Abstract: A single-wire loop alarm system in which the status of each alarm switch can be determined and which is relatively immune to the effects of noise. False alarms are avoided by incorporating an integrator in the loop potential test circuit; during polling, however, the integrator is switched out. Low-frequency noise is discriminated against by verifying the existence of an alarm condition; the loop current is doubled and, if the alarm condition is being caused by an impedance-introducing open alarm switch, the loop voltage change should double. Polling is made more reliable by relying on a "window" test, rather than a simple threshold test.