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The car boot is targeted first

An explosion would seem, by nature, to be uncontrolled.

But in the past few days officers have carried out several "controlled explosions" in the investigation into the failed car bombings in Glasgow and London.

It was also a method used two years ago on a car left by the 7 July bombers in Luton. The vehicle had a rucksack filled with explosives and the police subjected it to two controlled explosions.

So what happens? After suspicions are raised that a vehicle may contain explosives, a cordon will be placed around the car and all members of the public moved outside it.

THE ANSWER Clear the area Safely gain access to the vehicle Use a robot and a small charge to destroy the initiating circuit and detonator

See how it works

"The aim is to recover the explosives device intact and get as much [evidence] out of it."

First, the inside of the car must be accessed. Opening the boot by hand would be dangerous because of a potential booby trap, so a robot is used to minimise the risk to the bomb disposal officers, says Sidney Alford, an explosives expert.

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If the car has been parked for a few days and presents a very low risk, the explosives could be placed on the vehicle by hand.

Once the vehicle has been accessed, officers prepare for the main controlled explosion. The robot can use its camera to give experts standing at a distance a sight of what is inside.

The robot has a camera and a gun

"The British approach is almost always to try to get the initiating mechanism [eg a mobile phone] and disable it," says Mr Alford.

A policy used in Iraq is to block all mobile phone signals in the immediate area to prevent such devices being detonated remotely.

What the controlled explosion does is remove the initiating circuit and the detonator, says Hans Michels, a professor of safety engineering.

If the main charge goes off then the car would be destroyed

Dr Hans Michels

"If you have a device that is completely assembled and ready to go, it has an initiating circuit, either electrical or mechanical, which gives a high temperature or shock to set off the detonator and the detonator charge will set off the main charge."

A screen such as a heavy steel plate is placed between the initiator and the main charge, to minimise the damage, says Dr Michels.

A successful controlled explosion will leave the main charge intact and blow only the sensitive part of the device away.

The whole exercise can be carried out by the robot, which can also shoot the detonator and disrupt the device that way. That would still be classed as a controlled explosion.