Double wound transformers provide a means of isolating equipment from the mains supply and are a valuable aid to safety when employed to supply mains to hazardous areas and test benches. Transformer supplies can be dangerous if contact is made between both conductors.

Auto-transformers and variacs do not isolate. If these are wrongly connected the whole apparatus may be raised to live voltage with respect to earth. It is important to make sure that one side of the supply is taken from the same end of the winding as the neutral of the supply and not the live lead. It is important to take great care not to damage the winding of the variac.