An earth-leakage circuit breaker has a totalizing current transformer, a magnetic trip device and switchgear with main current contacts for the main leads. A current-signal generator, a current sensor, a time-signal generator and an electronic timer wherein the switching value and response time of the circuit breaker are measured at predetermined intervals, compared with reference switching values and response times and, if these reference values are exceeded, an alarm is promptly given. During the measurement, a locking device prevents the main current contacts from opening and/or when predetermined limiting switching and response times are exceeded, the locking device opens the main current contacts and subsequently acts as a lock preventing the contacts from closing again, which is equivalent to fail-safe behavior. In addition, the switching value of the circuit breaker is multiplied by its response time and the product, which may exceed a particular constant value, is used for monitoring purposes.