The Journal of Explosives Engineering - July/August 2022 - 24

100 years ago
type of blasting machine is particularly
well suited for much coal-mining work.
Leading and Connecting Wires
Connecting wires are used to join the
wires of electric blasting caps when they
are not long enough to reach adjoining
holes; leading wire is used for completing
the circuit between the connected
blasting caps and the source of current.
These wires, as well as the cap wires, may
be either copper or iron. Copper leading
and connecting wires are preferable and
should always be used on large shots. If
iron wires are used, they should be of a
size corresponding to the resistance of
the equivalent length of copper wire, inasmuch
as they have a much higher resistance
per foot. Insulated copper wire,
No. 20 or No. 21 Brown & Sharpe gage,
is suitable for connecting wire; No. 14,
Brown & Sharpe gage, insulated copper
wire either single or duplex is commonly
used for leading wire.
Duplex leading wire consists of two
insulated copper wires with an outside
insulation. It weighs approximately twice
as much as the same length of single
wire, and is often more convenient to
handle. Where the leading wire is apt to
be damaged by flying rock, however, it
is better to avoid duplex wire. Short circuits
are caused by damaged paired cables.
The insulation makes it difficult to
locate such short circuits in duplex leading
wires; it is, therefore, simpler and
better to use instead, two separate wellinsulated
wires of the proper diameter.
With this arrangement, the chance of
having a short circuit is reduced and any
injury to the wires more readily located.
It is better to suspend leading wires near
the top of tunnels or drifts. If they are
laid on the floor, moisture or contact
with rails and other metallic articles may
cause short circuits.
When connecting electric blasting
cap wires, the ends of the bare wires
should be scraped with a knife, the wires
joined with a long twist, and twisted together
tightly to keep down resistance
to a minimum. When connecting the
ends of the electric blasting caps or
the connecting wire to the main leading
wires, the detonator or connecting
wire should be wrapped tightly around
the end of the leading wire about 1 in.
(25 mm.) from the end; then the end of
the leading wire bent back and one or
two turns of the connecting wire made
24
around the loop.
Bare joints in the circuit should be
protected against short circuiting on wet
work. This can be done in mines by supporting
the wires from the roof, or, if this
is not practicable, by laying the wires on
stones or sticks on the ground. An additional
precaution, when the wires are
on the ground, is to wrap all joints with
tape.
APPARATUS FOR TESTING
BLASTING CIRCUITS
Each electric blasting cap should be
tested before it is used; it is also advisable
to test blasting machines occasionally
to determine their capacity. Electric
blasting caps and electrical connections
are tested with galvanometers and ohmmeters;
blasting machines are tested
with rheostats.
Galvanometers
A type of galvanometer commonly
used by blasters is illustrated in Fig. 2:
The cell and dial with a movable hand
are contained in a case of metal and
hard rubber, which is enclosed in a
leather case with straps for carrying over
the shoulder. A silver-chloride dry cell is
used. The current passes through highresistance
coils, so that when connection
is made to the galvanometer terminals
the current is less than one-tenth of
that required to fire an electric blasting
cap. When the galvanometer is properly
used, several thousand tests can be obtained
from one cell. Replacing an exhausted
cell is a simple operation.
To test an electric blasting cap or a
blasting circuit the wires are placed on
the terminals of the galvanometer and
the deflection of the needle is noted.
No definite figure can be given for the
distance the needle will deflect, as this
varies with the age of the cell. The blaster
soon learns the deflection to expect
from different circuits. As the cell is used
it constantly grows weaker, which causes
a gradual diminution in the deflection of
the needle on the galvanometer. When
one electric blasting cap is tested with
a new cell the needle should deflect almost
completely across the scale. The
greater the number of caps, the longer
the wire on each cap, and the greater
the length of connecting and lead wire
in the circuit, the greater will be the reThe
Journal of Explosives Engineering
sistance and, consequently, the less will
be the deflection of the needle. After
some practice, the blaster will know the
amount of deflection to expect from any
circuit that he tests. If the needle moves
too far across the scale, there are probably
short circuits or leaks; if it does not
move far enough, there is too much resistance
in the line, probably caused by
poor connections; if it does not move at
all, the circuit is broken. To find where
the trouble is, the galvanometer is connected
with different sections of the circuit,
until the part that contains the poor
connection, short circuit, or broken wire
is found.
One method of doing this is as follows:
Assume a series connection with
five electric blasting caps in which the
galvanometer test at the blasting machine
shows that the circuit is broken.
Connect the galvanometer in the circuit,
using the leading wire and the first blasting
cap. If the needle deflection shows
that this circuit is all right, include the
second cap in the circuit. Continue to
test, adding a cap each time until the
galvanometer again shows a broken circuit;
the trouble is then known to be in
the last cap or connection tested. If the
needle does not deflect at all, the cap is
broken; if it does not deflect far enough,
there is too high resistance, probably
caused by a bad connection; if it moves
too far, there is not enough resistance,
probably because of a leak or short circuit,
which can then be easily fixed.
July/August 2022
The Journal of Explosives Engineering - July/August 2022

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