Fire Alarm System

Last month in our series on the elements of fire prevention and fire safety we looked at sprinkler systems. This month we begin to look at the Fire Alarm system.

A Fire Alarm System is designed to alert us to an emergency so that we can take action to protect ourselves, staff, and the general public. They comprise the detectors and the alarm system itself. This month, we will look at the different types of smoke detectors.

Different Types of Fire Alarm Detectors

At the core of a fire alarm system are the detection devices, from sophisticated intelligent smoke detectors to simple manually operated break glass units, there are a wide array of different types, but we can divide them into groups including:

  • Heat detectors
  • Smoke detectors
  • Carbon Monoxide detectors
  • Multi-sensor detectors
  • Manual Call Points

Heat Detectors

Heat detector can either work on a fixed temperature basis, where it will trigger an alarm if the temperature exceeds a pre-set value, or they can work on the rate of change in temperature.

Commonly heat detectors work in a similar way to an electrical fuse, the detectors contain a eutectic alloy which is heat sensitive. When a certain temperature is reached the alloy turns from a solid to a liquid which in turn triggers the alarm.


Smoke Alarms and Detectors

There are three basic types of smoke detection alarms.

  • Ionization detectors contain two chambers, a reference to compensate for temperature, pressure, and humidity changes; and a second containing a radioactive source which ionizes air passing through the chamber. When smoke enters, the current drops triggering the alarm.
  • Light scattering detectors comprise a photocell and light source, separated from each other by a darkened chamber. When smoke gets into the chamber, it causes the light to scatter and fall on the detector triggering the alarm.
  • Light obscuring detectors work in a similar fashion to light scattering detectors except the smoke prevents the light reaching the photocell thereby triggering the alarm.

Other fire detectors include

  • Carbon Monoxide Detectors which sense CO2 generated by the fire and work on an electrochemical cell which detects CO2 but not smoke.
  • Multi-Sensor Detectors combine inputs from both optical and heat sensors and process them using a software built into the detector circuitry.
  • Manual Call Points such as Manual Call or Break Glass Call Point is a device which enables personnel to raise the alarm by breaking the frangible element on the fascia; this then triggers the alarm.

Contact Firebrand for your fire equipment needs, a safety audit, risk assessment or recommendations on a fire safety system for your building.