Control circuits constructed with electronic components, and particularly control circuits having fault protection, have for some time been a part of the state of the art and are used, for example, for controlling presses, punching presses, automated systems and the like. These control circuits pick up fault conditions by using self-monitoring logic circuits, for example, two-channel logic circuits with antivalence monitoring (see German Offenlegungsschrift 27 21 270), or by cyclic monitoring of safety-related components (see German Offenlegungsschrift 26 33 322 or British Pat. No. 1,527,857). Such control circuits are designed for monitoring certain operational conditions such as single-stroke operation with eccentric presses, and in this they guarantee a high degree of operational reliability.
The actuators ad/or the power amplifiers preceding the presses, which actuators and/or power amplifiers are driven by the control circuits, are predominantly provided with semiconductor components. These semiconductor components can be connected to a load voltage supply circuit in such a manner that, if the control circuits and the machines to be controlled are functioning properly, the actuators which cause movements will be driven.
In the case of a fault, however, as a rule the load volatage supply circuits are cut off. This cut-off is a result of action by the control circuit which is protected against faults. Under special operating conditions, for example, during setting-up, the control circuit can be used in such a manner that it is possible to operate the machine even though certain safety circuits (e,g, protective screen up) are deactivated. During such an operating condition the semiconductor components which switch the load voltage supply circuit are connected direactly into the active circuit. Therefore, a failure of a semiconductor component can lead to an unplanned movement, for example, in an automated system, which greatly endangers the operator during setting-up operations.