Fire alarm systems are the smoke detectors, heat detectors, alarm buttons and sirens in a building, wired to a central panel that decides when to raise the alarm. Sprinklers, which spray water on a fire, are a separate system, but they are connected to the same panel. In India, a large building needs a no-objection certificate (NOC) from the state fire service before people can occupy it, and the fire service checks the fire alarm system before it issues one.
A smoke detector usually works with a small light inside a chamber. When smoke enters, the smoke particles scatter the light onto a sensor. A heat detector reacts to a high temperature instead. It is used in kitchens, where cooking smoke would set off a smoke detector. A manual call point is the red box on the wall that a person presses on seeing a fire. A carbon monoxide alarm detects carbon monoxide, a poisonous gas with no smell, which can build up in a basement car park.
Older systems divide a building into zones, and the panel shows only that "zone 4" has an alarm. Most large buildings now use addressable systems, in which every detector has its own address. So the panel shows exactly which detector went off.
Imagine a mall in Ahmedabad with 1,800 detectors. At 9:40 p.m., detector 112 on loop 3, in a restaurant kitchen on the third floor, reports smoke. The panel first raises a pre-alarm, only in the fire control room. A guard is sent to check. If nobody cancels it within a few minutes, or if a second detector or a call point goes off, the panel raises the full alarm. Sirens sound on every floor, and a recorded message tells shoppers to leave by the nearest exit.
The panel then sends the alarm to other systems. The building automation system (BAS) stops the air handlers, sends the lifts to the ground floor and releases the exit doors. A critical event management system can send a message to every employee's phone and track who has reached the assembly point outside. Factories and plants also give workers portable gas detectors. A portable detector warns one worker about a leak near that worker, while the panel warns the whole building.
False alarms are a real problem. If the sirens sound every week because of burnt food, people stop leaving when they hear them. The two-stage pre-alarm exists for this reason. The panel's records also show the maintenance team which detectors are dusty or faulty, so that they can be cleaned or replaced on a planned visit.