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 gas level indicator incorporates two Taguchi-type sensing elements, one of which is to receive air to be sampled and the other of which is insulated therefrom. A comparator circuit receives the outputs of the two elements, and an indicator actuated by the output of the circuit announces the result of the comparison. Preferably, both sensing elements are mounted in a single housing, and an elongated pipe and a fan or aspirator provided to form a probe for drawing air to be sampled from a remote position to the one element.

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.