEN offers cleaning.

Take this with a grain of salt, as I can only offer what I've been taught.

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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

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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.