Page 434 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.34 THE WORK

(A) Single row of holes connected by detonating cord and detonated by electric blasting caps or caps
and fuse attached to detonating cord at A. Note detonating cord loop is tied to trunkline with right
angle connection.

(B) Double row of holes connected by detonating cord. Note detonating cord from back line is tied to
trunkline with right angle connection. Detonated by electric blasting caps or caps and fuse attached
at point A.
FIGURE 9.27 Detonating cord layouts.

if they are not in steady use, as they may deteriorate rapidly in damp storage. Testing is done by
means of a special rheostat which sets up a resistance in the line; and from one to four blasting
caps. If the machine will overcome the rheostat resistance of its rated capacity, and fire the caps
in addition, it is in good condition.
Newer-type blasting machines operate on the capacitor or condenser principle. Current is supplied
by flashlight-type dry cells, or by a 12-volt rechargeable nickel-cadmium unit. This modest cur-
rent is built up to high voltage in condensers, then discharged into the blasting circuit.
The high voltage may be built up when required, by pressing a Charge button or switch, then
used by depressing a Fire button, after a buildup time of 5 to 30 seconds.
Some models automatically build up voltage between uses, and can fire instantly.
Storage or dry cell batteries may be used in emergencies for shooting a small number of caps,
but they are not considered to be adequate or safe for regular use.

High Lines. Where electric lines (called high lines) are available, 220 to 440 volts may be used
for firing. Special switches are made for connecting into such lines. They automatically shunt or
short out the firing lines until the moment the switch is pulled.
Series. There are three basic types of circuit—series, parallel, and parallel series.
When the caps are arranged in series, Fig. 9.28, the current must have enough force, or volt-
age, to overcome in succession the resistance of the lead wire, the caps and their wires, and the
return lead wire, in addition to the variable resistance offered by connections between wires.
The voltage required can be calculated by Ohm’s law. This basic law states that the current, in
amperes, in an electric circuit will be equal to the potential, or voltage, of the power supply divided
by the resistance, in ohms, of the circuit. That is, if sufficient current is supplied at 110 volts to a
circuit with 10-ohm resistance, the flow of current will be 11 amperes. If the voltage is 6, the flow
will be only .6 ampere.
A single cap requires a current of about .5 ampere. A series of caps takes 1.5 amperes, with
sufficient voltage to overcome all resistances in the circuit.
The tables in Fig. 9.29 indicate the resistance of caps and wires commonly used. The supply of
current should be well over the calculated need, however. Minute differences in the bridge wires in
caps may vary their resistance, so that a weak current might burn some of them through and break the
circuit before all are exploded. Be specially liberal if the series includes both regular and delay caps.
Series circuits are easy to lay out, to hook up, and to test.