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In an explosive, pyrotechnic device or military munition, a fuse (or fuze) is the part of the device that initiates function. In common usage, the word fuse is used indiscriminately. However, when being specific (and in particular in a military context), the term fuse describes a simple pyrotechnic detonating device, like the cord on a firecracker, whereas the term fuze[1][2][3] is used to indicate a more sophisticated ignition device incorporating mechanical and/or electronic components e.g. a proximity fuze for an M107 artillery shell, magnetic/acoustic fuze on a sea mine, spring-loaded grenade fuze[1][2][3], pencil detonator or anti-handling device[4].
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The simplest form of fuse is the burning fuse, believed to date back to the 10th century and originating in China, this simple fuse consisted of lightweight paper filled with loose gunpowder, and served as a means of delaying ignition in fireworks. This simple form of burning fuse can still be found today in many modern pyrotechnics. A version of this simple fuse is called visco fuse, and consists of the burning core coated with wax or lacquer for durability and water resistance. The commercial and military version of a burning fuse referred to as safety fuse (invented by William Bickford) is a textile tube filled with combustible material and wrapped to prevent external exposure of the burning core. Safety fuses are used to initiate the detonation of explosives through the use of a blasting cap.
Modern day safety fuses are often used in mining and military operations, to provide a time-delay before ignition, and they more often than not are used to initiate an explosive detonator, thereby starting an explosive chain reaction to detonate a larger more stable main charge. Safety fuses are typically colored black (military) or fluorescent orange (commercial) to distinguish them from detonating cords such as Primacord, which are brightly colored or transparent.
Fuses are found in fireworks, model cannons, matchlock firearms, some improvised explosive devices and many forms of pyrotechnics.
As can be seen from the various accompanying diagrams, most countries use the 'z' spelling[5][6][7][8][9][10] to distinguish between simple burning fuses and more complicated munition fuzes, which contain mechanical and/or electronic components. The relative complexity of even the most early fuze designs (dating from circa 1915) can be seen in the cut-away diagrams at the end of this article. Modern fuzes for "smart weapons" are invariably computerised i.e. incorporate microprocessors such as gate arrays.
A fuze refers to a device used in munitions which is designed to detonate, or to set forces into action to ignite, detonate or deflagrate, the charge (or primer) under specified conditions. In contrast to a simple pyrotechnic fuse, a munitions fuze always has some form of safety/arming mechanism, designed to protect the user from premature or accidental detonation[11].
Types of fuzes include:
Combinations are common, e.g. the RPG-7 has usually an impact (PIBD) fuze and a 4.5 second time fuze; detonation occurs on impact, but not later than after 4.5 seconds.
Military weapons containing explosives have fuzing systems to ensure that they do not initiate (explode) prematurely. In general, the munition has to travel a certain distance, wait for a period of time (via clockwork, electronic or even a chemical delay), or have some form of arming pin/plug removed. Only when these processes have occurred will the arming process be complete. The multiple safety/arming features in the M734 mortar fuze are representative of the sophistication of modern electronic fuzes.
Safety/Arming mechanisms can be as simple as the spring-loaded safety levers on M67 or RGD-5 grenade fuzes, which will not initiate the explosive train so long as the pin is kept in the grenade, or the safety lever is held down on a pinless grenade. Alternatively, it can be as complex as the electronic timer-countdown on an influence sea mine, which gives the vessel laying it sufficient time to move out of the blast zone before the magnetic or acoustic sensors are fully activated.
In modern artillery shells, most fuzes incorporate several safety features to prevent a fuze arming before it leaves the gun barrel. These safety features may include arming on "setback" or by centrifugal force, and often both operating together. Set-back arming uses the inertia of the accelerating artillery shell to remove a safety feature as the projectile accelerates from rest to its in-flight speed. Rotational arming requires that the artillery shell reach a certain rpm before centrifugal forces cause a safety feature to disengage or move an arming mechanism to its armed position. Artillery shells are fired through a rifled barrel, which forces them to spin during flight.
In other cases the bomb, mine or projectile has a fuze that prevents accidental initiation e.g. stopping the rotation of a small propellor (unless a lanyard pulls out a pin) so that the striker-pin cannot hit the detonator even if the weapon is dropped on the ground. These types of fuze operate with aircraft weapons, where the weapon may have to be jettisoned over friendly territory to allow a damaged aircraft to continue to fly. The crew can choose to jettison the weapons safe by dropping the devices with safety pins still attached, or drop them live by retaining the safety pins as the weapons leave the aircraft.
Aerial bombs and depth charges can be nose and tail fuzed using different detonator/initiator characteristics so that the crew can choose which effect fuze will suit target conditions that may not have been known before the flight. The arming switch is set to one of safe, nose, or tail at the crew's choice.
Base fuzes are also used by artillery and tanks for shells of the 'squash head' type. Some types of armour piercing shells have also used base fuzes, as have nuclear artillery shells.
The most sophisticated fuze mechanisms of all are those fitted to nuclear weapons, and their safety/arming devices are correspondingly complex. In addition to PAL protection, the fuzing used in nuclear weapons features multiple, highly sophisticated environmental sensors e.g. sensors requiring highly specific acceleration and deceleration profiles before the warhead can be fully armed. The intensity and duration of the acceleration/deceleration must match the environmental conditions which the bomb/missile warhead would actually experience when dropped or fired. Furthermore, these events must occur in the correct order.
Note: some fuzes, e.g. those used in air-dropped bombs and landmines may contain anti-handling devices specifically designed to kill bomb disposal personnel. The technology to incorporate booby-trap mechanisms in fuzes has existed since at least 1940 e.g. the German ZUS40 anti-removal bomb fuze[14].
 Fuzes fitted to M107 155mm artillery shells, circa 2000 |
 Fuzed 81mm white phosphorus mortar shell in 1980. Note spelling of "fuze" on adjacent boxes |
 British AP Shell Mk XXII BNT (circa 1943) for BL 15 inch Mk I naval gun, showing base fuze |
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 British No. 63 Mk I Time and Percussion fuze, circa 1915 - used in shrapnel shells |
 British No. 100 Graze Fuze for high-explosive shell, World War I. |
 Hotchkiss base percussion fuze for artillery shells, circa World War I |
 British Time Fuze No. 31, World War I, used in trench mortars |
 British Percussion Fuze No. 110 Mk III, World War I, used in trench mortars |
 British No. 131 D.A. (Direct Action) Impact Fuze, Mk VI, World War I, used in anti-aircraft artillery |
 Percussion fuze for an artillery shell |
 British No. 16 D Mk IV N Base percussion fuze, circa 1936 |
 British No. 45 P Direct Action Impact Fuze, World War I, used in howitzer shells |
 Cut-away diagram of Japanese Type 99 Grenade showing fuze mechanism. Circa 1939 |
 Cut-away diagram of a US M2A4 bounding mine showing the M6A1 pressure/pull fuze. Circa 1950 |
 USSR pull-fuze designed for booby-trap or anti-handling purposes. Circa 1950s. Detonator assembly is inserted into explosives |
 Alternative design of USSR booby-trap pull-fuze, usually connected to a tripwire. Circa 1950s |
 USSR pressure fuze for booby-trap purposes e.g. victim steps on loose floorboard with fuze (connected to TNT explosives) concealed underneath. Circa 1950s |
 Italian TC/2.4 mine, showing central location of mechanical pressure fuze |
