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When I turn on my welding receptacle breaker (50A, 240V), it trips the main breaker. There is a flash/spark that comes from the connection of the welding receptacle's ground wire to the panel. One other ground wire is connected at the same spot (220V) as well as some copper wire that exits the panel and goes outside (... the ground?).

Other information: the panel is in very good shape, is about 10 yrs old, the welding receptacle has never been used before.

Thanks very much for any help!

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    The critical missing bit is the wiring configuration of the receptacle. Please edit to describe that or add a photo or diagram. It sounds like maybe there's inadvertent contact with the ground wire in the box. – isherwood Jul 28 '17 at 16:20
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    Is there anything plugged into the welding receptacle? – mmathis Jul 28 '17 at 16:21
  • How many amps is the main breaker? Say you use 45A for the welders, but the rest of your house is using 60A then a (60+45=105 trip) 100A breaker could trip. However, your main breaker could be much smaller. One the other hand it should be rare to draw 60A for any significant time. – cybernard Jan 21 '18 at 18:50
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Anytime there is a flash or spark on a connection it means it is loose. A tight connection does not spark.

Tighten the connection that is sparking. See if that changes anything. It probably won't.

The most likely reason the ground is sparking is there is a ground fault caused by a miswired receptacle or whatever is plugged in to the receptacle.

Unplug whatever is plugged in to outlet and if it still trips open the outlet and investigate the wiring. Something is seriously wrong here.

Good luck and stay safe!

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    Turn off the breaker before opening the receptacle, please. – Ecnerwal Jul 29 '17 at 1:37
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PYou have two big problems.

A squirreley ground

You can't put more than one ground wire in one hole in a ground bus(unless it's labeled to do so), and you probably can't do it with larger wires. The most important ground** on that entire bus (assuming it is a combo ground+neutral bus) is the bare wire that exits the panel and goes outside**. That is the grounding protection system. If it has problems, it will float your grounds, any ground fault will energize all your grounded places (chassis of machines etc.) compared to actual earth. A serious ground fault will float it a lot. And you've got a serious ground fault right now. So it really matters right now.

It sounds like somebody did some shoddy work and just stuffed the wires wherever, cranked down screws (if they did that) and hoped for the best. Nuh-uh. Fix all that and fix it right.

If you don't have enough room on your ground bus, you can buy additional ground buses from the panel manufacturer. Another (temporary) option is to pull several smaller grounds off, four at a time, and wire-nut them together with a pigtail, and put the pigtail on the ground bus. They even make ground wire-nuts that let you continue one wire through the nut. The pigtail should be the size of the largest wire, ideally do like-size wires together. Don't do this with neutrals.

A ground fault!

You are lucky that ground worked well enough to trip the breaker. You have a ground fault, or a "hot" wire coming in contact with ground. It may be in your wiring or your receptacle. Or it may be in the welder itself. Unplug the welder, and light up the circuit one more time. You better not see any sparks this time from the ground! If you do, you skipped the above step.

If it doesn't trip, you've now narrowed it down to the welder itself. I would first check its power cord and plug, and then off to the repair shop it goes.

** actually, as Tester101 mentions, the most important ground is the neutral-ground bond in the main panel. Since your ground bus is apparently overfull, and that's unlikely for only grounds, I assume this bus is being used for both neutrals and grounds. That makes the neutral-ground bond inherent, and so I am disregarding it.

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    I disagree. The most important ground "wire" (conductor), is the one that bonds the service neutral to the ground system. Without that, you have no ground-fault current path. If you don't have that, a short to ground can light up your entire grounding system, including everything connected to it (enclosures, chassis, etc.). The grounding electrode conductor is great, but definitely not the most important ground wire. – Tester101 Jul 28 '17 at 22:40
  • @Tester101 that would be true if that bond existed. But since his ground bus is apparently full, and I've never seen a ground bus that is too small for the panel, I assume this is bus is being used for both neutrals and grounds -- and thus that bond is inherent. – Harper - Reinstate Monica Jul 28 '17 at 23:45
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    "You can't put more than one ground wire in one hole in a ground bus." Most panel manufacturers label their panels for one or two wires under a screw on the ground bus. The Code limits neutrals to one wire per terminal but not the grounds. I had quite a discussion with an inspector over this but the upshot is most inspectors allow 2 wires per terminal screw, even if they are the neutral and the ground as long as it is the service panel. – ArchonOSX Jul 29 '17 at 8:53
  • @Harper Actually, it doesn't make it inherent. In fact, if the neutral bond was removed (because somebody thought it should not be there), all the neutrals landed on that bus would be floating. If that bus is not bonded to the service neutral, it can be quite dangerous. – Tester101 Jul 29 '17 at 19:42

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