In the quest of keeping their property safe, how far can homeowners go? Kathryn McCarthy looks at what the law will and will not allow.

When farmer Tony Martin was jailed for life in April 2000 for murdering a teenage burglar, it fuelled a widespread debate about the lengths an individual could go to in order protect their property.

Some people said he was entitled to the right to security and peace of mind, whereas others believed no one should be allowed to take the law into their own hands. There is a palpable sense of injustice in cases like this, but a balance must be struck between adequate home security and harmful initiatives that can lead to a criminal conviction for the home-owner and a liability exposure for the insurer.

With an array of security products now on the market - from electrified window grilles to trip wires - this creates a grey area in terms of what is legal and what is not.

"Barbed wire and the proliferation of other similar products in this part of the security industry is not as litigious as we think," says manager of risk management services at Hiscox, Peter Gwynn. "Provided health and safety issues are addressed, then such protections can be installed."

Royal & SunAlliance (R&SA) technical manager of risk control Steve Clark says problems arise when policyholders engage in DIY security measures. "For example, mains electrification, booby traps or rigged-up trip wires - these people are applying security devices in a non-secure way. It is not lawful to set man-traps, so in cases like these we would advise our insured to stop doing it from a liability point of view."

Some security initiatives can be less dangerous and present less of an exposure to risk for the insured. Hiscox advises policyholders to consider using plants to help ward off intruders. Prickly plants such as shrub rose, holly bushes and hawthorn are a good deterrent when planted under windows or around the perimeter of a house. Also, external lights can illuminate shadows and leave thieves nowhere to hide. Gravel paths and driveways are a good deterrent because of the noise made by driving or walking on them.

R&SA risk control survey manager Andrew Miller says security devices originating in commercial property security are also making their way into home security. These include smoke-generating devices, which fill an area with a vapour if security is breached, reducing visibility. "The problem with this," he says, "is that it could also present a danger of injury to the burglar, so unless there is clear signing it could lead to problems."

There are a number of devices insurers wouldn't be happy with if they were being used by a domestic insured. These include "disruptors", which are acoustic devices attached to an alarm system. "The volume and frequency of the noise creates so much pain it is impossible to remain in the environment," says Clark.

"The use of something like this would be unacceptable and we would ask them to stop, or we would consider coming off risk. You have to consider the innocent and, as 90% of alarm activations are false, it is too high a risk."

A private member's bill will be presented to Parliament this session, which may legalise the use of static devices such as electric fences, glass on top of walls and anti-climb paint. The move was prompted following a local council that ordered a 93-year-old to remove barbed wire from her house which was put up after she was burgled four times. The bill is unlikely to cause homeowners en masse to erect such devices, but the demand for systems that are legal and safe is growing.

Gwynn says drawing the line between security of one's house and those who seek to commit crime is an ever-evolving one. "It is one which inevitably the courts get involved in to adjudicate. Cases such as Tony Martin's are seen as excesses, but nevertheless indicate the frustration of the public when attempting to defend their property. However, a balance has to be struck and, in my mind, not always in favour of the criminal, since the courts will use sound common sense as to whether the defence used was reasonable or not."

The message is if a homeowner exceeds the level of reasonable force, they are liable for any injury. No one is at liberty to inflict harm other than in self-defence.

However, when an intruder is injured, there is a contributory factor to be taken into account. If they continue even after all the warning signs and if the installation is British Standard-compliant, they have far less potential to claim than if there were no warnings.