PlantFlow 3D › Guides › Fire alarm basics: conventional zones, addressable loops and EOL resistors
Fire alarm basics: conventional zones, addressable loops and EOL resistors
Easy fire alarm guide: detectors, call points, sounders, conventional zones with EOL resistors, addressable loops with isolators, and how faults are found.
What a fire alarm system does
Detectors (smoke or heat) and manual call points sense a fire and signal the fire alarm control panel (FACP). The panel switches on sounders and beacons, and outputs such as door releases, lift recall and fire-damper control (the cause and effect).
Conventional zones and the EOL resistor
In a conventional system each zone is a two-wire radial circuit. The last device has an end-of-line (EOL) resistor, for example 4.7 kΩ, so a small monitoring current flows when everything is healthy. A detector in alarm adds a low resistance, the current rises and the panel shows FIRE for that zone. A broken cable removes the EOL current and the panel shows FAULT; a short circuit shows a fault too.
The panel only knows which zone is in alarm, not which detector, so zones are kept small (often one floor or area).
Addressable loops
In an addressable system every device has its own address and the cable runs as a loop that leaves the panel and returns to it. The panel talks to each device digitally, so it shows exactly which detector is in alarm. Because the loop is fed from both ends, a single cable break does not lose any device. Loop isolators split the loop into sections so that a short circuit disables only a short section.
A loop has limits on the number of devices and on the current; where a site needs more, extra loop cards or a loop extender/power supply are added according to the manufacturer’s data.
Testing and faults
Weekly tests operate one call point per zone; detectors are tested with test smoke or heat. Typical faults: open circuit, short circuit, missing EOL, earth fault, a detector removed from its base, and sounder circuit faults. The standard to follow is the local one (for example BS 5839-1, NFPA 72 or EN 54 products).
Try it yourself
Open the Fire alarm tab and press “Learn step by step”. Test a detector with smoke, silence, reset, then cut a cable and see the 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.