Electric arc furnaces have become widely used as a means of melting quantity of material and of carrying out chemical processes requiring high temperatures. The furnace consists of a crucible containing the charge into which dip three carbon electrodes. The electric energy supplied by a 3-phase transformer, together with a limiting reactor, which is frequently connected in the transformer h.v. circuit.Over-voltages have been observed on several furnaces. In some cases these have been ascribed to switching surges, but others have remained unexplained. The author investigated an instance of this latter type on a 10-ton steel furnace. By considering the characteristics of the arc it can be shown that excess voltages can occur when one of the arcs in the furnace iseither extinguished or short-circuited by movements of the solid charge. The over-voltages derived by this means appear to explain the abnormal voltages which were measured.These abnormal voltages are uniformly distributed in the transformer and reactor windings and do not therefore call for local reinforcement of the insulation. The insulation between phases and to earth requires special consideration, and, in general, the insulation class adopted on arc-furnace transformers is higher than that used on power transformers for the same nominal voltage. If the connections in each phase are not perfectly symmetrical, the voltages generated are influenced by the phase sequence.