Abstract: Surveillance of signal changes, especially for self-checking control systems. It is essential in self-checking control systems to create equipment to monitor signal changes and allowing to detect all conceivable initial errors or defects and to allow beginning a dangerous state or a dangerous motion of a machine only when the input peripherals evince, not only for instance static signals, but after definitively at least one change of signal has taken place and after this input signal was successfully checked dynamically for its suitability in beginning the dangerous state or the dangerous motion of a machine. For that purpose a special circuitry is proposed in which at least one signal source as well as relays and relay contacts are arrayed in such manner that both self-checking signal detection and voltage-free transfer of at least one checked signal to a higher safety level shall take place.

Abstract: An electrical command or information device housing of the type adapted to be affixed to an assembly plate through a substantially circular bore hole thereof which selectively includes or excludes a radially inwardly opening keyhole slot or notch includes a rib which projects radially outwardly from the housing and is adapted to be received in the keyhole slot. The rig has high shear resistance in a direction substantially tangential to the housing, but includes at least one notch for effecting relatively low force break off of the rib upon the application of radial or axial forces thereto.