Abstract: In order to provide a dependable check on the reliable functioning of contacts of switches or relays which control a control device, to ascertain whether they occupy the desired rest contact position and/or have a sufficiently high mains voltage, the rest contacts of the two switches can be connected in parallel by way of a respective electrical resistor to a pole of the voltage source and both resistors can be connected in series to a charging capacitor which, upon the attainment of a given electrical charge or voltage (threshold voltage) activates the control device and in particular feeds a threshold switch or itself serves as such.