What is a smoke detector?
A smoke detector senses the particles a fire produces before there is much heat. It gives the earliest warning of the common detector types, which is why escape routes are usually covered by smoke detection.
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<img src="https://fireoptix.co.uk/glossary/smoke-detector/diagram.svg" alt="smoke detector — diagram from FireOptix" width="880" height="620" loading="lazy">
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<a href="https://fireoptix.co.uk/glossary/smoke-detector">What is a smoke detector?</a> — diagram by
<a href="https://fireoptix.co.uk">FireOptix</a>
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</figure>Early warning is exactly why they go on escape routes: the route has to still be usable when people are told to use it, and heat detection would signal after smoke has already reached it.
The same sensitivity makes them prone to responding to things that are not fires — steam, aerosol, cooking, dust from building work. Those are environmental problems rather than faults, and they are solved by siting and by type rather than by ignoring the alarm.
Different sensing technologies respond differently to smouldering and flaming fires, which is another reason siting is a design decision rather than a matter of spacing them evenly.
In practice
A smoke detector sited above a kitchen door.
It produces unwanted alarms whenever somebody burns toast, so it has been isolated at the panel semi-permanently.
Detector type and siting determine false alarm behaviour, and the usual response to a badly sited detector is to disable it rather than to move it.
The room ends up with no detection at all, which was not the decision anybody thought they were making.
What goes wrong in practice
- Fitted in or next to kitchens, bathrooms or plant rooms where false alarms are inevitable.
- Bagged during works with no record of which were covered or when they came off.
- Repeated unwanted signals tolerated until occupants stop responding to the sounder at all.