In this method, the voltage outage load-side is an opening of the circuit breaker, and the circuit breaker is able to interrupt the flow of electric current in an electric connection including an electric conductor. The auxiliary unit comprises a current sensor for measuring the intensity of the current flowing in the electric conductor and a first detection means for detecting the voltage outage load-side from the circuit breaker. The method comprises the following steps: measuring the intensity of the current flowing in the electric conductor, detecting the voltage outage load-side from the electric circuit breaker, determining a cause of the detected voltage outage load-side from the circuit breaker, said cause being selected from the group consisting in: an electric overload, a short-circuit and a voltage drop.