232

THE COLLIERY GUARDIAN.

January 31, 1913.

question of how the change is made is, the writer suggests,
of no interest for the present purpose. The point of interest
is the practical difference between high electrical pressure
and low electrical pressure. If a dynamo is used to convert
mechanical energy into electrical energy, and if it is so
designed that in the process of conversion a high electro-
motive force is produced, the dynamo is said to generate
high-pressure current, and a system of conductors connected
to the terminals of the dynamo would form a high-pressure
system. If in the preceding sentence the word “ low" be
substituted for the word “ high,” the system of conductors
connected to the terminals of the dynamo would form a
low-pressure system;

All the current-carrying parts of apparatus should be
surrounded by insulating material so that no point of
weak insulation—that is to say, no point of lower insulation
than the general standard—is introduced into the system
to which the apparatus is connected. When a point of
weak insulation is introduced, the standard of insulation of
the whole system is lowered, just as the strength of a chain
is lessened by a single weak link. When a conductor is
surrounded by insulating material so that there is no
leakage of current to neighbouring conductors, that con-
ductor is said to be well insulated. Insulation is made
thicker for high than for low pressures, or a different
material altogether from that suitable for very low
pressures may be required for very high pressures. Al}
insulating material should be chosen with special regard to
the circumstances of its proposed use, and it should be of
sufficient mechanical strength and so protected as to
maintain its insulating properties under the working
conditions both in regard to temperature and moisture
whatever they may prove to be and however they may
vary.

Against most insulating materials is brought the charge
that they are liable to deterioration. If the insulating
material has been wisely chosen, and the circumstances are
favourable, there may be little or no deterioration. If, on
the other hand, the insulating material has not been wisely
chosen, or if the circumstances are unfavourable, deteriora-
tion may be rapid. Colliery managers should endeavour to
acquire a general knowledge of the properties of the
different insulating materials in common use.

In the writer's view it is better, even in low-pressure
systems, to cover all insulating material in use below
ground by an earthed metallic outer covering, so that it
cannot in any circumstances be touched. If the pressure is
in excess of medium pressure, to fail to cover insulating
material in that way, whatever its quality may be and
whatever the circumstances, is to invite accident.

The insulating material having been carefully chosen,
there are two considerations if the insulation of the system
is to be maintained at a high standard, namely :—

(a.) Wherever it is applied, the insulating material
must be of sufficient thickness.

(b.) Where there are bare parts, the bare conductors
must be fixed and arranged so that sufficient surface of
insulating material is provided to prevent“ creepage ”—
that is to say, leakage from a bare live part to a neigh-
bouring conductor via the surface of intervening
insulating material, which may, on occasion, through
atmospheric changes, become damp.

A number of failures of electrical plant have been traced
to “ creepage." The common tumbler switch often proves
to be defective in this respect. Such switches are out of
place (though not infrequently used) below ground in
mines on circuits of 400 or 500 volts pressure, because their
“creepage surface" is usually far too small, even when
their construction is satisfactory from a mechanical stand-
point, which usually it is not. Apparatus covered with
poor insulating material or ill-provided with creepage
surface possesses low insulation resistance. Highly
insulated apparatus on the other hand possesses high
insulation resistance.

The idea is commonly held that to touch a bare live part
is necessarily dangerous. Generally—almost invariably
under underground conditions—it is; but it is perhaps
worth while to consider briefly the conditions which must
be fulfilled before an electric shock can be received. A
circuit mnst be completed through the person's body.

Touching a live conductor in a completely insulated
system does not provide any alternative path for the
current. What a miner touching a live part of a com-
pletely insulated system does in fact is to bring one pole of
the system, the pole which he touches, to zero (earth)
pressure—nothing more. No shock is felt unless there is
a path for current through the miner's body, vid earth, to
some other point or points on the system where the
insulation is defective. This consideration supplies the
reason why it is never safe—or hardly ever safe—to touch
a live part, even if provision has been made to insulate
every part and every point of the system from earth.
It is most difficult in practice to achieve that desirable
object. Equally with a system of which one point is
connected to earth, it is possible to obtain a shock between
poles. Almost equally with a system of which one point is
connected to earth, it is possible to obtain a shock (though
not usually a shock of the full pressure of the system)
between a live part and earth.

The writer discussed the conditions for obtaining an

electric shock in the case of a direct-current system (a)
with one pole earthed (the concentric system); and (b)
with neither pole earthed (the completely insulated system
in common use). There is a third system in which a point
midway on the pressure gradient between the two poles, a
point which may be called the “ mid-voltage ” point, is
connected to earth. This condition is very commonly
obtained in practice when a pair of lamps, or a pair of
resistances, is arranged to indicate any defect in the insula-
tion of the system. It is usual in such circumstances to
connect to earth a point between the two lamps or between
the two resistances (which are connected in series between
the two poles of the system), and the conditions are not
very different from those of the concentric system, as
regards liability to electric shock, if a live part be touched.
In each case one point of the system is intentionally
connected to earth. The difference is that whereas in the
concentric system if contact be made with a live part the
full pressure between poles is a measure of the shock to be
obtained; in. the case of a system the mid-voltage point of
which is connected to earth, one-half the pressure between
poles is a measure of the shock to be obtained. It is
assumed in each case that the person receiving the shock is
standing on earth and making contact with a live conductor
at one point only.

A moment's consideration will show that a person
standing on earth and touching one phase of a three-phase
system, the neutral point of which is connected to earth, is
in much the same position from the point of view of his
liability to receive an electric shock as a person standing on
earth and touching either conductor of a direct-current
system in which the mid-voltage point is connected to
earth—the case which has just been considered. Similarly
a person standing on earth and touching one phase of a
completely insulated three-phase system is in much the
same position from the point of view of his liability to
receive an electric shock as a person standing on earth and
touching either pole of a completely insulated direct-
current system. Farther, the exact equivalent of the
direct-current concentric system from the point of view the
writer is considering is clearly the single-phase alternating-
current concentric system with the outer earthed.

It is quite clear that in any concentric system with the
outer earthed, and in any alternating-current system with
one phase earthed, it is possible for a person standing on
earth to touch with perfect safety any conductor connected
to that pole or phase of the system which is itself connected
to earth—that is to say, the “ outer" conductor or the
earthed phase, as the case may be. In such circumstancee,
the person making the contact is only assisting slightly to
improve a connection with earth which by hypothesis is
already effectively made, and there is no reason, therefore,
why any current at all should flow through his body to
earth. Hence it is that the best way to prevent electric
shocks from being received by contact with any conducting
material which is not intended normally to carry live current
—for example, the armouring of a cable—is to connect it
to earth so effectively that it cannot become charged with
electricity to a pressure above that of earth—that is to say,
so that it cannot become charged with electricity at all.

The writer has discussed, quite briefly, the meaning of
two or three terms or expressions in common use, and the
conditions which must be fulfilled before an electric shock
can be received. If a colliery manager feels that he has
clear ideas on those points, he may next, it is suggested,
turn his attention to matters which are a little more
advanced, such as the making of a cable joint and the
working of such protective devices as may be in use at his
colliery. The simplest protective device is a fuse. There are
others less* simple, such as maximum cut-outs, no-voltage
releases, leakage cut-outs*, &c., and these latter, although in
no sense complex, will serve as a good illustration of the

* It is suggested to the writer by a mining engineer who
was good enough to read this paper in manuscript that it
might be appropriate to add a short note on the respective
merits as protective devices of fuses and maximum cut-outs,
on the one hand, and leakage cut-outs on the other. Fuses
and maximum cut-outs have the common characteristic
that they are called into action only in the event of the
current strength in the circuit exceeding a predetermined
value, namely, the value at which the fuse melts, or the
value at which the maximum cut-off set to operate. Hence
the protection afforded is in no way sensitive ; and, further,
in a circuit so protected a mere temporary overload might
cause the current to be cut off. Leakage cut-outs, on the
other hand, do not operate on an overload, temporary or per-
manent, so long as the flow of current is confined to the con-
ductors. They only operate when some portion of the current,
it may be a very small portion, elects to leave the conductors
provided for it and to return to the generator by some
other path—possibly by earth. When that occurs there is
a “ fault" on the circuit, and the balance which normally
exists at every instant between the current which enters
and that which leaves the conductors of a healthy circuit,
at every point thereof, is disturbed. Leakage cut-outs are
operated by such disturbances only; that is to say, they
operate immediately in case of a fault: they do not wait
before they operate until a fault has so developed that there
is an overload on the system due to it, as do fuses and
maximum cut-outs. If it is desired to cut off the pressure
from a circuit quickly should a fault occur—for example, in
the event of a fall of stone on a cable in an atmosphere
charged with coaldust—the advantage from a safety stand-
point which the use of leakage cut-outs confers (in con-
junction with the use of properly constructed apparatus) is
at once clear.

attitude which the writer suggests that bewildered miners
—and he has met a few who merit that description—should
adopt towards electricity. If protective devices are seen to
be of sound mechanical construction, and if in practice they
do what they are intended to do, it is suggested that, for all
practical purposes, this knowledge is sufficient.

Discussion.

Mr. J. Gregory said the paper fulfilled a very useful
purpose in making matters exceedingly clear to mining
men who had not the advantage of a highly technical
training on electrical questions. He was quite in
accord with Mr. Nelson in stating that it was not
necessary or possible for a colliery manager to go deeply
into the theory of how apparatus was designed. Motors
he took it, were the class of apparatus that was most
commonly called for, and it was far better to deal with
a good firm and to rely upon their calculations in the
design of the machine, specifying the duty that was
required and the conditions under which it was to work.
In the early days he had some little trouble in regard to
the output of machines. It seemed a simple matter to
say one wanted a 10-horse power motor, but if one
obtained quotations from English and Continental
makers, there were two different specifications—one for
intermittent duty and one for continuous duty ; and so
it was necessary that continuous duty should be
inserted in the specification. Further, the temperature
tests should be, say, of at least two hours’ continuous
duty on full load to ensure that the spefication was
complied with.

Dr. J. Cadman said that never before had it been
his privilege to hear in such simple language points
which at some time all of them had had to fathom.
There was one question he should like to ask
Mr. Nelson, and that was in regard to electrical
signalling. It was now suggested that all wires of
signalling apparatus should be insulated. Did he know
of any case in which electric signalling wires used for
bells had been the cause of an ignition in practice.

Mr. F. H. Wynne, after adding his appreciation of
the paper, said he wished to draw attention to the
definition Mr. Nelson gave of earthing. He did not
know whether everybody appreciated how important it
was that that connection should be effectively made, of
good materials and of a conductor that was likely to
carry the current that might come upon it.

Mr. W. Saint said he should like Mr. Nelson to
elucidate the point referred to by Mr. Wynne as to the
best way of making a thoroughly good earth at the
surface. Different people had various ways of doing it:
some advocated burying the earth plates in wet clay,
others in porous ground kept damp, and in one case he
had seen the plate lying at the bottom of a reservoir at
the colliery. It was no doubt very important that the
connections between the earth wires and the earth plates
should be made of a permanent nature. They (the
inspectors) had come across cases where the earthing
system had become very defective, with the result that
the whole of the outer covering of the cables became
alive throughout the pit, and such cases had led to
accidents.

Mr. Gregory said he would like Mr. Nelson to
explain a good method of testing the efficiency of the
earthing of non-current carrying parts—say, in the case
of a motor a mile inbye.

The President, on the subject of earthing, said he
had seen cases where the earth connection consisted of
a piece of iron, an old chisel or something that was no
use for anything else, with a piece of wire connected to
the end of it, and the piece of iron or chisel driven into
hard, dry ground. That was considered an efficient
earth connection. He had seen other cases where the
chisel or earth plate was connected to the installation
by means of a single wire of something like — in. in
diameter, and anyone carelessly kicking against it would
almost necessarily break it.

Mr. Nelson thanked the speakers for their
complimentary remarks. Dr. Cadman asked if he
knew of an explosion caused by signalling wires.
There was a South Wales case that was attributed
to bare signalling wires. The pit had only just
been sunk, and they were about 140 yards inbye.
They had electric signalling. A fall had taken place,
and the men were engaged in removing the fall
when they heard the electric bell ring. There was no
one at the other end to operate it. They took no notice,
but it rang again, and almost immediately there was an
explosion, which burned five or six of them. Everyone
who investigated the accident agreed that something had
brought the two wires into contact. They were bare
wires running on the same side of the road, and at the
point where they were brought into contact, or at the
bell itself, the gas had been ignited. They subsequently
experimented with the bell that was in use, and ignited a
mixture of coal gas and air quite readily. It was