coal trade bulletin - Clpdigital.org

44 THE COAL TRADE BULLETIN.
ERRONEOUS IDEA
in the minds of a great many people, is that
shocks produced by low voltage systems are harmless.
In some mines, high voltage wires are
labeled with a plate showing a lightning stroke
or skull and cross-bones, thereby scaring all who
come in close proximity to the line into giving it
the proper respect, while low voltage lines are not
marked in any way, are installed in a manner
ancl handled with a familiarity that breeds contempt
and accordingly the idea is generated in the
minds of those who come near to such circuits
that they are perfectly harmless. This idea of
the harmlessness of a low voltage system, is in
direct disregard of one of the fundamental laws
of electricity; namely, that the electrical current
which flows through any conductor, while directly
proportional to the voltage, is also inversely proportional
to the resistance. In other words, a low
resistance means a heavy current to just the same
extent as a high voltage means a heavy current
and it is the current that counts. It is the passage
of the electrical current that causes the electric
lamp to give forth light; it is the passage of
the current that causes a motor to drive its load;
it is the current that causes the blinding flash
when a short circuit occurs, which flash will in
jure a person who is near enough to it, and it is
the passage of the current through the body, and
nothing else, that kills. Some bodies have less
lesistance than others; hence, a low voltage will
kill some people or injure them, while it will produce
nothing more than an unpleasant sensation to
others.
Furthermore, the resistance which is offered to
the passage of an electric current
THROUGH THE HUMAN RODY
depends very greatly upon the manner in which
the contact is made between the body and the
terminals of the circuit. The larger the area
of the contact, the less the resistance and consequently
the greater the current and the more severe
will be the resulting shock. For example,
a contact made thiough the lamp on a miner's
cap which may come in contact with a trolley
wire, offers very little resistance to the passage
of the current. The soles of a man's shoes if
they contain nails, especially if they are damp,
will make very good contact with the ground.
Moisture on the skin at the point of contact has a
very great influence on the resistance of the contact.
A person might touch the terminals of an
electric circuit with his dry hand, particularly if
the skin is fairly thick, and feel no sensation whatever,
while if the skin should he moist he would
reecive a severe shock.
I do not mean to say that a low voltage system
is as dangerous as a high voltage one but wish
to impress the fact that low voltage systems should
be insulated and protected from accidental contact
with any part of a person's body who may
be passing or working in the neighborhood of the
circuit. The voltage on the systems where several
of the fatal accidents occurred in the mines
of Kentucky during the last two years, was 250
volts. It is, without doubt, true that many contacts
are made with systems of this and higher
voltages without injury, but when the conditions
are right, 250 and even 110 volts will produce
death. It is, therefore, important to insulate and
guard low voltage as well as
HIGH \OLTAGE LINES.
In fact, a bare electrical wire that is not effectually
guarded so that it is impossible for a person
to come in accidental contact with it, should not
be allowed inside of a mine. Trolley wires must
of necessity, be bare but they can be protected
from accidental contact by placing a board on each
side that extends well below the wire, the two
boards being not over three or four inches apart.
Even in the case of insulated wires, if they are
subject to mechanical injury or accidental contact
with a person's body, they should be boxed in or
protected in some way so that the insulation will
not be damaged and so that a person will not come
in contact with them. The principle that a wire,
no matter how well insulated should be treated as
bare, is one of the cardinal points in the rules of
the National Board of Fire Underwriters, covering
all installations over which their inspectors
have supervision and there is every reason for its
observance in mining installations as the danger
in this case is to human life rather than damage
to property by fire.
Armored cable is about the best conductor for
use inside of a mine. It is true that armored
cable is somewhat expensive but some extra expense
in first cost will result in a more satisfactory
system so far as operation and freedom from
accidents concerned. The armor of the cable
should be effectually grounded at frequent intervals
so that if due to any cause, however remote,
the conductor should come in contact with the
armor, a person standing on the ground and
touching the cable will not receive a shock.
Iron pipe is not a satisfactory protection for
wires in a mine as it
IS PRACTICALLY IMPOSSIBLE
to prevent the collection of moisture on the inside
of the pipe which wil] cause the pipe to rust and
will damage the insulation on the wire. The
eventual result will be contact between the conductor
and the pipe. Furthermore, as pipe is not
flexible any movement of the ground will likely
cause serious damage to the system.
Weatherproof insulated wire is very good for