Is the European Circuit-Breaker Rating System Really More Conservative than the American? [includes discussion]

A European power circuit breaker is given two interrupting ratings, one for the interruption of symmetrical currents only and one, usually about 25 per cent larger, for the interruption of asymmetrical currents only. In contrast, an American power circuit breaker is given a single rated interrupting current at rated voltage which applies to both symmetrical and asymmetrical currents, and which is numerically the same as the rated symmetrical current of a corresponding European breaker. The larger rated asymmetrical interrupting current of a European circuit breaker is not accompanied by a correspondingly larger rated making current and therefore can be used only when the current to be interrupted is increased without increasing the initial current. To use the full 25-per-cent extra, the relays and breaker must operate rapidly enough to separate the breaker contacts in about 2 cycles of a 60-cycle wave after the short circuit starts. If the relay time plus breaker opening time is 4 cycles or more, as is assumed for 8-cycle breakers in American-recommended application procedure, a typical circuit will not give an asymmetrical current which, at the time of contact parting, exceeds in value the rated symmetrical value, without exceeding the rated making current at 1/2 cycle. Consequently, in most cases this higher value of asymmetrical interrupting capacity, not accompanied by higher making capacity, can not be used.