The method involves measuring a current flowing through a primary line which comprises a monitoring resistance (R) connected in series with a reference resistance (R1). The circuit of the resistances is supplied by a voltage source (Uo, EMV) with an internal resistance or a current source (10). The quotient of the voltage drop at the reference resistance and the voltage drop (UR) at the primary conductor are monitored for a deviation from a nominal value, and are used for the activation of the alarm. The voltage drop at the reference resistance is determined from the difference between the voltage drop at the series connection and the voltage drop at the primary conductor (US).
