Page 433 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.33

FIGURE 9.26 Lighting a fuse with a match.

Fuse does not light readily with a match because of the small area of powder exposed, and the
likelihood of wax from the coverings being spread over the end while it is being cut.
If only a few fuses are to be lit, good results can be obtained by splitting each fuse with a knife or razor
blade, as in Fig. 9.26, bending the fuse so the opening is down. It may then be lit with a match. Care should
be taken to keep the fingers out of a line with the end of the fuse, as it will spit out a jet of flame.
The split fuse can be ignited more readily by having the opening horizontal or upward, placing
a broken-off match head between the halves, and squeezing them together, as in (B). The match
head gives a much hotter flame than the stick.
It is possible to buy a number of devices which simplify the lighting of fuses. The match lighter
is a short paper tube which fits over the end of the unslit fuse and is coated on one end with a com-
pound similar to that of a safety match head. This is readily ignited with a match or the edge of a
match box, and subjects the fuse end to intense heat.
This lighter throws a jet of flame which resembles that caused by ignition of the fuse. To tell
whether the fuse is actually burning, it is necessary to observe a moment later whether a thin stream
of smoke is issuing from it. If it is not smoking, the lighter should be removed and another applied.
The pull-wire lighter is of similar material, but clamps on the fuse and is ignited by pulling a wire.
The lead spitter is a coil of thin lead tubing containing black powder. A piece is cut off, lit with a
match, and the resulting hot flame used to then light the fuses. The lead melts back as the powder burns.
The hot wire is similar to a fireworks sparkler. It burns slowly with a very hot flame, and is the
safest and most dependable of the devices listed.
A burning fuse will light black powder by contact. When used to explode dynamite, it must be
connected to an explosive cap.

ELECTRICAL FIRING

Electrical firing requires a complete circuit from the power source through all the caps and back
to the power source. (See Fig. 9.27.) One cap, or several hundred, may be used.
A cap may detonate the charge through a primer in the borehole, or initiate a detonating cord
hooked up with a number of holes.
Source of Current. One standard source of electric energy for blasting is a blasting machine. This
may be a generator which delivers high voltage when a handle is pushed or twisted vigorously.
These devices are rated according to the number of caps in series which they can fire at one
time. Rating is usually conservative. Their efficiency should be tested occasionally, particularly