DETONATORS AND ACCESSORIES

High explosives are initiated by detonators or detonating fuses. A detonator is a small copper or aluminium tube containing essentially a small auxiliary charge of special explosive. A chemical reaction initiated by a flame or electric current in the special explosive can build up very rapidly into an explosion of sufficient intensity to project a detonation wave throughout a high explosion enclosing the detonator.

Detonators are of the following types:

1.  Plain detonators. These are fired by safety fuses, the spark or "spit" from the fuse causing the detonator to explode; these are sometimes called "ordinary" detonators.

2.  Ordinary electric detonators: These are fired by passage of electric current through the detonator. They are further subdivided as:—

(a)  Low tension detonators, and

(b)  High tension detonators (not generally used in mining).

Ordinary electric detonators are of instantaneous type, i.e. without any delay element. They are of copper or aluminium tubes.

3.  Delay detonators: These are essentially low tension electric detonators with a delay element, the object in their use being to phase the firing of shots, so that time and effort are saved in charging and firing several successive rounds of shots. These are subdivided as:—

(a) Half second or long delay detonators, and

(b) Milli-second delay detonators (also known as short delay detonators)

A plain detonator, i.e. non-electric, consists simply of an aluminium tube 6 mm dia. 37 to 50 mm long, filled 1/3 with A.S.A. composition and penta-erythritol tetra-nitrate (P.E.T.N). The A.S.A. composition consists of a mixture of lead azide (A), lead styphnate (S) and a little aluminium powder (A).   The A.S.A. composition initiates the base charge of P.E.T.N. which is a much more powerful explosive.

No. 6 detonator is suitable for normal requirements of mining work. No. 8 detonator is more-powerful than No. 6 but is not generally used.

In an ordinary electric detonator, i.e. non-delay type or instantaneous type, the priming charge and base charge are the same as for plain detonator, but they are tired, not by ignition of a safety fuse, but by passing electric current through a fuse head. The current ignites a flashing composition in the fuse head, which in turn, initiates the priming charge (Fig. 8.2)

The resistance of low tension detonators with a 45 m long shotfiring cable is about 7 ohms. Current required for ignition of the fusehead is 0.5 amps so that a single detonator can be blasted with minimum voltage of 3.5 V. The circuit continuity of a L.T. detonator can be tested by a galvanometer and simultaneous shotfiring of a number of detonators with series connections is possible.

Delay Detonators

In appearance and composition, this is like the L.T. detonator. The L.T. detonator described above is of instantaneous type; the moment voltage is applied to it, the detonator explodes and along with it the enclosing explosive. A delay detonator has a delay element introduced between the fusehead and the priming charge, (Fig. 8.2). The delay element consists of a copper or brass sleeve filled with a special composition which burns at a specified rate and the delay is obtained by varying the length of the sleeve containing the special composition.

Delay detonators of some manufacturers have antistatic sleeve over the fusehead as a protection against static electricity hazard.

The delay detonators and non-delay detonators are distinguished by the colours of lead wires. The delay period is marked on a tag attached to the wires. Moreover the delay number-is stamped on the bottom of tube.

In underground coal mines aluminium detonators are not permitted but only copper detonators should be used.

Permission from the D.G.M.S. is required before using delay detonators in underground coal mines. Delay detonators and non-- delay detonators should not be kept in the same box.

Advantages of Delay Detonators

(1) Reduced consumption of explosive as blasting is more efficient due to availability of a free face for each row or round of shots e.g. blasting due to No. 1 delay detonator gives a free face for the blasting effect of shots fired by No. 2 detonator.

(2) Increased fragmentation and ease of loading the rock or coal. Broken rocks from successive shots collide in air, thereby increasing the fragmentation.

(3) Considerable time is saved in that the whole round of shots is fired in a fraction of a second. This is the chief advantage. If individual shots, or even groups of 5or 6 shots simultaneously by LT. detonators are fired, the time required for inspection and clearance of fumes and gases between successive firings is considerable. In steep seams, the exertion involved in frequent trips for such examinations and connections is saved.

(4) The millisecond delay (short delay) detonators have been observed to produce less ground vibrations than the half second delay detonators and are therefore used in mechanised quarries where blasting of large diameter holes containing heavy charges is likely to produce excessive ground vibrations and damage to nearby surface buildings and other important engine foundations, etc.

Accessories

Safety fuse:

A safety fuse which looks like a cord consists of a core of fine grained gunpowder wrapped with layers of a tape or textile yarn and waterproof coatings. The burning speed is usually 100 to 120 sec/ metre. ICI manufactures a range of safety fuses to suit various Conditions, e.g.Double Bull brand for dry conditions, Blue Sump for damp conditions, OCPS (orange coloured plastic sheathed) and Blue Plastic for wet and very rugged conditions. IDL also manufactures safety fuse (yellow). When one end of the fuse is ignited, it carries the flame at a uniform rate to ignite gun powder or to detonate an ordinary detonator which in turn can detonate a high explosive.

Detonating fuse

For shallow depths, say less than 3 m, and for small number of holes, a detonator is inserted in the cartridge itself and detonated by ignition of safety fuse or in the case of elec. detonator, by an exploder. For a large no. of holes blasted at a time in mechanised quarries and in U/G coal mines electric detonators are used. A deep hole in a quarry needs a long length of detonator leads and to avoid this it is common to use a detonating fuse like cordtex (trade name of ICI). The fuse consists of a core of PETN enclosed in a tape which is wrapped with cloth. The fuse is then completely enclosed in a tubular cover of plastic material which is white for Cordtex and orange for Geocord detonating fuse ( ICI) The detonating fuse looks like a plastic cord; its external dia. is about 5 mm and weight about 20 g per metre length. It has a velocity of detonation of 6500 m/sec. and it is practically instantaneous in its action.

A large number of shots connected with detonating fuse can be blasted by a single detonator. A detonating fuse can be used under water, as moisture cannot penetrate the fuse through the plastic cover. Water may however penetrate into the core through the cut ends which can be guarded against by sealing them with tape or water-proofing compound.

A detonating fuse is often used for demolition operations.

Nonel

The Nonel system of detonation is developed by Nobel AB of Sweeden. Primers of explosives with Nonel detonators inserted in them are charged in the blast-holes and the Nonel tubes are bunched for convenience of connection to the mains blasting system. Upon 'initiation, the shock wave passes down the plastic tubes, the insides of which are coated with reactive substance that maintains the shock wave at a rate of approx. 2000 m. per second which has sufficient energy to initiate the primary explosive or delay element in a detonator. Since the reaction is contained in the tube, this has n'o blasting effect and acts as a signal conductor.

Nonel means non-electric detonator. The flexible plastic tube has 3 mm external and 1.5mm internal diam. The tubes are available in pre-cut lengths. One end of the tube is fitted with a non-electric delay detonator which is crimped to it in the factory while the other end is sealed. The end having detonator is lowered down into the blast-hole while the sealed end projects outside the hole. The sealed end is intituled by detonator or detonating cord.

The advantage of the Nonel system lies in its extreme resistance to accidental initiation by static electricity, stray current, radio transmission, flame, friction and impact. It is also immune to misfires caused by current leakage in conductive ore bodies and eliminates the need for complicated electrical circuit testing and shot-firing equipment.

Raydet manufactured by IDL Chemicals, is just like Nonel.

Raydet

Raydet is a non electric initiating device combining the versatility and advantages of electric detonator and detonating cord. It consists of a plastic tube carrying a very small quantity of explosive material on its inner surface. A high strength no. 8 instantaneous or delay detonator is crimped to one end of the raytube. When initiated, a low order shock wave travels through the tube and initiates the detonator. Raydet can be initiated by a detonator or a detonating cord. A tag indicates the delay number of reydet and a tape fastening the tube in a coil indicates the tube length. Length of tube varies from 3 m to 45 m. The delays are from no. 0 delay to no. 15 delay; No 0 delay is instantaneous. No. 1 delay is 50 ms and No. 15 delay is 625 ms.

When using the raydet, do not cut factory sealed end of ray tube and do not connect two raytubes. One raytube will not initiate another.

Detonating Relays

In opencast workings, detonating relays using detonating fuse for initiation provide a non electric delay firing system. This method avoids the electrical connections which are required when using delay detonators. A detonating relay is essentially an assembly of two open-ended delay detonators coupled together with flexible neoprene tubing in an aluminium sleeve suitable for crimping into a detonating fuse.

 

Inside the detonating relay, the construction is symmetrical with the delay element at either end so that the detonation wave may pass in either direction. The delay interval for each detonating relay varies from 15-45 millisecond. In use, the main or branch line of detonating fuse is cut at the point where a delay is required, and the detonating relay is then crimped between the two cut ends of the line. By judicious selection of the points at which the detonating relays are inserted, any delay firing sequence can be arranged (fig. 8.3). Being non-electric in nature, detonating relays are insensitive to stray current and static electricity,

Cord relays

Delay detonating relays manufactured by IDL Chemicals are known by the trade name Cord Relays

A cord relay has a dia. of 11.5 mm and length of 152 mm. They are available with two delay periods, 15 ms and 25 ms.

Under the Explosives Rules, the various explosives and accessories are classified under the following headings:

Class 1 — Gun powder.

Class 2 - Nitrate mixtures (e.g.-GN/l, Powerflo, Godyne, Pentadyne, etc.)

Class 3 — Nitro compounds, e.g. Blasting'Gelatine, Special Gelatine, O.C.G. permitted explosives, Permex, Gelonite, Powex 80, TNT, gun cotton, PETN, etc.

Class 4 — Chlorate mixtures.

Class 5 — Fulminate.

Class 6 — Ammunition safety fuse, detonating fuse, detonators, delay detonator relays, etc.

Class 7 — Fireworks.

Class 8 — Liquid Oxygen Explosives

Circuit Tester

In electric shot firing before any attempt is made to fire the shots, the circuit is sometimes tested to make sure that there is no open or short circuit. Such testing should be done by approved apparatus and it is important that the current passed during testing should be limited, so that there is no possibility of accidental explosion of the detonators. In addition, ail testing must be done from a safe place and safe distance from the blast site.

An instrument to test continuity of an electric circuit for blasting is the bias to meter manufactured by 1DL chemicals Ltd. It is an electronic solid state circuit tester and is available in two ranges:

(a) 0 to 100 ohms for underground coal mines.

(b) 0 to 1000 ohms for other applications.

Crimper

A crimper is a pair of pliers to crimp or press the end of a plain detonator tube on a safety fuse inserted into it so that the fuse cannot come out of the detonator. It is dangerous to crimp the lube end with teeth.

Shot firing cables

During electric shot firing the leads for the detonator are connected to long shot firing cables (not less than 30 m in length to fire the shots from a safe distance. The cables are twin-core and insulated to withstand at least 250 V.

Other accessories for shot firing include.

(i) Wooden stemming rod to stem the holes.

(ii) A wooden dolly weighted with lead or brass for deep holes in mechanised quarries.

(iii) A scraper made of brass to clean the holes and detect cracks.

(iv) A pricker made of brass, aluminium or wood to prick the cartridge prior to inserting the detonator or detonating fuse.