Improvements in Safety Devices for Use with Electric Cables.
Year: 1898

page_text: PROVISIONAL SPECIFICATION.
Improvements in Safety Devices for Use with Electric Cables.
JOHN AMBROSE FLEMING, Professor of Electrical Engineering in University
College, Gower Street, London, W.C., in the County of Middlesex, do hereby
declare the nature of this invention to be as follows:-
It is well understood that in systems of electric supply both by alternating and
continuous currents when operated with long cables especially with those of the
concentric type, occasions arise due to electrical actions in which a sudden
increase in electrical pressure or voltage occurs in some portion of the cable
system. This may be due to sudden stoppage of the current or to effects pro-
duced by the interaction of capacity and inductance which are generally com-
prised under the term electric resonance. The insulating material with which
the conductors of the cable is surrounded is able to withstand electric pressure
or voltage up to a certain value depending on the nature and thickness of the
di-electric.
If subjected to pressure beyond this point it breaks down. Hence the insu-
lation of a cable which may well be able to stand the ordinary working electric
pressure or voltage may break down if a sudden rise of pressure occurs. This
rise of pressure may be only momentary and yet it may be sufficient to crack or
injure the di-electric so that the cable subsequently fails under the normal
working pressure.
My invention has for its object improvements in devices intended to prevent
such failure in cable insulation. I have devised a safety device which enables
this sudden electric pressure to relieve itself by a special path provided for it,
and so prevent the rupture of the cable insulation." The device is of such a
nature that if the ordinary current flowing in the cable attempts to follow on
in this special path the latter is interrupted and the current flow is arrested.
My device also includes an arrangement by which the special path is renewed
or reinstated after each blow-off so that a relief circuit is again established and
the cable is again prepared for any abnormal rise of electric pressure.
My apparatus is as follows:-
In it there are a number of fine metallic or fuse wires or wires capable of
being fused by a relatively small current of electricity. These may be contained
in tubes packed or not with a non-conducting powder such as chalk, lime,
magnesia or steatite. These fuses or fuse tubes to any required number are
arranged vertically round a cylinder or radially in a suitable frame. One end
of each fuse wire is in metallic connection with a metal or carbon knob, ball or
spark point and these latter are arranged so as to be in close contiguity to one
or more corresponding knobs, balls, or spark points which are in metallic con-
nection with one another and with one metallic conductor of the cable to be
protected. The other end of the fuse wire is in connection with a metal rod
lever or trigger piece which latter is upheld or pulled by a suitable metallic
spring so as to strain the fuse wire. These last metal pieces will be called the
D

page_text: trigger bars or pieces. The trigger pieces are held on a suitable frame and are
arranged in a circle or in any other convenient manner.
There is a metal radial arm of such length that when pivotted in the centre
it moves round like the hand of a clock and rests against one of the trigger
pieces. A strong spiral spring causes this radial arm to press strongly against
one of the trigger pieces. The centre of this radial arm is in connection with
the other metallic member of the cable to be protected or with the earth.
:―
The action of the apparatus is as follows:-
Starting from one metallic conductor of the cable to be protected we have
several paths or circuits which may be called relief circuits. These circuits
comprise, first a spark gap between the metallic or carbon knobs, balls, or
points; second, a-fuse wire in series with the spark gap, and third a metallic
circuit consisting of the radial arm and a connecting wire joined to the other
conductor of the cable or to the earth. This last or third portion can, however,
only be associated with one of the spark gap and fuse circuits at a time.
The apparatus is set by giving the radial arm a twist against the torsion of
the spring and adjusting the length of the arm so that it rests against and
presses on one of the trigger pieces.
If then a sudden rise of electric pressure takes place in the cable, it is pos-
sible so to adjust the spark gap between the balls that this extra pressure will
cause an electric spark to jump across the gap, whereas the normal working
pressure would not do so. This spark breaks down the air gap and relieves the
extra electric pressure harmlessly, thus preventing it from damaging the cable
insulation. The spark, however, causes the ordinary or normal pressure to send
a current following along the same path. This current, in one or two moments
melts the fine fuse wire and so breaks the circuit. The corresponding trigger
piece being released by the breaking of the fuse wire is drawn up or down by
its spiral spring. The radial arm is then by that action liberated and permitted
to fly round and make contact with the next or contiguous trigger piece and so
again provide a metallic path broken only by a short spark gap between two
balls and offering a relief path between the two conductors of the cable or
between one conductor and the earth. When in turn this second fuse is melted
by a discharge the radial arm advances again one more step and completes a
third path, and so on.
When only one fuse is left, the radial arm may be made
to complete an electric circuit in advancing the last stage of its journey, and
so ring a warning electric bell or indicate in any other way that the reserve of
fuse wires is nearly exhausted.
A vane may be attached to the radial arm and move in a dash-pot full of oil
or mercury, and so retard or control the sudden movements of the radial arm.
The spark gap interval may be filled with a non-conducting solid or liquid so
adjusted that it is ruptured electrically by a less pressure than that which
would injure the cable di-electric.
The apparatus may be arranged to act as a lightning protector to preserve
electric cables from "destruction and puncture by lightning. In this case a
safety device of the above kind is connected between the earth and each metallic
conductor of the cable.
The apparatus acts as an electric safety valve to blow-off a sudden electric
pressure which would otherwise injure or destroy the insulation of electric cables,
but it prevents the passage along the same path of any current generated by the
normal working pressure of the cable by the rupture of a fuse, and it renews
this fuse as often as it is broken, up to the number of the reserve of fuses. The
above arrangement of parts may be varied to suit convenience, but the essential
nature of the invention is the combination of one or more spark gaps each with
a fuse wire or wires in series with it the fuses being successively placed as the
prepared relief path by the automatic action of a revolving radial arm controlled
by a spring or weight or gravity. This arm successively makes contact with
each fuse in turn as the adjacent and preceding one is broken by being melted

page_text: by a current rush, and so always leaves one complete relief path broken only by
its adjusted spark gap, ready to be broken down and used as a relief path for
any momentary extra electric pressure occurring between the two conductors
forming the cable or between either of them and the earth.
Dated this 20th day of October 1898.
Improvements in Safety Devices for Use with Electric Cables.
I, JOHN AMBROSE FLEMING, Professor of Electrical Engineering in University
College, Gower Street, in the Administrative County of London, do hereby
declare the nature of this invention and in what manner the same is to be per-
formed, to be particularly described and ascertained in and by the following
statement:-
It is well known that in systems of electric supply both by alternating and
continuous currents when operated with long cables especially with those of the
concentric type, occasions arise due to electrical actions in which a sudden
increase in electrical pressure or voltage occurs in some portion of the cable
system. This may be due to sudden stoppage of the current or to effects pro-
duced by the interaction of capacity and inductance which are generally com-
prised under the term electric resonance. The insulating material with which
the conductors of the cable are surrounded is able to withstand electric pressure
or voltage up to a certain value depending on the nature and thickness of the
dielectric. If subjected to pressure beyond this point it breaks down. Hence
the insulation of a cable which may well be able to stand the ordinary working
electric pressure or voltage may break down if a sudden rise of pressure occurs.
This rise of pressure may be only momentary and yet it may be sufficient to
crack or injure the dielectric so that the cable subsequently falls under the
normal working pressure.
My invention has for its object improvements in devices intended to prevent
such failure in cable insulation.
I have devised a safety device which enables this sudden pressure to relieve
itself by a special path provided for it and so prevent the rupture of the cable
insulation. The device is of such a nature that if the ordinary current flowing
in the cable attempts to follow on in this special path, the latter is interrupted
and the current flow is arrested. My device also includes an arrangement by
which the special path is renewed or re-instated after each blow-off so that a
relief circuit is again established and the cable is again prepared for any
abnormal rise of the electric pressure.
In the accompanying drawings Fig. 1 is an elevation and Fig. 2 a plan of one
form of my invention and Fig. 3 is an elevation of a modification. Like parts
are lettered to correspond in all three figures.
The apparatus shown in Figs. 1 and 2 is constructed and arranged as follows: -
In the apparatus there are a number of fine metallic or fuse wires A or wires
capable of being fused by a relatively small current of electricity. If found
necessary or desireable these wires A may be contained in tubes packed or not
with a non-conducting powder such as chalk, lime, magnesia or steatite.
These fuses or fuse tubes, to any required number, are arranged vertically
round a cylinder B as shown, or they may be arranged radially in a suitable
frame. One end of each fuse wire A is in metallic connection with a common
conducting wire D and this latter, by the wire M, is in connection with a metal

page_text: or carbon knob, ball or spark point C which is arranged so as to be in close
contiguity to one or more corresponding knobs, balls or spark points C¹ C³,
which, by suitable wires are in metallic connection with one another and with
one metallic conductor N of the cable to be protected.
The other end of each fuse wire A is in connection with a metal rod, lever,
or trigger piece, E which latter is upheld or pulled by a suitable metallic
spring G so as to strain the fuse wire. These last metal pieces will be called
the trigger bars or pieces F. The trigger pieces F are pivotted on or to a suit-
able frame H and may be arranged in a circle as shown or in any other con-
venient manner.
There is a metal radial arm J of such length that when pivotted in the centre
of the cylinder B, or of the frame H, it moves round like the hand of a clock
and rests against one of the trigger pieces F. This radial arm J is secured to
one end of a strong spiral spring L the other end of which is secured to a fixed
central post K on the top of the cylinder B or on the frame H. The torsion of
this spring L will cause the radial arm J to press strongly against one of the
trigger pieces F.
The post K is in connection with the other metallic member of the cable to
be protected or with the earth.
The action of the apparatus is as follows:-Starting from the metallic con-
ductor N of the cable to be protected several paths or circuits are provided which
may be called relief circuits. These circuits comprise first, a spark gap between
two or more metallic or carbon knobs, balls or points C, C¹ C2, secondly a fuse
wire A in series with a spark gap through the wire M, and thirdly a contact
metallic circuit consisting of the trigger piece F, radial arm J, spring L, post K
and a connecting wire E joined to the other end of the cable or to the earth.
The last or third of these circuits can however only be associated with one of
the spark gap and fuse circuits at a time.
This
The apparatus is set by giving the radial arm Ja twist against the torsion of
the spring L and adjusting the length of the arm so that it rests against and
presses on one of the trigger pieces F. If then, a sudden rise of electric pressure
takes place in the cable it is possible so to adjust the spark gap between the
balls C, CC2 that this extra pressure will cause an electric spark to jump
across the gap whereas the normal working pressure would not do so.
spark breaks down the air gap and relieves the extra electric pressure harm-
lessly thus preventing it from damaging the cable insulation. The spark how-
ever caused the ordinary or normal pressure to send a current following along
the same path. This current in one or two moments melts the associated fine
fuse wire A and so breaks the circuit. The corresponding trigger piece F, being
released by the breaking of the fuse wire A, is drawn up or down by the spiral
spring G. The radial arm J is then by that action, liberated and the torsion
of its spring L causes it to fly round and make contact with the next or con-
tiguous trigger piece F and so again provide a metallic path broken only by a
short spark gap between two or more balls and offering a relief path between
the two conductors of the cable or between one conductor and the earth. When
in turn this second fuse is melted by a discharge the radial arm J advanced
again one more step and completes a third path and so on. When only one
fuse is left the radial arm J may be made to complete an electric circuit in
advancing the last stage of its journey and so ring a warning electric bell or
indicate in any other way that the reserve of fuse wires is nearly exhausted.
A vane may be attached to the radial arm and move in a dash pot full of oil
or mercury and so retard or control the sudden movements of the radial arm J.
The spark gap interval may be filled with a non-conducting solid or liquid so
adjusted that it is ruptured electrically by a less pressure than that which would
injure the cable or dielectric.
In the modification shown in Fig. 3 the springs are dispensed with and
counterweighted levers substituted. In this form the conducting wires M lead-

page_text: ing from the spark ball C is connected to an insulated metallic bar D¹, and to
this bar one end of each of the fuse wires A is connected, their other ends
being each connected to the weighted end G₁ of a lever F. The weighted ends
of these levers when released, fall into insulated metallic cups or forks y' and
the bearings f¹ of each of the levers F is in metallic connection, by means of a
wire, with the cup or fork g of the lever next in the series; the end lever
being in metallic connection with the conductor E leading to the other terminal
of the cable or earthed. The action of this form of my invention is analogous
to that already described with reference to Figs. 1 and 2, except that as the
fuse wires are ruptured the circuit produced by the subsequent sparking will
pass through the succeeding fuse wire A, and its lever F¹, thence through the
wire to the cup or fork g of the lever next in the series, and through this
lever to its bearings f¹, thence through the next wire f, and so on to the
conductor E.
: )
Either form of the apparatus may be arranged to act as a lightning protector
to preserve electric cables from destruction and puncture by lightning. In
this case a safety device of either kind is connected between the earth and each
metallic conductor of the cable.
The apparatus acts as an electric safety valve to blow-off a sudden electric
pressure which would otherwise injure or destroy the insulation of electric
cables, but it prevents the passage along the same path of any current generated
by the normal working pressure of the cable by the rupture of a fuse, and renews
this fuse as often as it is broken up to the number of the reserve of fuses.
The arrangement of parts may be varied to suit convenience but the essential
nature of the invention is the combination of one or more spark gaps each with
a fuse wire or wires in series with it, the fuses being successively placed as the
prepared relief path by the automatic action of a revolving radial arm actuated
and controlled by a spring or springs, or of a falling weight, which successively
brings each fuse in turn into circuit as the adjacent and preceeding one is broken
by being melted by a current rush, and so always leaves one complete relief
path, broken only by its adjusted spark gap, ready to be broken down and used
as a relief path for any momentary extra electric pressure occurring between
the two conductors forming the cable or between either of them and the earth.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is
1. A safety device for use with electric cables consisting in the arrangement
of a number of fuse wires in series with one or more spark gaps to which one of
the conductors is connected and in metallic connection with the other conductor
said fuse wires being so placed as to successively act as relief paths for excessive
electric pressure in the cable substantially as described.
2. A safety device for use with electric cables comprising a spark ball or balls
or the like connected to one conductor and to a number of fuse wires arranged
to successively carry the excess electric pressure passing the spark gap to the
other conductor of the cable or to earth as the preceding wire is broken by the
current passing therethrough substantially as described.
3. A safety device for use with electric cables comprising a spark ball or
balls or the like connected to one conductor and to a number of fuse wires
arranged to successively carry the excess electric pressure passing the spark gap
said fuse wires being connected to trigger pieces making contact with a radial
arm which latter moves by the torsion of a spring, connected to the other con-
ductor of the cable or to earth as one of the wires is fused by the current
passing therethrough, and its trigger piece removed from the path of the radial
arm, substantially as described with reference to Figs. 1 and 2 of the accom-
panying drawings.
4. A safety device for use with electric cables comprising a spark ball or balls

page_text: or the like connected to one conductor and to a number of fuse wires arranged
to successively carry the excess electric pressure passing the spark gap. each of
said fuse wires being connected to an insulated weighted lever which, when its
wire is fused, is automatically cut into the circuit of the succeeding fuse wire
in the series, the last weighted lever being connected to the conductor of the
cable or to earth, substantially as described with reference to Fig. 3 of the
accompanying drawings.