What Every Firefighter Should Know About Explosives - Part 2

Firefighters should know about the triggering mechanisms of such bombs, as well as about bomb threats and bomb searches.


Many bombing incidents involve improvised explosive devices (IEDs), or homemade bombs. Firefighters should know about the triggering mechanisms of such bombs, as well as about bomb threats and bomb searches. Triggering mechanisms of IEDs can be located inside or outside of small or large...


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Many bombing incidents involve improvised explosive devices (IEDs), or homemade bombs. Firefighters should know about the triggering mechanisms of such bombs, as well as about bomb threats and bomb searches.

Triggering mechanisms of IEDs can be located inside or outside of small or large bombs. Bombs can look like explosive devices or be disguised as such objects as books, briefcases, letters or packages.

There are a variety of triggering mechanisms. It is important to leave untouched any suspicious-looking object and to notify the appropriate bomb disposal authorities:

  • A tremor switch is a device that senses vibrations around bombs.

  • An anti-probe switch prevents any metallic object, like a knife, from cutting into a bomb. Surrounding the bomb are two conductive layers of aluminum foil, with a non-conductive layer of such material as paper in between. A metallic object that cuts through the two conductive layers completes the circuit, detonating the bomb.

  • A baro switch completes the circuit when a bomb is subject to a certain atmospheric pressure. The most common one is a balloon encased in a container. As the balloon expands, it pushes a conductive plate into two electrical leads and completes the circuit.

  • A thermo-static switch operates at a given temperature and can be as simple as a wall thermostat.

  • A collapsing circuit detonates a bomb if any wire in the circuit is cut.

  • A magnetic switch operates if any metallic item is passed over or near the bomb. A simple stud finder will work.

  • A photo-electric switch operates if a light is shone into a package or if a package is opened, exposing the inside to light. Photo-electric switches can be bought in almost any electronics store.

  • An anti-opening switch will detonate a bomb if the package is opened or if a drawer or door is pulled open. A simple method is to use a clothespin with two thumbtacks wired to its open jaw. The jaws are held open by a wooden peg. The peg is either glued to the top of a lid or tied by a string to a door or drawer. As the peg is pulled out, the two jaws snap together and the circuit is complete, detonating the bomb.

  • A timing switch can be nothing more than a clock which, at a set time, activates the rest of a circuit. This allows the bomber time to place the bomb without setting it off prematurely. Once the bomber has hidden the bomb, he sets the timer to allow him enough time to leave the area. When the timer runs down, the bomb is armed. A second timing switch is used to activate the bomb.

Booby Traps

Several other items have been used against police officers and firefighters.

A firefly, which is technically not an explosive, consists of a gelatin capsule filled with calcium carbide and sodium that is dropped into a gas tank of a police car or fire truck. When the water in the gas tank dissolves the capsule, the calcium carbide mixes with the water to form acetylene gas. The sodium reacts with the water to burst into flame and ignite the acetylene gas. The resulting explosion ruptures the gasoline tank.

A shotgun shell with a BB taped over its primer and a fin or streamer on the top is sometimes dropped from roof tops onto firefighters below.

Fire extinguishers can also become explosive devices. For example, during an arson fire in a San Antonio, TX, pizza parlor, the author discovered a fire extinguisher filled with gasoline. Fortunately, there was no pressure since the powder had been removed.

Bomb Threats & Searches

The increasing number of bombings involving IEDs requires the public to know more about bomb threats and bomb searches. Today, any individual or organization may be involved in a bomb threat or in a bomb search.

Those in authority must be especially prepared for bomb threats. They should understand the motives of bombers and the two ways bomb threats are communicated.

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