Intruder alarms

An intruder alarm installed and used properly can deter burglars - though it's not a substitute for good locks and security. The law now requires that alarm bells or sirens cut out after 20 minutes; some alarms may continue with a flashing light.

An alarm consists of three main elements - the detector devices, the control panel and the warning device, usually a bell box. Detection devices include:

• magnetic contacts - for use on external doors and accessible windows
• sonic detectors - triggered by unusual sounds like breaking glass
• passive infra-red - recognises any change in the ambient level of infrared energy (heat) in a room.

If you decide to have an alarm professionally installed, get specialist advice and a number of quotes. Use a company affiliated to an independent inspectorate:

• AISC
• Integrity 2000
• NACOSS
• SSAIB

You can install a system yourself for as little as £100, although it may not provide the same level of protection as a professionally installed system, and can't be linked to a central monitoring station. Your insurance company may insist on professional installation, so check with them first.

Badly fitted alarms can create problems in themselves: don't fit one unless you're experienced and competent at DIY. The best type to install yourself is a radio alarm, which doesn't need any wiring.