Traditional 4 wire system
Traditional 4 wire system
There are three kinds of loops detectors, sprinker, and signaling devices.
Detector loops conductors loop through each device, that is two conductors from the central control box side come in and two conductor to the next device or end-of-line (EOL) resistor go out. The wires must be cut and terminated at each device.The polarity of the devices must be preserved unless they are simple pull stations or other NO contact devices. Devices that use power, smoke detectors for instance, require proper polarity. No TEE taps are allowed. To reset the electronic detectors, the central control box interrupts the supply to a loop for a short time.
The central control box looks for three possible signals:
1) OPEN -- trouble -- loop comprised
2) resistance roughly of the EOL -- OK -- loop is ok, no alarm condition
3) SHORT -- alarm -- a device has signaled fire.
Sprinkler loops send to the central control box:
1) OPEN -- trouble -- loop comprised
2) resistance roughly of the EOL -- OK -- loop is ok, no alarm condition
3) other resistance level -- trouble -- sprinkler valve may be closed
4) SHORT -- sprinkler in operation -- alarm condition
The alarm loops also loop through each device as above. The EOL value may be the same as detector loop, or much lower. The central control box normally supplies the signalling loop with reverse voltage. To initiate the alarm signals, the central box reverses the voltage (to normal) which causes the alarms to sound and/or flash. The individual alarm devices are furnished with a diode (or its equivalent) which detects the polarity. When the central control box does not send out the alarm signal, it looks for two possible signals:
1) OPEN or SHORTED -- trouble -- loop compromised.
2) resistance roughly of the EOL -- OK -- loop is ok, no alarm condition
The central box also looks for ground faults on these loop. It also looks for loss of commercial power. Boxes are typically supplied with 24 hour? battery supplies trickle charged from commercial power.
The central control box often also controls the power to:
1) large HVAC equipment controls to shut it down on alarm
2) fire-door magnetic holders to close doors on alarm conditions (not supervised, fails safe with doors closed).
3) Magnet door locks to release door locks on alarm conditions (not supervised, fails safe with doors unlocked).
This is a brief summary of the 100--200 pages of instructions usually provided.
NOTE: every new central control box must come with "complete" instructions. Read them until you understand them before doing an installation. Old central control boxes should have the instructions available for them at the work site, if not contact the manufacturer for a copy.
Devices like light beam smoke detectors usually have additional requirements which will be in their instructions.
There are also interconnections for hood extinguishing systems, hood exhaust fans, kitchen makeup (air from outside to inside) fans....