One of the oldes debates around.

For years I was disconnecting batteries until I was talking to a mate who is a welding indtructor at TAFE and he explained to me that the current from the welder is basically only interested (as such) in it's own earth.

May sound stupid but thats the jist I got from his highly technical explnation while I was doing some welding on the tray of his ute.

As long as the earth is as close to the weld as possible it is fine as long as your not welding an alternator bracket or battery tray.

If you were doing major work welding any where near a battery I would sugest taking it out and away from the vehical, Hydragen and stuff like that is a bit explosive, and always open the bonnet before welding on a car and it makes it easier to put the fire out when you hit a fuel line (another long story).

Also 3 phase stuff would be more than likly to damage electronics rather than single phase.