Be sure the welding machine frame is properly grounded, and double-check the grounding connections. Never use pipelines carrying gases or flammable liquids or conduits carrying electrical conductors as grounds. Don't ground to a building structure that is a great distance from the weld. It may break under load years later, perhaps with fatal results. If you must weld a suspended workpiece run grounding cables from the workpieces on both sides of the weld to the same ground as the welder and use a nonconductive sling rated for the load.
Wet floors are dangerous and can cause electrical shock.
Make sure the insulation is sufficient on higher open-circuit voltage. Do not change the polarity switch when your machine is under load. Arcing because of high current can burn the switch contact surface and can seriously burn you. Electrode holders should be stored where they cannot make contact with personnel, conductors, fuels or compressed gas tanks. If you are not going to be welding for a few minutes, disconnect the power source and remove the electrodes.