Although the oxidation reactions that release energy in explosive reactions are energetically possible, they do not occur spontaneously.
There is usually some small barrier that must be overcome by the input of energy that will start the reaction, which then will continue by itself until completion.
Sometimes only mechanical force is required like in the case of nitroglycerin. In other situations, it requires heat like from a match or electricity.
For safety considerations, explosive materials are separated into three categories:
Sensitive or Primary Explosives: those which will detonate easily;Intermediate Explosives: those which require slightly more energy to detonate;Insensitive or secondary explosives: those which require relatively more energy to detonate.
Common explosives and their uses:
Primary explosive materials are used to detonate the entire explosive device. That is, they are usually connected to some external device which starts the detonation.
In this capacity, the primary explosive is called the fuse. The energy from the explosive detonation of the primary material is used to set off the booster which in turn sets off the main charge which is made up of secondary (insensitive material).
This combination of a small quantity of sensitive material used to set off a large amount of secondary material is known as the explosive train. It is called a train, because the events occur in sequence. The main charge must be made up of insensitive material for the safety of those handling the device.
In practice, the fuse is rarely stored with the device until it is required for use. In this manner, the device remains relatively safe, since it is only made up of secondary (insensitive) material and cannot be detonated.
Once the fuse is installed, the entire device requires great care in handling to prevent inadvertent detonation. Often, the device is configured so that the explosive train must pass through a small physical port that connects the fuse to the main charge.
This port can be blocked until the device will be used. As an example, the port may consist of two rotating plates with off-center holes. When the plates are aligned, the two holes will line up and permit operation.