Smoke Detectors
Smoke alarms save lives and every household should have at least one.
There are different types of smoke detectors and different legal requirments to have them.
Basic detection
What Sort of System should I have?
The 3 basic types of system used in dwellings that are 3 stories or less, have a floor area of less than 200m2 and that are NOT Houses of Multiple Occupation are
Grade F
This is the most basic protection and consists of one or more battery powered detectors.
Pros - Easy and cheap to fit.
Cons - The batteries are often removed and not replaced, either due to false trips or low battery power.
Grade E
Mains powered interlinked detectors with no battery backup/standby power
Pros - There are no batteries to replace
Cons - They do not work in a power cut, occupiers can turn them off by turning their power supply off. Cost of installation
Grade D
Mains powered interlinked detectors with battery/standby backup
Pros - Works in a powercut.
Cons - Batteries will still need to be replaced (unless the unit uses an integrated rechargable backup), cost of installation.
Grades A, B and C
Grade A, B and C systems are fire detection systems that have a central contol panel. Whilst they are in principal no more difficult to wire up than normal interlinked smoke alarms they are usually used in offices, HMO, schools etc and so are not covered in this article.
Legal Requirements and other recommendations
Whilst there is no legal requirement to have smoke alarms in older houses, houses built after 1992 were required to have grade D or E systems installed. Houses having some types of building work carried out (eg loft conversion, 2 storey extensions) are also required to have grade D or E systems.
Where to Locate Detectors
What rooms should they be in
In an ideal world, all rooms would have a detector for the fastest alert, giving extra valuable minutes to escape. The low cost of detectors makes this practical now.
One per floor is the next best option, and is popular. The detector is put somewhere central, in a corridor or open space, so it can detect fires in adjoining rooms as soon as possible.
The least effective configuration is a single detector on the ceiling at the top of the stairs.
Whereabouts in the room should they be
They need to be placed where normal air currents will reach them. The usual guidance is 30cm from any corners and lightfiting, however the manufactueres instructions must be followed. Don't put them too close to a ceiling fan, or dust buildup will hinder them.
Kitchens
Kitchens are harder to cover than other rooms, due to the burning of food and rapid temp rises due to cooking. A fixed temperature heat sensor is the best option in an interlinked system; although its detection ability is relatively poor, it won't suffer the numerous false alarms that other types will. Optionally an additional standalone ionisation alarm positioned at the far end from the cooking area will give early alerts before a fire makes signisificant progress, or often when its about to break out but hasn't yet.
Where minimum cost cover is the plan, an ionisation detector can be placed at the furthest end from the cooking, and if false alarms occur, jsut move it a bit further away until false alarms stop.
Types of detector
The type of detector used is a compromise between the ability to detect the type of fire that may occur and to eliminate false alarms. Smoke detectors alarm much faster than heat detectors, and are used in most rooms & circulation spaces. Heat detectors are used where false alarms could occur with smoke detectors, such as in garages, kichens and laundry/utility rooms.
Types of Smoke Detector
Smoke detectors may be used in any room or area of a dwelling, other than kitchens, bathrooms and shower rooms.
Ionisation
Ionisation smoke detectors are the cheapest available detectors. They are sensitive to cooking fumes and should not be placed in rooms next to kitchens.
Ionisation detectors are better at detecting fast flaming fires are less sensitive to smouldering fires or fires some distance away.
Optical
Optical smoke detectors detect light being scattered by smoke particles. They may give a false alarm if exposed to steam and should not be located close to poorly ventilated bathroooms.
They are more sensitive to smouldering fires and are recommended for use in the circulation spaces (BS5839-6 2008).
Dual sensors
Ionisation and optical are sensitive to different types of fires, so combining both in one alarm gives the best possible detection speed.
Types of Heat Detector
Heat detectors are used in places that are unsuitable for smoke detectors such as kitchens and boiler rooms. Heat detectors take longer to respond to a fire than smoke detectors and are a compromise to reduce false alarms
Fixed Temperature
Fixed temperature heat detectors operate when they reach a preselected temperature (often 58degC). These are the most common type of heat detector in use and would be used in areas where rapid temperature fluctuations may be experienced (eg a kitchen when cooking.) The downside is that it takes a blaze to reach 58C.
Rate of Rise of Temperature
Rate of rise heat detectors operate when the temperature rises at an abnormally fast rate (about 7 deg C a minute), rather than just at a fixed temperature and can operate at a lower temperature than a fixed temperature detector. They may however not respond to slow burning fire and often have a built in fixed temperature sensor as well.
These detectors are no good for kitchens where opening an oven door will cause a rapid change in temperature.
Carbon Monoxide Detectors
Some CO detectors may be used as part of a fire detection system. CO detectors are most sensitive to smouldering fires and fires in which the rate of burning is controlled by the supply of oxygen. Although not often used inside dwellings as a means of fire detection thay may be suitable for certain store rooms and boiler rooms as they are unaffected by steam and dust.
Electrical Installation Guidelines
Grade E systems require the detectors to be powered from their own MCB/RCBO at the main CU. Grade D systems may be run from a local lighting circuit but should incorcorate a means of isolation to allow maintainance on the detectors and leave the lighting circuit powered.
Neither type of system should be powered from an RCD that is also used for sockets outlets.
The interconnecting cable is usually 3 core and earth cable, however T&E plus a single is sometimes used. When installed in conduit then single cables would be used. RF interlinked detectors are available that do away with the interconnecting cable
Batteries
Keeping spare batteries makes replacement instant. Batteries usually need replacing every 1 to 2 years. 10 year lithium batteries are available that makes this unessesary. Mains powered detctors are available that use capacators or rechargable batteries and so need no battery replacement
Chirping Alarms
When the battery is reaching the end of it's life then the alarm will chirp every few minutes to warn you. Annoyingly this most often happens at night when the temperature drops reducing the voltage of the battery. Since usually all the detectors sound the same, if you don't get up and find it, it will only annoy a few more nights. Just get it done.
Maintenance and Replacement
Test regularly using the test button and not with a flame. Detectors should be cleaned with a vacuum cleaner every 6 months to keep down the buildup of dust. Some detectors have a lifespan of about 10 years and usually have a replacement date on them.