Experience in service and at short-circuit testing stations has demonstrated that the behaviour of circuit-breakers when breaking shortcircuit currents depends on a number of factors which have been termed conditions of severity. They include circuit power-factor, recovery voltage, earthing conditions, and current asymmetry, and are specified in certain clauses in current testing specifications.The paper deals with a further condition of severity which has been acknowledged but is not specified in specifications, namely the restriking- voltage transient and its important controlling attribute, the ate of rise of restriking voltage. The aim of the paper is to place this severity factor in its correct perspective by stating its effect on the performance of plain-break oil circuit-breakers, oil circuit-breakers fitted with arc-control devices, and air-blast circuit-breakers. Compared with the relatively small effect that rate of rise has on plainbreak and arc-control oil breakers, the importance of the effect of rate of rise on the performance of air-blast breakers is emphasized, and recommendations are made as to the utilization of this important factor in the rating, testing and selection of these breakers.A method is proposed for testing certain types of high-voltage airblast breakers whose breaking capacities are above those for which testing stations are normally equipped.