Mutual interlocks are known in particular as accessories for circuit-breaker configurations with, for example, two switches arranged alongside one another. The interface between a circuit breaker and an interlock is in this case frequently formed by a control-disk/plunger mechanism. The control disk may be integrated in a switch-internal drive shaft; the interlock is driven, and the switch is prevented from being switched on, via this drive shaft. In this case, the plunger enters the circuit breaker and forms the connecting element between the switch-internal control disk and the switch-external interlock. A blocking device with two plungers is used for mutual interlocking of the two switches. When a circuit breaker is switched on, its plunger is moved by the control disk from its nil-point position to its opening position. At the same time, the other plunger is moved, for example by positive coupling, from its nil-point position to its interlocking position. When the plunger has reached the interlocking position, the associated circuit breaker is blocked, that is to say it is no longer possible to switch the circuit breaker on. For correct operation of the interlock, it is necessary for the contact surfaces of the control disk and the plunger to each be located at precisely predetermined positions. Any discrepancy is possible only within tight tolerances, which leads to considerable sensitivity to manufacturing tolerances and wear. The positions of the control disk and plunger must in each case be readjusted for correction.
The “floating” nil-point position is responsible for the tight tolerances. Although this is defined by the control-disk/plunger mechanism, there is, however, no physically usable reference for this. When the mechanism is in the rest state, then the plunger is in each case held in the nil-point position by the interlock. If parts and components of the interlock do not correspond to their nominal value (shape and position tolerances, wear), then this influences the position of the plunger, that is to say it deviates from its nil-point position.