When a low‐pressure arc, considered as an electrical conductor, is “overloaded,” it opens circuits with a suddenness which can create surges of many thousands of volts. Starting from the theory of the arc for moderate current densities, four factors are seen to be possible causes of such a limitation, namely : the longitudinal pressure gradient, a pressure difference arising in double sheaths, the transverse pressure gradient, and the magnetic pinch effect. These are discussed relative to the scant experimental material available and found to be of the right order of magnitude to form the basis of an explanation in various cases.