Fuse, safety

Safety fuse is made from fine grain blackpowder, although certain processes use mill cake, or indeed blackpowder ingredients themselves. The normal process is illustrated further in Fig. 13.1. 

The semi-fuse thus produced is waterproofed by passing through troughs of molten bitumen or by coating also with plastic.For higher grade safety fuse, a further countering of textile, such as cotton, is followed by a final varnish, which may be coloured for identification purposes. 

To obtain the best results, the fuse powder should have a relatively low potassium nitrate content, and a typical formula would be 

Such a powder gives less smoke than the 75% nitrate composition and eliminates side sparking. 

During manufacture it is important that the fuse should not be allowed to stand in the molten bitumen, or the core may be penetrated. The time for such faults to develop depends on the construction of the fuse, and is shorter the lower the potassium nitrate content of the powder and the 

The primary safety considerations in the formulation, manufacture, and use of explosives are that they must never explode prematurely but always explode when desired. Often this balance is achieved with an insensitive main charge being initiated by the least amount of a more sensitive explosive. This concept is expanded to that of an explosive train in which a series of decreas-ingly sensitive explosives is ultimately sufficient to initiate an insensitive main charge. The sensitivity of explosives to initiation provides a common means of classification  

Other explosives may be added to explosive trains to sensitize certain components or add a time delay. There are as many combinations as necessary to 

Mercury fulminate detonators were widely replaced due to their extreme sensitivity. Contemporary simple nonelectric detonators contain a small quantity of primary explosive (often lead azide) which, once ignited by a fuse, ignites the more powerful base charge of a secondary explosive (an additional low explosive may be used between the fuse and the primary explosive). 

Often another controlled volume of low explosive is placed between the matchhead and the primary charge. This bums for a specified period introducing a delay (from a few milliseconds to a few seconds) into the detonator. However, any low explosive delay device syn. delay electric igniter) introduces imprecision because the chemicals are affected by moisture, temperature, and time. High precision delay electric detonators implant the match-head directly into the primary explosive and control the delay with an integrated circuit within the detonator. 

Detonators for ammunition include primary explosives to detonate the main charge. They may or may not be co inected to a fuze. 

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Abschmelzdraht, m. fuse wire, fusible wire. Vbschmelzeu, v.i. melt off, melt. fuse, separate by melting seal off, seal. — v.i. melt off. Abschmelz-konstante, /. fusion coeflScient. -achweissung, /, flash welding, -sicherung, /, (Elec.) safety fuse. 

Miner s Safety Fuse (Meches de Surete). See Bickford Safety Fuse... 

Although safety fuse is reliable and relatively constant in burning speed, a number of shots fired with this aid will naturally detonate at slightly different times. This can be an advantage, particularly if the times of detonation are varied by altering the length of safety fuse, or by ensuring... 

Many other methods of making lead azide in a safe form have been described, but the only one to have found commercial importance consists of replacing the dextrine by a small proportion of gelatine. When properly made this form of lead azide is as safe to handle as the dextrinated form and has improved sensitiveness to flame. It can therefore be used by itself in electric and delay detonators, but not in plain detonators as it is not ignited with certainty by safety fuse. 

The initiating explosive used must ignite with certainty from the spit of a safety fuse. It must be remembered that the intensity of the spit can be reduced if the safety fuse is not cut squarely and also that the fuse may in practice not always be fully inserted into the detonator. Lead azide by itself is not sufficiently easily ignited to give a satisfactory plain detonator and it is therefore used in admixture with lead styphnate, which is very readily ignited by flame. The proportions of such mixtures vary from 25 to 50% of lead styphnate. Mercury fulminate and diazodinitrophenol are sufficiently sensitive to flame not to require such additives. 

The normal initiating charge in a British detonator is lead azide modified with gelatine. In the case of plain detonators a small proportion of lead styphnate is added to the azide to ensure satisfactory ignition from safety fuse. 

As the blackpowder core of a safety fuse bums, it produces gases which must escape. At the same time the heat of the combustion melts the bitumen and plastic and thus produces side venting through the textile layers. This results in the production of an increased but constant gas pressure, determined by the equilibrium between gas generation and gas lost sideways. As the rate of burning of blackpowder depends markedly on the pressure, it is this process of equilibration which determines the speed of burning of the fuse. 

With a properly constructed safety fuse the core is almost impermeable to gases, but should faults occur, either during manufacture or due to mishandling afterwards, cracks may develop which enable the hot gases to... 

It follows also that the actual rate of burning of safety fuse depends on the ambient pressure. Indeed, if the pressure is reduced to less than about a fifth of an atmosphere the burning ceases altogether. In deep mines the extra pressure can be sufficient to give an increase in burning speed of safety fuse. Compared with the effect of pressure other influences on the burning speed are small. Temperature has little effect and humidity also has little effect unless the fuse is kept for a prolonged period at a humidity sufficient to cause deliquescence of the potassium nitrate in the core. 

The gas evolved by safety fuse consists mainly of carbon dioxide and nitrogen with some carbon monoxide and oxides of nitrogen. The amount of gas produced is likely to be 15 to 20 ml per cm of fuse. 

Because of the extreme importance of regularity and freedom from failure which must characterise safety fuse, detailed and exhaustive tests must be carried out on the product. Certain controls are obvious, namely, measurement of powder charge and of burning speed, both before and after immersion in water. Other tests usually carried out include resistance to cracking on flexing at low temperatures round a mandrel, coil tests in which the fuse is bunched into flat or complex coils and freedom from failure after immersion in water is determined. New types of fuse are usually tested to indicate the amount of smoke produced and also to determine the adequacy of the end spit. The end spit is the projection of particles of burning powder from a cut end and is of importance because it provides the mechanism which enables the fuse to ignite a detonator. 

For some purposes, particularly in fireworks, instantaneous or almost instantaneous transmission of flame is required without detonation such as is characteristic of detonating fuse. Instantaneous fuse is used for this purpose, deliberate use being made of one of the possible faults in safety fuse, namely, the rapid speeding up which can occur if the core of the safety fuse is porous. 

If the outer covers of safety fuse are pierced, e.g. by falling rock, water can penetrate and stop the combustion of the fuse. Much work has been done in efforts to obtain a waterproof fuse which will obviate this difficulty. This work, although not successful in its original purpose, has led to the development of igniter cord which has proved of considerable value in secondary blasting (see p. 148). Two types of igniter cord are manufactured, fast cord with a burning speed of 3 s m1 and slow cord with a speed of 30 s m-1. 

Most blackpowders contain 75% of potassium nitrate, 15% of charcoal and 10% of sulphur. For safety fuse, however, blackpowders are made with reduced amounts of potassium nitrate. Also manufactured are sulphurless powders, containing approximately 70% of potassium nitrate with 30% of charcoal. These sulphurless powders are used for ignition purposes where sulphur could cause corrosion of metallic components. 

End spit. The flash of burning material ejected from safety fuse when burning reaches a cut end. 

Igniter cord. A cord for igniting safety fuse. 

Safety fuse. A fuse which propagates by slow burning. 

The techniques described are not so complex as to require a chemical laboratory or machine shop however, many of them do assume access to basic demolitions and incendiary supplies, such as explosives, time (safety) fuse, detonating cord, blasting caps, and flammable materials, and to a few household tools. 

Explosives may be detonated electrically or nonelectrically. A nonelectric firing system will consist of a blasting cap, a length of time (safety) fuse or a firing device attached to the cap, and a means of activating the system match, fuse lighter, delay mechanism, or trip wire. The electric system requires an electric cap which has two wires attached, perhaps additional wire, and a battery or batteries to provide the current which activates the cap. 

Time fuse, or safety fuse, which is used in most nonelectric firing systems, is available in a multitude of varieties, both in color and rate of burning. 

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