PlantFlow 3D › Guides › Conventional fire alarm zones and EOL supervision
Conventional fire alarm zones and EOL supervision
How a conventional fire alarm panel detects alarms and cable faults using an end-of-line resistor, and what silence, reset and disable do.
A zone is a supervised circuit
In a conventional system each zone is a two-wire circuit from the panel through all detectors and call points, ending in an end-of-line (EOL) device, often a resistor. In the normal state a small, known current flows through the EOL. The panel measures that current all the time.
Alarm, open and short
When a detector or manual call point goes into alarm, it connects an alarm resistance across the line. The zone current rises into the "fire" band and the panel signals FIRE for that zone. If the cable is cut, the EOL is lost and the current falls towards zero: the panel shows a FAULT, because devices beyond the break can no longer signal. A short circuit gives a very high current, also a fault.
The thresholds and resistor values depend on the panel manufacturer. PlantFlow 3D uses typical teaching values and shows the zone current on a meter so you can see each band.
Sounders and outputs
Sounder circuits are supervised the same way. On fire, the panel drives the sounders and operates outputs from the cause-and-effect matrix, for example releasing fail-safe door locks, stopping air handling units or signalling a monitoring centre.
Silence, reset and disable
Silence stops the sounders but keeps the fire indication. A new alarm in another zone re-sounds them. Reset clears the panel only after the device that caused the alarm has been restored. Disabling a zone removes it from service for maintenance and must be recorded, because that area is then unprotected.
Try it yourself
Open the Fire alarm tab, apply test smoke to a detector, then cut a zone cable and compare the zone currents for normal, fire and fault.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.