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Butcher, Unless it got broke or removed, the typical garden variety 220 volt AC Buzz Box Welder metal case SHOULD ALREADY BE GROUNDED. That 3 wire cord and plug consists of two hot phase wires (220 VAC across them) plus an equipment grounding safety conductor. That equipment grounding conductor (often green or bare) should already be bonded to the welders metal case and it provides a low impedance return current path back to the panel (via cord and receptacle) to trip the breaker and clear the fault so you dont get shocked should you come in contact with the metal case and a hot phase wire got shorted against it. NOTE Mother earth ground IS NOT THE SAME as the equipment grounding conductor. If the welder had no equipment grounding concuctor and one of its hot phase wires shorted to the case, the circuit breaker wouldnt trip out, so if you touched the case and were standing on like wet conductive earth or concrete, you could get enough curent flow through your heart to be lethal. Even if you tied the welders case to mother earth (but had no safety equipment ground as described above), since the earth is not a very good conductor (depends on moisture and mineral content etc etc) theres a chance there wouldnt be sufficient current flow to trip the breaker and again, its a potential hazard. Thats the reason you want the case bonded to a low impedance (unlike mother earth high impedance) copper wire grounding conductor back to the panel via the cord and receptacle, so the breaker will immediately trip should the case become energized!!!!! !!!!! !!!! Tying it to mother earth IS NOT THE ANSWER as far as total safety is concerned, it needs to be tied to a copper wire (grounding conductor) that leads back to the panel so the breaker trips if a hot wire touches the case BEFORE YOU DO !!!!! !!! ALSO Neutral IS NOT THE SAME AS THE EQUIPMENT GROUND !!!!! !!! The white Neutral is a current carrying grounded conductor while the green/bare equipment ground is a non current carrying conductor intended ONLY to carry fault current to trip the breaker. Just because the equipment ground would work like a Neutral DOES NOT MAKE IT A NEUTRAL NOR MAKE IT NEC LEGAL. Except for the range/dryer exceptions it is NEC illegal and unsafe to use the equipment ground as a Neutral to carry ordinary phase current. Hope this helps, take care be safe n God Bless John T Retired Electrical Engineer
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