Abstract: A failure sensor for a gas detector comprises a direct current amplifier having a feed-back path between its output and its input. The feed-back path includes a negative feed-back circuit. A connection is provided in the feed-back path for connection to a gas detector and is arranged to influence the feed-back factor. There is also provided a sensor for sensing differential voltage between the connection and the feed-back path and the output of the amplifier and for providing an output signal indicative of a fault condition where a predetermined differential value occurs.

Abstract: A three phase ripple control system for feeding a control signal onto each phase line employs an injection transformer having a primary winding and three secondary windings. A control signal generator is connected in circuit with the primary winding and serves to provide a signal thereto. A respective inductor is connected in series in each phase line and is connected in parallel with a respective one of the secondary windings. The inductors have a value of inductance arranged such that their impedance at line frequency is substantially smaller than the magnetizing impedance of the transformer. The secondary winding thereby introduce a common control signal onto each of the three phase lines.