The release unit includes a current converter (WE) and a switching transistor (TR) which is controlled via a voltage comparator (SK). The converter and the transistor accomplish short-circuiting of the input (NE) of the power supply. The voltage across a measuring resistance (MW) is measured using microprocessor (MP). The voltage comparator provides ready-to- measure signal (MS) which controls a time circuit (ZS). The time- circuit allows the power supply input to be short-circuited for a minimum duration. The ready-to-measure is simultaneously supplied to the microprocessor for determining the measuring resistance voltage value during the short-circuit duration.
