For current to flow, there has to be a full circuit. That means two wires from the power source have to connect to the "load".
As I mentioned, in the case of AC mains one of these wires ( the neutral ) is already earthed, at the power station or at a local transformer. If there is a breakdown of insulation inside the load, the second wire - live - could accidentally connect to the metallic chassis of a piece of equipment. If anybody touched or held it, they would risk an electric shock, since current would then flow from the live through their body to earth - which is neutral. To prevent shock, any equipment that is not double-insulated is intentionally earthed, and any current taking that route back to the power source, instead of via the neutral conductor, will trip an RCD. This may be described as "taking an easy path to earth", but it only happens because the power circuit is specifically designed for that to happen.
If the power source is not AC mains, or it is the secondary of a double-wound transformer ( such as a mains isolating transformer, or your spot welder ), none of the above scenario applies. Neither of the two wires from the power source connect to earth. It is "fully-floating". If insulation breaks down, or is intentionally absent, one power source wire could connect to earth, but the second wire would still float, so no current would flow.