a is connected via an electrical lead 24 and the clothing strip 20 b u2032 is connected via an electrical lead 25 to a counting device 26 the counting device 26 is able to determine the number of contacts between the card clothing 4 a and the clothing strip 20 b u2032 per unit of time an electrical power source for example a battery 27 is present in the lead 24 the counting device 26 is connected via an electrical lead 28 to a contact comparator 29 to which furthermore a limit value setter 30 is connected the comparator 29 is able to compare the number of contacts determined by the counting device 26 with a number of contacts preset in the limit value setter 30 the output of the comparator 29 is connected to the input of an electronic control and regulating device for example the machine control 31 when a limit for the number of contactings per second is exceeded the card k is switched off by a shutdown device 32 in those respects the apparatus corresponds to that of fig3 the metal clothings 4 a and 20 b u2032 act like a switch in an electric circuit the battery 27 can produce for example a low voltage of 5 v in the electrical circuit in the example of fig5 in the lead 24 there is a device for measuring capacitance 36 which is connected via a lead 41 to a capacitance comparator 37 to which furthermore a capacitance limit value setter 38 desired value setter is connected via lead 42 the capacitance comparator 37 is able to compare the actual capacitance c 1 or c tot measured in the circuit with a preset desired capacitance.