Imagine yourself standing barefoot on the piece of metal with the ground clamp attached. Now further imagine (to insure good connection) that you touch the electrode to your tongue. Current will flow through your body. The machinery has no way to tell that it isn't just welding. There is no ground fault since all the primary current is returning. DO NOT TRY THIS YOU COULD BECOME SERIOUSLY DEAD.

There are 2 circuits to welding machine (Gas & Diesel machines not with standing) Primary and secondary.

The first provides AC power to the machine to produce the lower voltage higer (generaly DC voltage)amp welding current. They are just a transformers (converter or inverter nowdays)

GFCI should be required on the primary side, this protection is only good if you come in contact with the primary leads. as for the secondary side follow the instructions in the other post.

Seriously though for conventional welding the secondary voltage is not high enough to push welding amps through the human body, Mr Ohms says its against his law. How ever laws were made to be broken so they say. I hope you are familiar with Ohms Law.

There must be something to it though, after all you do hold one end of the circuit in one hand and brase your self by holding on to the other side of the circuit with the other hand.