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.