When a fire is detected, either by a smoke detector, a heat detector, or a call point, the fire alarm system sound an alarm throughout the building, or in selected area and notifies external organizations and personnel, such as the fire brigade or the buildings security and safety personnel. When a fire is detected output devices will be activated as required.

Class B wiring is the more popular of the two, and allows a free run of loop wiring between the control panel and devices in the field.  This method is cheap, quick, and flexible, but its main shortcoming is in large fire alarm systems, where a short in a loop may disable many devices.

This can be overcome by Class A wiring.  Fire alarm system wired in Class A is suitable for fire detection that require an extra level of redundancy that is not provided by Class B wiring. Class A allows wiring the detectors and call point such that a single short or open in the SLC loop will not disable the detection process. Fire alarm systems that are wired in Class A return the wires via a different path to the control panel. Class A wired fire alarm systems include line isolators that automatically isolate any short in the wiring, while the rest of the loop functions without disruption. The control panel will notify of this condition so that maintenance personnel can rectify the problem.

Class A fire alarm system are slightly more costly than Class B wiring, due to extra hardware and wiring required, however the redundancy and peace-of-mind they provide easily justify the additional cost.

Class A is suitable to almost all fire alarm systems – all that is required is proper wiring.