Page 438 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.38 THE WORK

FIGURE 9.32 Connecting wires.

expensive wire, to reduce damage to the ends. If not enough surplus wire is available, connecting
wire may be brought on the job on a spool and cut as needed.
The use of scrap wire involves a risk of misfires due to breaks inside the insulation, which is
not justified by the small value of the wire saved.
When the last cap has been connected, the whole series should be rechecked, to make sure that
no hole has been omitted, that no loose ends of wire are lying around, and that connections are
tight. It is good practice to squeeze each connection with pliers at this time.
Bare connections may be propped up on sticks or rocks where necessary to keep them out of
water, or from contacting wet ground. If any connections are unavoidably wet, they may be tightly
taped or smeared with water-resistant grease.
If the shot is only of a few caps in a limited area, and the electrical source is of ample power,
precautions against bare wet joints are necessary only if the water has a high mineral content.

Electrical Hazards. Electric caps are supplied with a connection between the ends of the wires
which is called a shunt. This prevents the accidental buildup of opposite electric charges in the
two wires, which might pass enough current through the cap to explode it.
Such charges may be caused by the near presence of electrical machinery or transmission lines,
a radio transmitter, stray currents in the ground, thunderstorms, or static electricity from dust
storms or escaping steam. Such currents in lead wires may often be detected by inserting a No. 47
radio pilot lamp in the circuit instead of the cap. If it glows, conditions are unsafe.
The best precaution to take in blasting near an electrical hazard is to use fuse caps and Primacord.
However, certain precautions can be taken which will reduce the danger of using electric caps.
Lead or other wires should not run parallel to electric lines.
The shunts should be left on cap wires until they are connected into the blasting circuit, and
the circuit should be shorted until ready to fire.
The two-cap layout in Fig. 9.33 illustrates the method to be used. The lead wire is shorted at
each end. A wire is connected to one lead, and to one leg of a cap. A connection is made from the