I've been meaning to ask this for a while now. First let me explain my setup. On the far wall of my garage I have my 240V outlet which s about 1ft away from the breaker panel. My work area is on the other end of my 3 car garage so to get power to the welder I use a 6ga 30ft RV extension cable. The outlet is a 4 prong outlet with Phase 1, Phase 2, Neutral, and ground. The RV extension cable is the same. The welder has a four prong plug on it now but the earth ground lead is not plugged in.

Now the issue. If everything is plugged in and I flip the switch on the back of the welder the breaker in the panel trips. If I first turn on the switch and plug the welder into the RV extension cable the breaker in the panel trips. If I first turn on the welder and plug the RV extension cable into the outlet there is a little bit of flash arcing but everything is fine.

Anyone run into this before? Ideas?