All optical smoke detectors have a second light source as redundancy. If one LED fails, the detector can detect smoke, keeping the same sensitivity
A failure signal warns to replace the faulty detector. All loop devices have built-in isolators. If a short circuit or a wire breakage occurs, they sense the voltage drop or the line resistance change and the nearest isolators open, isolating the faulty loop segment. Not a single signal will be lost, as signals go from c to a and from d to b.

At the same time a trouble signal warns the operator to inform the maintenance staff to repair the fault. A redundant standby board processor unit replaces the main board Loop interface processor if a failure occurs.

A trouble signal warns to replace the faulty Loop Interface.A redundant standby central CPU replaces the main Central CPU if a failure occurs.
A trouble signal warns to replace the faulty CPU board. The FAP Display and Control panel is redundant of the FAM Monitor and event processing.