The following link gives you the info you need. Phil W

Try this http://www.twi.co.uk/content/jk28.html Look at the paragraph on power source and earthing. The answer appears to be "it depends". Note I am not an expert on welding!

It is doubtful that you should earth the secondary side of a high current transformer back to the public supply unless there are special controls and requirements in place. Often the structures being welded are actually earthed anyway. There is no benefit for protection against electric shock from the primary (supply side) as this should be covered without the welding set even turned on at its controls as the set provides the same hazards from the primary (supply side) whether its operating switch is in the on or off position. There is still a voltage present up to the body of the unit. The secondary - welding side is more often than not a separated circuit. One has to be very careful earthing the secondary of the welding transformer as it could cause currents to flow in the installation which could overload the fixed wiring main and equipotential bonding conductors, circuit protective bonding conductors, and cause stray currents to flow elsewhere in the installation which could damage electronic equipment and other appliances. You could also fall foul of the electricity supply regulations by causing interference to be transmitted back onto the grid as noise, spikes etc, these can cause inadvertent operation of the DNO protective devices as well as the protective devices within our installation, and may cause nuisance tripping on RCD devices as the current diverted from the welding circuit, transformer secondary to true earth will not be related to that derived from the transformer primary which is referenced to true earth, this would cause a supply imbalance, thus leakage current causing the RCD to trip (if fitted) this is a common scenario where circuits are incorrectly designed. HTH Paul

It depends on how old the welding kit is, and what standard it was built to. If the secondary is isolated and has double or reinforced insulation, then it is preferable that the workpiece is not earthed for reasons described above. The welding set may be marked as BS638, or EN50.060 or EN60.974 or IEC974. If it is not built to this standard then the workpiece should be earthed so that if the insulation breaks down between primary and secondary the earth can clear the fault, as the workpiece will become live at the mains voltage. I suggest you get hold of a copy of HSG118, Electrical safety in arc welding.