Last edited 08 Jul 2021

Smoke detector

Smoke detector.jpg


[edit] Introduction

According to the fire and rescue service, they are called to more than 600,000 fires a year in the UK, which result in more than 800 deaths and 17,000 injuries.

Fire can be detected by; heat detectors, flame detectors, smoke detectors and multi sensor detectors, or an alarm can be triggered at manual call points.

Smoke detectors (also known as smoke alarms) are self-contained safety devices that can be placed around a building with the purpose of detecting smoke that may be associated with a fire and sounding an alarm to alert occupants.

Approved document B defines a smoke alarm as 'A device containing within one housing all the components, except possibly the energy source, necessary for detecting smoke and giving an audible alarm.'

The Smoke and Carbon Monoxide Alarm (England) Regulations came into force in October 2015. This requires that private sector landlords install at least one smoke detector on every storey of rented properties, as well as a carbon monoxide alarm in every room that contains a solid fuel-burning appliance such as a coal fire or a wood burning stove.

It is the landlord’s responsibility at the start of each new tenancy to ensure alarms are working properly and failure to comply can result in a local authority fine of up to £5,000. The regulations do not specify which particular type must be used, but leave it up to landlords themselves to make an informed decision about which is most suitable for their property.

The Fire and Rescue Service also strongly recommend that smoke detectors are installed in other buildings, suggesting that you are twice as likely to die in a house fire where there is no smoke alarm than a house where there is.

The essential guide to providing adequate fire protection in all types of domestic buildings (including houses in multiple occupation and sheltered housing) is BS 5839 Fire detection and fire alarm systems for buildings. Enforcing authorities and contractors use this standard by following grades, from Grade F up to Grade A, that categorise how comprehensive a fire protection system needs to be depending on how great the fire risk.

[edit] Types of smoke detector

There are a number of different types of smoke detector available.

[edit] Ionisation smoke alarm

This is generally the most cost-effective option. Small particles of smoke produced by fast-flaming fires will be easily detected by this sort of detector, but they are slightly less sensitive to slow burning, smouldering fires that give off a lot of smoke relative to the amount of flaming.

They work by the means of an ionisation chamber being open to the air. Smoke particles inside the chamber disrupt the normal reactions between ions and electrons and shut off the electric current that is otherwise maintained. The circuit detects this and triggers the alarm.

[edit] Optical smoke alarm

This is a more expensive option but is generally more effective at detecting smoke produced by slow-burning fires. The detector is fitted with an infrared beam that shines across an opening in the bottom of the device from a light-emitting diode or LED. The beam shines onto a photocell that generates electricity as long as light falls on it, and so allows a circuit current to be maintained. If a fire starts and smoke particles interrupt the beam, the electronic current is broken and this triggers the alarm.

[edit] Heat alarm

These are more suitable for installing in kitchens where ionisation and optical alarms may be too sensitive. This is not actually a smoke alarm, rather the increase in temperature from a fire is detected. The disadvantage is that a relatively small area of room space can be detected by one device, so several alarms may be needed for a large kitchen.

[edit] Combined detectors

The most common combination detectors are:

Each type of detector is powered by either a battery, mains electricity or a combination of the two. Detectors can be interlinked throughout a property, so that smoke detected in one room will trigger the alarms of others.

‘Hush’ buttons can be found on most modern alarms which allow for a reduction in false alarms, most commonly during cooking. The alarm will still trigger should there be an unusual level of smoke produced.

Standard smoke detectors require batteries to be replaced every 12 months, although some can be fitted with sealed 10-year batteries which are more expensive but mean that batteries don’t need to be changed.

All new buildings must have mains-powered alarms installed, together with a back-up battery. Detectors can also be fitted with an escape light which comes on when the alarm is triggered. For people with hearing difficulties, smoke detectors are available that can be fitted to pillows, making them vibrate if triggered, or a strobe light effect.

[edit] Placement of detectors

The number of smoke detectors required will depend on the particular building, but the general rule is that the more that are installed, the higher the level of protection. Detectors may be installed in:

Heat alarms are most suitable for kitchens and garages, whereas landings are most suitable for ionisation alarms. Optical alarms are recommended for bedrooms and living rooms. Bathrooms are not suitable for any kind of detector due to steam.

One smoke detector may be sufficient for a one-floor property, whereas at least one should be fitted on each level of a multi-storey domestic property, with them all preferably interconnected.

Detectors are commonly screwed onto ceilings, and should be fitted as close to the centre of a room as is possible. The general rule is that they should not be fitted within 12 inches of any wall or light fitting. They must be fitted somewhere that is going to make it easy for occupants to hear.

[edit] Related articles on Designing Buildings Wiki

[edit] External references

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