Abstract: An alarm system in which a pair of transmission lines are disposed in a loop to define two channels of transmission from a controller to a group of interconnected detectors, the improvement comprising a spaced group of line fault isolators interleaved with respective sub-groups of detectors and connected across the lines for sensing a short circuit on the transmission lines so as to isolate the short circuit condition; the isolators including an arrangement for switching, in response to a signal transmitted from the controller, those isolators which are physically located adjacent to the short circuit without interfering with the operation of other detectors in the loop.

Abstract: A strobe light circuit is provided for flashing a strobe flash unit at a desired frequency. An inductor is repetitively connected and disconnected across a d.c. power line by a switch means so that energy is stored in the inductor during the period when the circuit is complete. The flash unit and a capacitor are connected in parallel so that the capacitor can discharge its stored energy to the flash unit when the voltage across the capacitor exceeds the threshold firing voltage of the flash unit. The parallel combination of the flash unit and the capacitor is in turn connected in series with a diode, and the resulting series circuit is connected across the inductor with the diode being connected in polarity such that current will not flow from the power line through the flash unit or the capacitor.

Abstract: In an electronic horn for alarm systems of the type in which an alarm condition is indicated by a reversal of polarity in the d.c. power supply, an improvement provides a pair of terminals for connection to said d.c.