Loop Isolator: Role on an Addressable Fire Alarm Loop

Loop isolator is a small device fitted at intervals along an addressable fire alarm loop that automatically disconnects a faulty section of cable from the rest of the loop, preventing a single short-circuit fault from disabling every device beyond it.

An addressable loop without isolators behaves badly under cable faults: a short-circuit anywhere on the loop pulls the supply voltage down across the entire run, and every device on the loop disappears from the panel's view. Isolators solve this by sensing the abnormal current draw of a short and opening their internal switches, partitioning the loop into the faulted segment (which is reported but disconnected) and the healthy remainder (which keeps polling normally). When the fault clears, the isolators automatically reset and restore the segment.

Installation standards in the UK and Ireland set a maximum number of devices that can be permitted to fail under any single short-circuit fault, which in turn drives the maximum interval between isolators on the loop. Most modern detector bases include an integral isolator, eliminating the need for separate inline modules and ensuring that every device is individually protected. Counting devices between isolators at design stage is one of the simpler but most often skipped calculations on a small addressable project.

For the loop architecture itself, see addressable fire alarm systems. For the loop concept in generic terms, see addressable loop.