J

THE COLLIERY GUARDIAN

■AND ' ■ -

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXI.

FRIDAY, MAY 19, 1916.

No. 2890.

Outside System of Electrical Shot=firing.*

By H. H. CLARK, N. V. BRETH, and C. M. MEANS.

The purpose of an outside shot-firing system is to
provide means for firing the shots when no one is in
the mine, in order to eliminate completely any risk to
life that may attend blasting. The system is intended
as a, safety measure, and for that reason has been
investigated by the Bureau of Mines.

An outside firing system consists of a source of electric
power outside the mine, conductors leading from the
surface to all working places, and a system of switches
for keeping the lines free from stray currents and from
the firing. current. until all men have withdrawn from
the mine. The most complete systems employ switches
at or near the entrance to rooms and working places, at
the mouth of each cross entry, and' at the point under-
ground where the shot-firing circuit enters the mine.
At that point there is also established a break in the
circuit to guard against .discharges of lightning. The
break is usually ■accomplished by providing flexible con-
ductors, about 6 ft. long, so arranged that they can be
readily disconnected from the circuit and locked in the
open-circuit position until all other underground con-
nections. and switches have been closed. In addition
there are provided on the surface a switch for connecting
the shot-firing system to the power supply, and another
switch to be operated for the actual firing of the shots.
This system allows the firing circuits to be completed
step by step, beginning at the working places iand pro-
gressing toward the mine entrance. The underground
circuits are therefore not complete until the switch at
the mine entrance has been closed and the 6 ft. break,
mentioned above, has been bridged across. The system
as a whole is not connected to the source of electric
power until it has been ascertained (by a suitable check-
ing system) that all men are out of the mine. Even
then it is still necessary for the man commissioned to
fire the shots to close a locked control' switch mounted
in an enclosure to which he alone has access. Thus the
discharge of blasts by stray currents is‘ prevented by the
room and entry switches, the discharge of blasts by.
lightning is prevented by the 6 ft. break at the mine
entrance, and the discharge of blasts by accidental con-
nection to the shot-firing generator is prevented by all
these switches, as well as by the two switches that are
placed on the surface.

Outside shot-firing systems are already installed, in
some of the mines of Utah, New Mexico, Oklahoma,
Alabama, and Kansas.

The essential details of one of the outside shot-firing
systems now in Use are as follow :—	■

Check Cabin.

The check cabin is placed at the manway entrance
to the mine, and has a single attendant, whose duty
it is. to see that each man receives his identification
check when he enters the mine, and deposits it when he
leaves. (In some systems there is no attendant, but
only a check board.) The cabin also contains the shot-
firing switch/installed in a locked box.

Source of Power.

The 500 volt direct-current power used for firing shots
is supplied from a power house generator of large
capacity, the connecting line from which is, controlled
by a locked switch at the mine. (In other cases
220-250 volt current, from various sources, is used.)

Shot-Firing Circuits.

The main shot-firing circuit runs from the locked
switch that connects with the power house to the check
cabin, and thence overland on poles to a. borehole that
opens into the 27th entry of the mine near’ the main
slope. From this point the main circuit follows the

*From “ Shot-Firing in Coal Mines1 by Electricity Con-
trolled from Outside.” Technical Paper 108, United States
Bureau of Mines.	•

slope. At each cross entry a branch circuit is tapped,
without switches, from the main circuit. The branches
run to the face of the entry. Short deads ending in
plugs , are tapped from the entry circuit at each room
entrance. The room circuit can be opened or closed by
removing or inserting the plugs in sockets in which, the
room wires terminate.

The main circuit from the power house to the borehole.
is No. 10 (B. and S.) rubber-covered copper wire; 530 ft.
of No. 4 (B. and S.) duplex machine cable is used -in the
borehole. Starting at the bottom of the borehole and
extending for 1,800 ft., No. 6 (B. and S.) rubber-covered
iron wire is used. The rest of the main circuit and all
cross entry circuits are No. 10 rubber-covered copper
wire.1 The room1 circuits are No. 14 rubber-covered
copper wire.;	'

The circuits on the - main and cross entries are sup-
ported on porcelain knobs fastened to wooden cross arms
secured to the roof. The wires in the rooms are hung
on the props without the use of insulators.

Switches,

There are only six switches in the entire shot-firing
system. They, are all installed, in the main circuit..
The switches are placed in locked boxes, situated as,
follows :—One double-pole switch in the power house;
one single-pole shot-firing switch in the check cabin;
one double-pole switch at the 27th entry near the bottom
of the borehole through which the shot-firing circuits
enter the mine; and double pole switches in the main
circuit at other entries. Connection to the switch near
the bottom of . the borehole is made by flexible leads,
which can be disconnected so as to leave a gap 8 ft. long
as a protection against lightning. There are no room
switches, the plug connection previously described serv-
ing that, purpose.	.	.

Method of Operation.

The shot-firers distribute the detonators during the
day, and the miners are allowed to connect the
detonators to the room wires. When the miner leaves
his room after having connected his shots, he inserts in.
the sockets in which his room wires terminate the plugs
that are connected to the circuit in the entry. When all
' the .men are in the man trip, the shot-firer unlocks and
’ closes the last in-by switch in the main circuit, and
closes the other switches as he comes to them on his
outward trip.. When the shot-firer gets outside he and
the check man go over the check board twice, to make
sure that all the men are out of the mine. When all the
men have been, accounted for, the shot-firer proceeds to
the power house, where he unlocks and closes the switch
that connects the shot-firing circuit to the power circuit.
He then returns to the check cabin, where the shot-firing
switch’is placed, and unlocks and closes the. shot-firing
switch. The switch is left closed for eight seconds, and
is then opened and locked, as is-also the switch that
connects to the power circuit. ■	-

Having completed these operations, the shot-firer goes
off duty. The switches inside the mine are opened in
the morning by the assistant mine foreman.

Miscellaneous Details.

The shots are connected in multiple, and the whole
mine is shot at once. The coal is undercut and is shot
down with permissible explosives. The average number
of shots fired per day is 158. The average proportion of
missed shots is about 3 per cent. There are three shot-,
firers, who are on firing duty a week at a time. The act-
ing shot-firer extends and repairs the circuits;-the other
two distribute the detonators. It is the practice of one of
the. shot-firers to close the shot-firing switch only once
when the shots are fired; the other two shot-firers make
a practice of closing the .switch twice. Seemingly, each
has the same degree of success. ..	'• -

Cost of Operation.

The cost of operation and maintenance of the shot-
firing, system varies between 0’5 tq 1’5.5 cents, per ton of
coal mined, according to the system'adopted.

Construction and Practices Recommended.

As the firing circuits of an outside , firing system are a
permanently installed network of wires. that extends
over a large area, and is exposed to various untoward
influences and possible mishandling by many people,
various switches and safety devices must be used in
connection with .the system.

It is also essential that effective means should be
provided for insuring that no one is- in the mine when
the shots are fired.	•	\

Factors of Success.

In order to ensure safe and successful operation; an
outside system must be so installed as to ensure freedom
from : (1) Accidental connection with its source of
power; (2) stray currents from other sources of elec-
tricity; (3) connection to the earth; (4) short circuits
between the .two sides of the shot-firing circuits; (5)
open circuits when,shots are to be fired.

Freedom from' accidental connection to the source of
power requires that several switches placed at different
points shall be connected in series with the line, and
that these be closed in predetermined order, and only
by the men authorised to do so.

Freedom from stray current requires that only pro-
perly insulated conductors be used for the shot-firing
circuits, and that .such .circuits be properly installed ;and
protected from roof falls and where crossing or passing
near other electric circuits.

Freedom from ground connections is desirable because
such connections . allow the firing current to be
diminished by leakage, and at the same, time provide a
path by which stray' currents may reach the . firing
circuits...

Freedom from short circuits on the firing line or from
even moderate leakage between the two sides of the
firing circuit is essential to the proper distribution of
current to the detonators or fuses. . -

Freedom from open circuits in the system at the time
of firing is manifestly necessary to the discharge of all
the shots.

Specifications for an Outside System.

Such a system may be divided into five parts—the
checking system, the source of power, the shot-firing
panel, the distributing circuits, and the switches.

Checking System.

It is essentia! that a system for checking men in and
out of the mine should be used. This need not be com-
plicated, but should be efficient and accurate. It is
manifest that a system based upon the memory of an
individual cannot be as safe as a system that gives
visible evidence of the number of men that are under-
ground. .

Source of Power.

The main requisite of the source of povrer is sufficient
capacity. It may supply alternating or direct current,
and may be a generator or a transformer, but its capacity
must be great enough to provide the necessary current
to the detonators. That will depend upon. the..number,
of detonators to be discharged, but a momentary.capa:
city of not less than '30 kw. should be sufficient for the
average size mine. The voltage -required will depend
upon the length of the distributing circuits and the size
of wire used, as w^ell as upon the;number of detonators
to be fired. Under most conditions 250 volts . will
suffice, but where distances are, great, and a large
number of detonators are to be fired,tit may be neces-
sary to use 500 volts. If a special generator is provided,
for shot-firing, it should be compound wmind.

Shot-Firing Panel.

The shot-firing panel may be regarded as the outside
terminal of the shot-firing system, as well asthe means
of putting the system into operation. The Bureau
recommends that the panel should be provided with two