Sorry to resurrect but wanted to explain the scenario here as there is a lot of misunderstanding in the thread.
The OPs welder is a gasless inverter mig. It sounds likely that it's a non isolated inverter circuit as shown below and the neutral is wired directly to the 'negative'
Being gasless the earth clamp is connected to the 'positive'. So when you clamp the earthed staircase the current runs to earth. These designs can be dangerous as when IGBTs fail they often fail short circuit and you have 220v on the tip. Now an RCD should shut that off in a couple of milliseconds so I'd wonder what state the wiring is in.
View attachment 414086
Except that a machine wired like that would not get electrical approval.
If some did slip through and were used on a building site (for example), it would only take one reverse wired power plug to cause a short and trip the breaker.
I am assuming that Vin is the mains and you haven't shown the rectifier.
Jack