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How Do Fire Alarms Work?

How Do Fire Alarms Work?

We all know what a fire alarm is. But have you ever stopped to think 'how do fire alarms work'? You have? Excellent, because this week we will tell you all you need to know in our blog. 

How Do Fire Alarms Work: The Basics

A fire alarm system is the ‘head office’ for all the fire alarm components in a property.
It is made of 4 sections


  • Smoke Alarms & Heat Detectors: Basically, the parts that detect heat and smoke
  • Sirens and Lights: These (obviously) produce the loud alarm noises and any flashing lights
  • Fire Control: For example, fire extinguisher systems or sprinklers
  • Electricity supply unit and all accompanying cables

The alarms and heat detectors, when a particular amount of smoke or heat is detected, activate. This then activates the sirens and fire control systems. All of this being powered by the electricity supply unit.

So, lets look at how smoke alarms work…

Smoke Alarms

Every fire alarm system has a smoke alarm and how they work depends on which of the two types of smoke alarm they are:

Ionisation Smoke Alarm

This type of smoke alarm has two electrodes (tiny plates) that connect to the positive and negative parts on a battery.
They also contain a very small amount of radioactive material (most commonly, an isotope called Americium-241)
What this material does is to convert molecules in the air, into positive and negative ions. This means they now have an electrical charge.

The negative ions go towards the positive electrode and the positive ions to the negative electrode. All of this completes a circuit.

All good so far.

However, ions really, really, like smoke. If smoke gets into the alarm, they immediately leave the electrodes and become best friends with the smoke. This breaks the circuit and activates the alarm.

image of fire alarm for blog by Mayfair Security on How Does A Fire Alarm Work


They are more effective at detecting small amounts of smoke produced by fires that burn quickly, not as effective with fires that are slow to burn.

Ionisation smoke alarms are the cheaper of the two but are known to be a bit too keen to activate in or near kitchens.

Optical Smoke Alarm

Optical Smoke Alarms work in a different way. Inside these alarms is an LED light beam (a bit like a laser pointer or tripwire) that goes from one end of the alarm to the other, and a light sensor.

If smoke gets into the alarm it trips the light beam, sending it in all different directions, including straight into the light sensor. This activates the alarm.

They cost more than ionisation alarms but are much better with big smoke particles that come from slower burning fires.

Heat Alarms

As the name suggest, they detect increases in heat, not smoke. Inside these alarms is a temperature gauge, which activates the alarm if heat causes the gauge to reach a certain temperature.

You can have both smoke and heat alarms in one fire alarm system to utilise the benefits of both.

If there is more than one alarm in a property, they can be linked. So, if an alarm goes off in a room at the top, it activates all the other alarms in the building.

Fire alarm systems can also be activated manually, we’ve all seen the ‘break glass here’ panels in properties.

Most systems should also have a back up power battery system, in case of a power cut or damage to the electrical supply.
Fire alarms can also be set up to send an alert to the fire brigade. Or monitored twenty-four hours a day by a Fire Alarm Monitoring Centre.

All smoke and heat alarms will need to be regularly maintained to ensure they are working correctly. Most will come with a regular maintence package from the supplier.

Fire Alarms For The Hard Of Hearing

If you are hard of hearing or deaf, there are alarm systems that have a pad that vibrates if the alarm is activated and with flashing lights and escape lights. Escape lights show a path to an exit and help you to see where you are going if it is dark or the room is filling with smoke.

image of fire alarm for blog by Mayfair Security on How Does A Fire Alarm Work

So, there we are. We hope that we have given you all the information needed to explain how fire alarms work!

How Mayfair Can Help

Here at Mayfair we provide fire alarm systems and services for both domestic and business use. You can have a look at what we can offer on our Fire Alarm page on our website.

Get In Touch

If you would to speak to us about fire alarms for your business or home, we have a team of experts ready to answer any questions you may have.

You can send an email to Alternatively, telephone us on 0800 917 9385 or use our online contact form here.

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