Fusehead

The leading wires are attached to a fusehead, originally a separate entity and containing a bridgewire surrounded by coatings of flashing composition. 

The flashing device, without leading wires, is known as a fusehead and its construction and properties will be considered first. 

The first successful type of fusehead was invented by Krannichfeldt in Germany. This sandwich type of construction is used in many countries, including Great Britain, and is illustrated in Fig. 10.2. 

The manufacture of a sandwich fusehead proceeds in the following manner. Brass or other metal foils are fixed on each side of a sheet of pressboard with a suitable adhesive. The pressboard is then stamped into combs of the shape shown in Fig. 10.3 and steps are cut in the tips of the heads. Fine resistance wire is stretched across the heads and soldered to the foil on each side of the pressboard. These operations were originally all carried out by hand now many are carried out mechanically. 

The first dip given to a fusehead is known as the flashing composition and is of particular importance. Originally copper acetylide was used for this purpose, but it has been superseded by more stable materials. Three common compositions are based on lead picrate, lead mononitroresorcinate and a mixture of charcoal and potassium chlorate respectively. These materials are suspended in a solution of nitrocellulose in amyl acetate and amyl alcohol, known as Zapon. One or more dips, with intermediate drying, give a layer of suitable thickness. 

Zapon. The fusehead is finally given a coat of a nitrocellulose solution as a protective layer and this coat may well be coloured with pigment so that the type of fusehead can readily be identified. 

For detonators to be used where electrostatic charges may occur, it is desirable to prevent sparking from the fusehead to the case, should a high voltage be generated on the leading wires. For this purpose, an insulating sheath is inserted, either into the detonator tube or else immediately around the fusehead. 

When an electric current is passed through a fusehead, the sequence of events shown in Fig. 10.6 occurs. After a time known as the excitation time, the fusehead ignites and this may or may not cause rupture of the bridgewire. The time it takes for the bridgeware to be broken is known as the lag time and this may equal the excitation time, or be as long as the... 

The curves of Fig. 10.7 apply only to fuseheads made to the same specification. If fuseheads are made with different bridgewires or different flashing dips, the corresponding curves could be completely different and there may be no point at which the minimum lag time of one type of detonator exceeds the maximum excitation time of the other. Under these conditions series firing would not be practicable and it follows that it is not feasible to use different types of detonators in a single series firing circuit. 

Some delay compositions are difficult to ignite and ordinary fuseheads may not be adequate for this purpose. In such cases special fuseheads are used which commonly contain cerium powder, or some similar additive which burns with the evolution of a large amount of heat. 

In the design of the detonator attention must also be paid to the effects of the liberation of gas which, though small, is still sufficient to require attention. As the speed of the delay composition is affected by pressure, it is necessary that the free space in the detonator should be carefully controlled. Also, the plug which seals the leading wires in place must withstand this pressure for more than the delay period of the detonator. This task is made more difficult by conduction, along the metal walls of the detonator shell, of heat liberated by the fusehead and the delay composition. Plastic plugs in particular are liable to soften and be ejected from the detonator with probable failure of burning of the delay column. 

Delay detonators for use in coal mines must be constructed so as not to ignite methane/air mixtures even if iired accidentally outside a cartridge of a blasting explosive. This requires a suitable selection of fusehead and the provision of the delay element in a form which will not produce large particles of hot slag on burning. In the British design the delay elements are... 

Excitation time. In firing electric detonators, the interval between the application of the current and the firing of the fusehead (see p. 112). 

Zapon. A solution of nitrocellulose used in fusehead manufacture. 

Electric detonators are also used for detonation of high explosive charges. They are similar in design to other types of detonators except for the presence of an electric fusehead consisting of a bridgewire made of chromium and nickel. The bridgewire is covered by a heat-sensitive pyrotechnic mixture protected by varnish insulation. Standard fuseheads have electrical resistance of 1.2 to 1.4 ohms and... 

Figure 10.1 Wirebridge fusehead (bottom), quickmatch (middle) and piped match (top).

Lead mononitroresorcinate (LMNR) fusehead initiating substance... 

When fired electrically, fireworks are connected to wirebridge fuseheads. The fuseheads can inadvertently ignite if subjected to radio hazard (RAD HAZ) when in the vicinity of radio frequency sources such as cellular (mobile) telephones or walkie-talkie radios. Further advice on this should be sought from the makers of the wirebridge fuseheads and of the communication equipment because the characteristics of all these items can vary to some extent. 

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