suitable devices indicated only symbolically in the drawing the control switch 14 of the thermostat 13 can be shorted by a by pass line 19 which is connected at one end between the conductor 11 of the connecting wire and the control switch 14 to the connecting line 12 and ends at the other end to the fixed contact of the switch 18 at the timer 15 within the timer 15 there is a line 20 the one end of which is connected to the movable contact of the switch 18 while its other end is connected between the control switch 14 of the thermostat 13 and the drive 16 of the timer 15 to the connecting line 12 a branch of this line 20 is connected to the movable contact of the double throw switch 17 the fixed contacts of which form the respective end points of two parallel lines in which the drive 21 of the refrigeration compressor and a heater element 22 serving as the defroster heater for the evaporator of the refrigerating unit are disposed next to each other both lines which can be controlled by the double throw switch 17 are connected to the conductor 11 39 of the connecting cable in the circuit diagram shown all switching members i e the control switch 14 of the thermostat 13 as well as the double throw switch 17 and the switch 18 of the timer 15.