Loss of excitation and control and subsequent damage to exciter windings in an excitation system for dynamoelectric machines wherein one or more rotating diodes fail in a shorted mode is prevented through the use of an RC circuit for detecting AC voltages in an exciter field winding for the purpose of directly operating a circuit breaker to remove excitation to the field windings and supply or to activate a timer for such purposes while temporarily shorting the AC current through the exciter field winding.