Abstract: A method of discriminating an arc detected in metal-clad equipment includes detecting an arc, generating an arc-appearance signal, transmitting the arc-appearance signal to a computer, and determining a time lapse T. The time lapse T is determined according to the formula t.sub.o -t.sub.a ; t.sub.o is an instant at which the computer receives a circuit breaker trigger order, and t.sub.a is an instant at which the arc-appearance signal is first acquired by the computer. The arc is determined to be an internal arc (1) if the time lapse T is positive, or (2) if the time lapse T is negative and its absolute value is less than or equal to 10 ms. On the other hand, if the time lapse T is negative and its absolute value is greater than 10 ms, variation of the arc-appearance signal is analyzed to discriminate the arc.