Rating and testing of power circuit breakers

IN THE PAST FEW YEARS, with field testing continuing at ever-increasing magnitudes of short-circuit capacity, the controversy over the symmetrical versus the rms basis of rating has taken on more significance. The European claim is that American circuit-breaker ratings must be discounted by a factor of two-thirds, and American manufacturers claim a rating based on the highest possible single-loop offset wave brought about by synchronized pretripping. Although these high American ratings are sanctioned by the American Standards Association, there is no proof that a circuit breaker thus rated is capable of interrupting a symmetrical current of the same rms value. Yet, the definition of rated interrupting current as the highest rms current, including the d-c component, which the circuit breaker shall be required to interrupt, indirectly implies that the two ratings shall be equal.