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great for that system to be considered. Miners who handle explosives and prepare and fire shots should be properly trained and should be made responsible for carrying out regulations and safe procedure. Where blasting is done electrically, the the actual firing often can be done by a boss from a switch controlling a level or section of a mine. Blasting should be done only at regular blasting periods, except in case of emergency, when permission must be obtained from the boss. There may be two regular blasting periods for each shift, immediately before lunch time and immediately before the end of the shift. Miners should be out of their working places when shots are going off near by to minimize the danger from rock falls due to vibration or from walking into unguarded shots. These periods reduce the danger of returning too soon and should give ample time for smoke and gases to clear. Men should be checked out by bosses during these periods.

ELECTRICAL BLASTING VERSUS FUSE BLASTING

Detonators are used to explode dynamites or other high explosives by the energy liberated as a combination of concussion and heat when the detonator is fired. Commercial detonators have been developed in later years to be less sensitive and more efficient than the older capsules of mercury fulminate.15 A blasting cap for use with fuse consists of a metal casing closed at one end. The pressed charge of initiating explosive in the closed end is exploded when the safety fuse inserted in the open end burns down to the charge. The explosive in modern detonators consists of layers of different explosive mixtures or charges; there may be first a base charge at the end, next a priming charge, and at the top an initiating charge. Various explosive materials are used by different manufacturers to make these charges.

Electric blasting caps have largely the same fundamental constituents as fuse detonators with an electrical heating unit for igniting the explosive charge. A short length of fine electric filament wire is in contact with the igniting portion of the charge, and the whole detonator is sealed to make it impervious to moisture. The fine wire filament or bridgewire is generally an alloy of platinum and other metals, or nickel, chromium, and iron. The bridgewire is made incandescent immediately by the application of a small electric current, thus causing the ignition charge to burn and, in turn, detonate the priming and base charges. Movement of the terminals of the leg wires in the detonator may ignite the charge by friction; therefore, the leg wires should not be jerked or worked loose.

Delay electric blasting caps usually have a powder train inserted between the bridgewire assembly and the explosive charge. The interval at which delay detonators fire after the electric circuit is closed depends on the burning of the powder train; it may be spaced from 1 to 20 seconds.

Electrical blasting is used widely in shaft sinking and driving long raises and is general practice in wet places. The number of mines in which it is used for all classes of work is increasing. The advantages of electrical blasting are:

All persons can be in a place of safety before firing is done.
Misfires are reduced and delayed shots largely eliminated.
Heavy smoke and gases from burning fuse are eliminated.
More efficient blasting can be done in wet places.

15 Grant, R. L., Structural Features of Typical American Commercial Detonators: Bureau of Mines Rept. of Investigations 3696, 1943, 30 pp.

The dangers or disadvantages of electrical blasting are:

Possible premature application of current to the blasting circuit.
Stray currents may be encountered.

For blasting with fuse the advantages are:

Less equipment is needed.

The cost of supplies in ordinary blasting is slightly lower; where long fuse lengths are used the difference in cost is negligible. Blasting may be done safely where stray currents can not be eliminated easily.

Disadvantages of fuse are:

Men may overstay the time limit in attempting to light fuse.
There are more chances of misfires and hangfires.

Premature shots are more frequent.

More smoke and fumes are produced.

There is greater danger of fire unless special fuse lighters are used in place of matches or open lights.

The foolish practice of short fusing is all too easy to follow. The advantages are with electrical blasting; and it is probable, therefore, that this method gradually will supersede further the use of fuse. POWER SOURCES AND POWER CALCULATIONS FOR ELECTRIC BLASTING

POWER SOURCES

In electric blasting the usual sources of current are power circuits, blasting machines, or storage batteries. Power circuits may be alternating or direct current and include mechanically driven portable generators. The capacity of the power source, in volts and amperes, may determine the method of connecting the shots to be fired. The type of power available may also limit the number of shots that can be fired at one time by a particular power connection.

The characteristics of electric blasting caps make it necessary to supply current above a minimum strength (0.5 ampere) to assure the firing of all shots. A strong electric current will heat the bridgewire of an electric blasting cap to the temperature necessary to fire it almost instantaneously, while a weak current will take longer. With extremely weak current the time might be several minutes. As all electric blasting caps vary slightly in sensitiveness, one of the more sensitive caps may fire enough in advance of the others to break the circuit before the rest of the caps have received enough current to fire them. This trouble may occur when too many detonators are connected in the same circuit but is eliminated by allowing for a sufficiently strong current to reach each cap to compensate for any slight variation in sensitiveness. The wiring circuit should be planned so that all caps will receive a generous excess of current.

OHM'S LAW

The current that will flow in any electric circuit will equal the potential of the power supply divided by the resistance of the circuit. This is Ohm's law, which is expressed by the equation,

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Rcircuit resistance, in ohms.

METHODS OF CONNECTING BLASTING CIRCUITS

The two primary methods of connecting blasting circuits are series

and parallel. Combinations of both may be used for large groups of shots or under special circumstances, but consideration of these combinations is not needed for ordinary blasting.

SERIES CIRCUITS

A series circuit is one in which the leg wires of the caps are connected to each other in a continuous circuit and the two free end wires are connected to the lead wires from the firing circuit. In this

12 electric blasting caps connected in "series"

and fired by a blasting machine

FIGURE 14.-Electric blasting; series connections.

type of connection each cap receives the same current, since there is only one path for the current through every cap in the series (fig. 14). This method has the advantages that less wire is needed for making connections, that broken wires or connections can be readily detected with a circuit tester, and that the resistance of the circuit can easily be computed. Disadvantages which may arise are that some caps might fail to fire if the current supplied is too weak for that circuit, or if current leaks from uninsulated joints of leg wires so that only part of the current reaches the bridge of some caps.

PARALLEL CIRCUITS

A parallel circuit is one in which one leg wire from each cap is connected to one side of the blasting circuit and the other leg wire of each cap is connected to the other side of the circuit. In this kind of connection each cap offers a path for the current which will divide equally through each cap if the resistance is the same in each (fig. 15). One advantage of a parallel connection is that each cap is independent of the other caps in the circuit and variations in sensitiveness will not affect the success of a round. Tracing wires from hole to hole is not necessary as in checking a series connection; counting the

wires connected to each bus wire will give a check on the holes connected. Joints do not need to be insulated, as with series connections, but no direct short circuits can be allowed, as too little current would reach the bridge wires to fire the caps. Disadvantages of a parallel circuit are: A high-amperage current is necessary, the entire circuit cannot be tested with a circuit tester although each cap can be tested during loading, and more wire may be required than with series connections.

The parallel method reduces the probability of misfires and is Power line

Double-throw switch

Shunt

12 electric blasting caps connected in "parallel"
and fired from a power circuit

FIGURE 15.-Electric blasting; parallel connections.

preferable where practical, but it cannot be used with a blasting machine because of the greater requirement of current.

COMBINATION CIRCUITS

Various combinations of series and parallel connections can be made which are used to accommodate the requirements of the blast to the power supply available. By using a suitable combination, such as the parallel-series connection, it is possible to fire more than 50 caps at one time with a single application of the current by a power circuit (and with certain limitations by a blasting machine) (figs. 16 and 17).

Graded parallel-series circuits are made by placing any number of caps in a first series and increasing the number progressively in each added series circuit. These circuits are connected in parallel to the power source. This method supplies much stronger current to the first series of caps, because of its lower resistance, than to the second series and more to the second than to the third. Thus, the different series will fire progressively if application of the current is continued. The

time interval between the firing of the several series is negligible as a rotation factor, and use of the method is controlled by patent.

CALCULATIONS OF POWER FOR ELECTRIC BLASTING
POWER REQUIREMENTS FOR SERIES CIRCUITS

The current supplied with a series connection should be at least 1.5 amperes; this requirement does not increase with an increase in the number of caps, but as the resistance is increased in proportion to the number of caps there must be a corresponding increase in voltage.

SERIES-IN-PARALLEL CONNECTION
Forty electric blasting caps connected
"series-in-parallel" and fired by a power line.
in the above layout there are 4 caps in each
series circuit and 10 series circuits-in-parallel

FIGURE 16.-Electric blasting; series-in-parallel connection.

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PARALLEL-IN-SERIES CONNECTION

Thirty electric blasting caps connected

"parallel-in-series" and fired by a power line.
In the above layout there are 10 caps in each

parallel circuit and 3 parallel circuits-in-series

FIGURE 17.-Electric blasting; parallel-in-series connection.

The principal requirement of a series connection is a voltage high enough to overcome the resistance of the circuit; the required wattage (volts times amperes) is low.

When firing is to be done with a blasting machine, the total resistance of the circuit must not exceed the capacity of the machine. A blasting machine is a portable generator of definite limits and is rated according to the number of 30-foot copper-wire electric blasting

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