Page 422 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.22 THE WORK

But the greatest danger of accidental firing of electric caps comes from electricity. Even with
their wires shorted by a soldered shunt in the original package, a very powerful nearby current
might detonate them unexpectedly.
After the shunt is opened to connect to other wires, there is danger from any stray electric current,
even from radios.
The content of a cap is small, but one can blow off fingers and toes, and flying particles of the
copper case may cause injury to personnel within a radius of 30 feet. The most serious danger in
an accidental explosion of caps, however, is that of setting off primers or nearby heavy explosives.
Caps should be buried before exploding for test purposes.

Primacord. Primacord is a detonating (exploding) fuse, made up of a core of an insensitive high
explosive, pentaerythritol-tetranitrate, that is called PETN for short, surrounded by a protective
wrapping. Primacord is detonated by means of a blasting cap. The explosion travels along it at a
rate of about 21,000 feet per second, and detonates any cap-sensitive explosives with which it is
in contact. See Fig. 9.15.
It is produced in a number of types that are classified according to explosive content in grains
per foot, and/or the type of protective covering around the explosive core.
Standard types usually have 50 to 60 grains per foot. Lighter grades, including “E-Cord” with
25 grains, are used chiefly for secondary and very shallow blasting. Heavier ones, with 100 to 400
grains, are used for continuous column initiation of ammonium nitrate–fuel oil mixtures, or for
cutting into short lengths for use as primers.
Wrappings are rated on the basis of strength and water resistance. All are water-resistant, but
plastic-coated ones are essentially waterproof except at cut ends. The plastic is also resistant to
oil, an important point when using an AN-FO mixture.
E-Cord and Primacord with 45 or more grains will fire when wet with water or oil if it is ini-
tiated from a dry spot. Wet Primacord can be initiated only with a very powerful primer, such as
80 percent gelatin dynamite or a special booster. It will not maintain detonation through a knot
connection.
Cut ends of Primacord will pick up some moisture from capillary attraction, usually only a few
inches in. If the cord is lower than the water level, it may in time become soaked all the way through.
Textile-reinforced Primacords are usually used in ordinary downholes, wire reinforcement in
rough and jagged ones.
The individual lines in the holes, called branch or down lines, are usually connected to the
blasting cap by a trunk line of Primacord. Fastening is done by simple knot connections. It is

FIGURE 9.15 Primacord is supplied in boxes and rolls.