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THE COAL TRADE BULLETIN. 57
SAFEGUARDING THE USE OF ELECTRICITY IN MINES'
By H. H Clark, Electrical Engineer of the United States Bureau of Mines
This subject has been discussed so often (hat
one can scarcely write or speak about it without
fearing that he will be accused of plagiarism.
However, in order properly to introduce the subject,
I have no recourse but to rehearse a statement
of conditions and results that I am sure
many of you could repeat in chorus were you
so inclined.
Electricity must be safeguarded everywhere
that it is used. The conditions that exist underground
make the use of safeguards more essential
there than almost anywhere else.
Electric shock is the greatest danger in connection
with the use of electricity in mines, because
conditions existing underground are so
favorable to its occurence. Men are often obliged
to work in more or less uncertain light near
bare wires that are carrying dangerous potentials.
The fact that the men are standing on the earth
practically connects them to one terminal of the
electric generator.
The most dangerous pieces of electrical equipment
underground is the trolley wire. It is
necessarily bare and extends for long distances
throughout a mine, often less than a man's
height from the track rail. Sometimes the making
up of triiis of cars must be done near low
hanging trolley wires. All bare wires offer the
same danger that trolley wires do, although not
to the same extent.
Apparatus that has accidentally come in contact
with the underground side of an electric
circuit is almost as dangerous as the trolley
wire. A severe and even fatal shock may be
obtained by coming in contact with the frame
of a motor or a switch box that has become
charged with electricity or "alive" as it is usually
termed. It is also
1CISSI1ILE TO OBTAIN SHOCKS
from the frames of locomotives and cars if track
sanding, poor bonding or similar causes has put
a high resistance to earth in the path of the
current.
The danger from fires caused by electricity arises
principally from defective installation and careless
upkeep or from injuries to equipment resulting
from falls of roof or similar causes. Fires
may be started by unrelieved short circuits, or
grounds, by the blowing of open fuses, and by
the overheating of resistance. Incandescent lamps
can produce heat enough to ignite combustible
materials if the dissipation of the heat from the
bulbs of the lamps is allowed to become restricted.
"Address delivered before the American Institute of Mining
Engineere. in New York. Feb. 17-20. 1914.
The fire danger is more remote than the shock
danger, but it may affect a larger number of men.
Explosions may be caused by the ignition of
explosives, gas, or coal dust. Accidents due to
the ignition of explosives by electricity are of
two kinds: Those that occur while handling and
transporting explosives near electric circuits, and
those that are incident to the detonation of explosives
by electrical means.
Some very mysterious accidents of the first
kind have occurred, but 1 believe that most of
these have been caused by hauling explosives in
metallic- packages, or else so near the locomotive
that flashes or sparks from the trolley wire
have come in contact with the explosives.
Electrical shot-firing accidents are usually
caused by the premature ignition of shots after
holes are charged. It is not the best practice to
shoot electrically under conditions that require
one side of the detonating circuit to be connected
to the earth, because wherever grounded systems
of power distribution are used unexpected differences
of potential exist in the earth in tinvicinity
of such circuits. If, therefore, one side
of the detonator be purposely grounded
AN ACCIDENTAL GROUND
on the otlier side may connect the detonator
across a potential sufficient to cause ignition.
Premature ignitions have been reported which
seemingly have been caused by the conditions
just described.
For all practical purposes it may be assumed
that sparks that occur around such apparatus and
circuits as are used for power and light in a
mine are capable of igniting gas. The ignition
of gas by incandescent lamps has been investigated
by the Bureau of Mines quite thoroughly
with carbon filament lamps and to a lesser extent
with tungsten filament lamps. The results
of the investigation seem to indicate that certain
of the larger sizes of carbon filaments will ignite
gas and that tungsten lamps of 25 watts or more
are almost certain to ignite gas when broken.
The study of the ignition of coal dust by electric
arcs and electric flashes has been carried on
to some extent by investigators in Europe. The
results of their experiments indicate that electricflashes
can ignite coal dust suspended in the atmosphere.
The Bureau of Mines has planned
similar investigations to proceed as soon as funds
are available.
Everyone connected with mining work knows
that the natural conditions surrounding underground
installations of any character are such
that accidents are likely to occur and this is
;