Abstract: An electrical fail-safe filter consists of a transistor amplifier with a frequency-selective positive feedback circuit such as a resistance capacitance network shunting the collector and emitter of the transistor such that failure of any component of the feedback circuit results in a reduction in the overall gain of the stage. The filter is described in its application to a fail-safe receiver for a railway signalling system.

Abstract: Signals such as track signals in a railway signalling system are monitored by antiphase sequential application to an enabling input of a control circuit which starts the operation of a timer, providing an output signal only if the control circuit is enabled for a time interval during which all the signals being monitored are present sequentially. Absence of any one of the signals in this interval results in a fail-safe condition, resetting the timer, which then fails to provide an output. In the application to a railway signal monitoring system absence of any one signal would be indicative of occupation of an associated track section, resulting in a `danger` condition being signalled.

Abstract: A fail-safe timing circuit has a timing capacitor which is charged progressively in steps through a switching arrangement to cause a biasing potential of a threshold device to change progressively. This biasing potential is normally outside the switching range required to cause changeover of the threshold device, and is only brought within this range after a timed interval determined by the capacitor. If the capacitor fails in any sense, the biasing potential fails to reach the switching range and the threshold device is not triggered.