Abstract: A controller has two shunt lines connected in parallel with safety relays. For testing their switching capability, the safety relays are reversed to their idle positions and the change in voltage is monitored on their idle contacts. If voltage is missing, an error signal is issued. During the test, the parallel shunt line is closed so that the safety power line is not interrupted. Switching amplifiers having a response and action time which is a fraction of the preset response time of the safety relays control the safety relays. For testing the electrical control, the drives of the safety relays are switched to currentless and the change in voltage is monitored on the drives. If the change in voltage is inadequate, an error signal is issued. The safety relays remain in their operating positions during the test.

Abstract: A measuring instrument (11) for fill level sensors (4) having a measuring electrode (6) and a compensating electrode (9). The measuring instrument (11) has a bridge circuit whose one bridge branch (21) is formed by two series-connected test resistors (23, 24). The other bridge branch (22) is formed by the measuring resistor (17) on the measuring electrode (6) and the compensating resistor (18) on the compensating electrode (9). Between the two bridge branches (21, 22) a diagonal line (26) with a test switch (27) is provided. The measuring instrument (11) is controlled by the control circuit (13) so that the diagonal line (26) is closed outside of preset test time periods, and interrupted at preset test time periods. The end of the diagonal line (26) located between the two test resistors (23, 24) is connected to the base potential (25) of the measuring circuit (12).