A class wiring is a specified requirement of SANS 10139 but isn’t this a waste of cable and difficult to implement on site?
The purpose of “A” class wiring in conjunction with short circuit isolators is provided for integrity of the detection circuits.
By running the loop cables out of the panel in one direction, keeping them apart from the return cable to the panel by coming from a different direction reduces the probability of the cables being cut or damaged. The added facility of installing short circuit isolators in between detection zones ensures if a cable goes short circuit only that zone / area of the building will be affected and lose communication. This facility is achieved by the panel communicating outward via the “A” leg to communicate with all the devices, if the isolator is activated the system reverses communication and talks down the “B” leg ensuring communication in both directions.
Installers fool the panel by running just one cable and linking the “A” output and the “B” output together so when the panel tries to communicate down the “B” leg it is redirected back down the “A” leg which is damaged. A large part of the loop could be out of order and not possible to communicate with.
There are exceptions to the “A” class wiring rules where the cable loops are routed separately and may not be run in the same conduit. These are:
- For a distance that does not exceed 3 m and where the outgoing and return conductors enter or exit the initiating device, notification appliance, or control unit enclosures.
- Where looped conduit systems are provided, single conduit drops to individual devices is permitted. i.e. drops to a manual call point.
- Where looped conduit systems are provided, single conduit drops to multiple devices installed within a single room not exceeding 100 m2 in area are permitted. i.e. 3 detectors in a small room.
In short you cannot run your cable route down a centre trunking and feed off and back to a detector in the same conduit if the distance is more than 3 m, nor can you run the outgoing loop in the same conduit, tray, or duct as the return loop.
By Keith Norgate – FSIB Fire Systems