Improvements in and in connection with Electric Cables.
Year: 1893

page_text: COMPLETE SPECIFICATION.
Improvements in and in connection with Electric Cables.
I, GEORGE GATTON MELHUISH HARDINGHAM, of 191, Flect Street, in the
City of London, Civil Engineer and Chartered Patent Agent, do hereby declare
the nature of this invention (a communication to me from the Firm of Felten &
Guilleaume, of Mülheim-on-the-Rhine, in the German Empire, Cable Manu-
facturers,) and in what manner the same is to be performed to be particularly
described and ascertained in and by the following statement:-
One difficulty attending the employment of electrical currents in places where
combustible substances or inflammable gases are present is that, should the con-
ductor happen to break, the sparks emitted are apt to cause ignition of the said
substances or gases, and thus under certain circumstances as for example in coal
mines to involve serious danger to life. The present invention relates to means
for obviating this danger.
In a cable constructed according to this invention, there is combined with the
main conductor an additional or "safety" conductor, made of less ductile metal
than the main conductor. This safety conductor serves, in conjunction with a
switch-device, to close the circuit whereby the main conductor is connected with
the source of electrical energy. Should the main conductor be strained to a point
rendering fracture imminent, the safety conductor, owing to its inferior ductility,
breaks, whereupon the current passing through the same is interrupted and the
switch in the main circuit is opened. The current passing along that conductor
is thus intercepted or diverted before fracture of the main conductor can take
place. The invention is also adapted to guard against danger from sparking
should the cable suffer injury by crushing.
In the accompanying drawings,
Fig. 1 represents in elevation a cable adapted for carrying out the present
invention; the same comprising a main conductor a surrounding a safety
dis another
conductor but separated therefrom by insulating material b.
insulating covering and e an outer protective covering. The main conductor a
is made of copper and the safety conductor of steel, bronze or other metal,
which, unlike the soft and ductile copper, will bear comparatively little elongation
without breaking. In the event of the cable being subjected to excessive
tensile strain, the independent current traversing the safety conductor.c is first
interrupted in consequence of that conductor breaking, the effect being that the
main current is diverted from the principal conductor before the latter breaks.
Where there is reason to apprehend a crushing of the cable, the mode of con-
struction illustrated in Fig. 2 is preferred. In this instance, the safety conductor c c
in the form of a bi-part steel tape is wound round the insulated copper conductor a
in a coarse pitch double spiral. Where such a cable is liable to damage from
concussion or crushing only (as distinguished from excessive tension) the main and
safety conductors may respectively be made of metal of equal ductility; but when
it is desired to provide in addition against the effect of tension the safety conductor
is composed of metal having less ductility than that possessed by the main
conductor.
The method of connecting a cable such as described with reference to Fig. I is
illustrated in Fig. 3; D representing the dynamo supplying current to the main
circuit H, B the battery supplying current to the safety circuit S, and A A' the
switch-device adapted, on the interruption of the safety circuit, to intercept
communication through the main conductor.

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Throughout the line, all the safety wires (c, Fig. 1) are included in one circuit S,
to which current is constantly supplied by the battery B; this current being just
strong enough to retain the armature A of the switch or cut-out in position
for enabling the other, or more powerful, current to traverse the main circuit H.
When the safety current is interrupted, the armature A¹, being no longer attracted,
falls away and thereby instantly effects the interception of the main circuit H.
Thus, when a cable of the improved construction, forming the main electric line, is
strained to a point threatening it with rupture, the safety wire c, being less ductile
than the main conductor a, breaks, interruption in the main circuit H ensuing
before the copper conductor a itself breaks.
с
-The method of connecting a cable-of the construction hereinbefore referred.to.
with reference to Fig. 2 is illustrated in Fig. 4. As there are in this instance two
safety conductors c', and consequently two safety circuits S S', a cut-out A
is connected to each of these conductors; current being supplied for both cut-
outs by one battery B common to both. In this arrangement, upon either of
the two safety-wires c c being broken, the current is diverted from the main
conductor a before the latter suffers fracture, and protection afforded against
sparks being emitted on such fracture taking place.
To provide at once against both kinds of accidents, viz., those due to
"excessive tension and those due to crushing, a cable, constructed in accordance
with Fig. 1, may be combined with a special safety-switch such as shown in Fig. 5.
In this figure, D designates the dynamo, H the main circuit, S the safety circuit,
A the cut-out or switch, B a battery, and R R two relays, provided with double
contacts and respectively; the provision for affording safety coming into
operation both when the current is cut off and when it is considerably intensified.
The contacts
of the relays are electrically connected with one pole,
and the tongues with the other pole of the battery B. The safety-wire c,
or in other words the safety-circuit S, is fed in the manner illustrated in the
drawing, ie., by part of the tension of the main current, and by the aid of a
resistance W, the safety-current travelling in the opposite direction to that of the
main current, as indicated by the arrows. It flows through the coils of the relays,
and, while its power is normal, retains the tongues in the intermediate position,
overcoming for the purpose the resistance of the usual relay-springs; the battery-
circuit being, in this position, open. If the cable be so far strained that it is about
to break, the safety-conductor will in all cases break first; the relay-springs
thereupon become released and close the circuit B¹ of the battery B, by causing
the tongues to come into contact with the contact-pieces (for example) of
the relays the armature A' is thus attracted, and the main circuit H interrupted.
Should, on the other hand, the cable be crushed and damaged, so that a connection
is established between the main conductor a 'and the safety-conductor c, an
intensified current will pass from H through the safety conductors S, the coils of
the relays will be excited to a greater extent, and the tongues z 2 will meet the
contacts the battery-circuit B' thereupon becoming closed, the armature A!
attracted, and the main current H interrupted as before.
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Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, as communicated to me by
my foreign correspondents, I,claim :-
1. An electric cable comprising a main conductor and a safety conductor the
latter being of such a character as to withstand less tensile strain than the main
conductor; said cable being designed for use in conjunction with means whereby,
upon the safety conductor breaking, the current is diverted from the main
conductor, before the latter breaks, substantially as herein described.
2. An electric cable comprising a main conductor and a safety conductor, and
designed for use in conjunction with means whereby, upon the safety conductor
being forced into contact with the main conductor, the current conveyed by the
latter is instantly interrupted, substantially as herein described.

page_text: 3. The combination, with an electric cable comprising a main conductor anda
safety conductor, conveying independent currents, of a switch whereby on the
safety conductor breaking the current conveyed by the main conductor is instantly
interrupted prior to the rupture extending to that conductor, substantially as
described with reference to Fig. 3 of the accompanying drawings.
4. The combination, with an electric cable comprising a main conductor and ja
safety conductor conveying independent currents, of a switch A, relays R R' and
a resistance W arranged or operating substantially as described with reference to
Fig. 5 of the accompanying drawings, and whereby, on the safety conductor being
either broken or crushed into contact with the main conductor, the current
conveyed by the main conductor is instantly interrupted.
Dated the 1st day of November 1893.
G. G. M. HARDINGHAM.