Electrical Circuits, Especially those whereon ne CurrentS Produced by Continuous or Alternating Current Transformers are Employed.
Year: 1887

page_text: Date of Application, 12th Nov., 1887
Complete Specification Left, 12th Sept., 1888
Complete Specification Accepted, 12th Feb., 1889
Electrical Circuits, Especially those whereon ne CurrentS
Produced by Continuous or Alternating Current Transformers are
Employed.
I, PHILIP CARDEW, Captain in Her Majesty's Corps of Royal Engineers, of
Brompton Barracks, Chatham, do hereby declare the nature of this invention to be
as follows:-
This invention relates to improvements in safety devices for use in connection with
electrical circuits whereon induced, or secondary, currents are employed, and has for
its object the provision of means whereby should the current on the lamp or supply
circuit become through any accident of a dangerously high potential the circuit will
be broken thereby avoiding the risk of injury to the person which might otherwise
occur.
It is well known that when transformers are employed for converting a high
tension primary current into a low tension secondary current of greater quantity
there exists a risk of injury to persons handling the lamps or wires in the secondary
circuit should even an imperfect contact become established between the primary and
secondary conductors of the transformer, the secondary circuit, although no difference
or fault in its working would be apparent, becoming charged with the potential of the
point of contact on the primary circuit and thus establishing a condition which might
be injurious, or even fatal, to any person touching a portion of the secondary circuit
while any portion of his body was more or less in contact with the earth.
Now to provide a simple and efficacious method of overcoming this risk I combine
together, as hereinafter described, the following parts; namely, first, a body, or
apparatus, which acts as a nearly perfect insulation for currents of low, or moderate,
tension, but which permits of a comparatively free passage to currents when the
difference of potential is sufficiently high to be dangerous to human life; second,
a small electro magnet used as a relay; third, a circuit closing device; and, fourth,
a circuit breaking apparatus.
For the body, or apparatus, I may employ a small Geissler, or partially, exhausted
glass tube containing platinum wires sealed into the two ends, as is well understood;

page_text: or it may conveniently be formed as a small oblong metallic box, whose lid has
a circular hole near one end. Into the hole is screwed a piece of ebonite or other
insulating material, and through this which forms an insulating annulus is screwed a
piece of metal whose lower portion, inside the box is enlarged so as to form, or
it may be attached to, a flat plate, parallel, and in close proximity, to the bottom
of the box, the distance between the two being capable of adjustment by screwing the
plate up or down. On the upper part of the insulated piece of metal and exterior to
the box is fixed a terminal while a second terminal is attached to the box itself. At
the bottom of the box is laid an elongated piece of aluminium or tin-foil formed
of two circular portions united by a very narrow neck, one portion being directly
underneath the insulated metallic plate.
In place of the box as above described a strongly constructed condenser may be
used.
To one terminal of the Geissler tube, or of the condenser, or to the uninsulated
terminal of the metallic box as the case may be, is attached a wire making good
connection with earth, while the other terminal is connected to the small electro-
magnet.
The circuit closing device consists generally of a metallic spring fixed at one end
and whose other, platinized, end tends to inake contact with a platinum stop but
is prevented from doing so by a piece of thin wire soldered to the spring and to the
support of the armature of the small electro-magnet.
This armature when attracted also completes an electric circuit, the spring or lover
to which it is attached, having a platinized contact point which comes into contact
with a platinum stop connected electrically with the stop of the contact making
apparatus. These stops are also connected to a terminal on the box containing the
apparatus, another terminal being attached to the spring of the contact making
apparatus. These two terminals are connected to the two poles, or terminals, to which
the secondary coil of the transformer is connected through the contact breaking
apparatus, and from which the mains for supplying the lamps, or charging, are led.
The other end of the coil of the relay magnet is connected to the spring of the
contact making apparatus and to the terminal in connection therewith. Now should a
contact be established between the primary and secondary coils of the transformer,
and should the secondary circuit in consequence become charged to a dangerous
difference of potential from the earth a sufficient current will pass from it by the
terminal connected to the relay coil, through this and the partially exhaused tube,
box, or condenser as the case may be, and so to earth as to attract the armature of the
relay magnet, thus causing it to complete a circuit by which a current driven by the
electro-motive force of the secondary coil will pass by the spring of the contact
making apparatus, the thin wire, the spring or lever of the relay magnet armature,
the platinum stop, and the terminal of the apparatus to which it is connected. This
current will at once fuse the thin wire, releasing the spring of the contact making
apparatus which will make its contact and thus short circuit the secondary coil of the
transformer through the contact breaking apparatus which will then break, or open,
the circuit of the secondary coil at each side, thus cutting off all connection between
it and the lamps and leads to them, so securing the safety of persons handling them
until the faulty transformer is removed.
The separate circuit breaking apparatus may be replaced by causing a spring,
retained in tension or compression by a fine wire, to sever the connection on the
fusing of the fine wire by the passage of the current through it, the release of the
spring bringing into action mechanism which shall sever the circuit direct, or with-
draw contact pieces from mercury cups.
The contact breaking apparatus may consist of a safety fuse on each side of the
secondary coil, and to ensure these being fused simultaneously on the passage of a
sufficient current they may consist of thin metallic foil stretched over a small wooden,
or other insulating, cylinder containing gunpowder or other explosive; or it may
consist of a magnetic cut-out similar to Conynghame's, but arranged to break both
sides of the circuit simultaneously.

page_text: When the metallic box enclosing the piece of aluminium foil is employed in place
of the Geissler tube the action is as follows:-The foil in the box is normally in
contact throughout with the bottom of the box and with earth, but there is no contact
with earth made on the secondary circuit of the transformer at any point: Should,
however, the potential of either side of the secondary circuit differ sufficiently from
that of the earth, owing to a contact with the primary circuit, to cause danger to
persons handling the leads or wires, the portion of the foil under the insulated plate
will become attracted by it and will rise into contact with it, the other portion
remaining in contact with the base and earth. A momentary current will thus be
produced sufficient to actuate the relay magnet. The action of the condenser is of
course somewhat similar to that of the Geissler tube, or box, but as it establishes, even
under normal conditions, more or less of a leak on the circuit, its use may be found
detrimental.
The apparatus herein before described will in general be found sufficient to ensure
the safety of the whole of the secondary circuit, since no two points on this can differ
from each other in potential by more than the clectro-motive force of the secondary
coil, or say, 50 to 100 volts. For greater security, however, it may occasionally he
desirable to duplicate the arrangement, in which case the Geissler tube may be made
with a wire let into the centre as well as one at each end, this wire being connected to
earth, and the others each to a relay magnet and thence to the two sides of the
secondary circuit; or for the same purpose, the metallic box may be provided with an
insulated plate at each end, these being connected as above described, the relay
magnets in either case being each arranged to close the circuit through the fuse wire
and actuate the contact making apparatus.
It will be understood that the apparatus hereinbefore described may also be used on
circuits in which the difference of potential is dangerously high although no trans-
forming apparatus may be employed therewith, and it will further be understood that
in speaking of the potential of any point in an alternating current circuit the mean
difference of potential between this point and earth in each alternation is meant.
Dated this 12th day of November 1887.
F. SEATON SNOWDON,
Agent for the Applicant.

page_text: Improvements in Safety Devices for Use in Connection with
Electrical Circuits, Especially those whereon the Currents
Produced by Continuous or Alternating Transformers are
Employed.
I, PHILIP CARDEW, Captain in Her Majesty's Corps of Royal Engineers, of
Brompton Barracks, Chatham, do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
This invention relates to improvements in safety devices for use in connection with
electrical circuits whereon induced, or secondary, currents are employed and has for
its object the provision of means whereby should the lamp, or supply circuit become,
owing to partial or complete contact, charged from the primary circuit to a potential
dangerous to life, or likely to cause injury, a complete severance of this connection
will be effected automatically by disconnecting either both primary, or both secondary,
mains from the transformer.
It is well known that when transformers are employed for converting a high tension
primary current into a low tension secondary current of greater quantity there exists
a risk of injury to persons handling the lamps or wires in the secondary circuit
should even an imperfect contact become established between the primary and secondary
conductors of the transformer, the secondary circuit, although no difference or fault
in its working would be apparent, becoming charged with the potential of the point
of contact on the primary circuit and thus establishing a condition which might be
injurious, or even fatal, to any person touching a portion of the secondary circuit
while any portion of his body was more or less in contact with the earth.
Now to provide a simple and efficacious method of overcoming this risk I combine
together, as hereinafter described, the following parts; namely, first, a body or
apparatus, which acts as a nearly perfect insulation for currents of low or moderate
tension, but which permits of a comparatively free passage to the current when the
difference of potential is sufficiently high to be dangerous to human life; second, a
small electro-magnet used as a relay, which is not, however, necessary in all cases;
third, a circuit closing device ; and fourth, a circuit breaking apparatus.
For the body or apparatus, I may employ a small Geissler, or partially exhausted
glass tube containing platinum wires sealed into the two ends, as is well understood;
or it may conveniently be formed as a shallow cylindrical metallic box, whose lid is
provided with a circular hole. Lnto this hole is screwed a piece of ebonite, or other
suitable insulating material, and through this, which forms an insulating annulus
is screwed a piece of metal whose lower portion, inside the box, is enlarged so as to
form, or it may be attached to, a flat plate parallel to, and normally in close proximity
with, the bottom of the box, the distance between the two being capable of adjust-
ment by screwing the plate up or down, a small scale fixed to the lid of the box

page_text: serving to indicate the position of the flat plate by means of an ivory disc fixed to the
screw and moving up or down with it.
On the upper portion of the insulated piece of metal and exterior to the box is
fixed a terminal, while a second terminal is attached to the box itself. At the bottom
of the box, in a shallow groove or depression formed therein, is laid an elongated
piece of aluminium, or tinfoil, formed of two circular portions united by a very
narrow neck, one of the circular portions being directly underneath the insulated
metallic plate. In place of aluminium, or tinfoil, paper coated with some substance
of a sufficiently high conducting nature may be employed. To allow of the position
of the foil being easily inspected in the groove or channel the lid of the box is provided
with a glass window.
The circuit closing device consists generally of a metallic spring fixed at one end
and whose other, platinised, end tends to make contact with a platinum stop but is
prevented from doing so by a piece of thin wire secured to the spring, but in some
cases insulated from it, and to a fixed support.
The circuit breaking device may consist of a safety fuse on each side of the primary
or secondary coil and to ensure these being fused simultaneously on the passage of a
sufficient current they may consist of thin metallic foil stretched over a small wooden,
or other insulating cylinder, containing gunpowder or other explosive; or wires,
strips, or other fusible and flexible conductors may be used, to the centres of which
is suspended a weight, insulated from them and arranged so that if one wire fuses
the other must immediately break by the sudden increase of weight thrown on it, in
conjunction with its high temperature; or the device may consist of a magnetic cut
out, similar to Conynghame's, but arranged to break both sides of the circuit
simultaneously.
When the metallic box enclosing the piece of aluminium, or other, foil is employed
the relay magnet is generally dispensed with as it is unnecessary, and the action is
then as follows: (see Figs. 1, 2, 3, and 4 of the accompanying drawings). The foil
in the box is normally in contact throughout with the bottom of the box and with
earth, but there is no contact with earth made on the secondary circuit of the trans-
former at any point. Should, however, the potential of the secondary circuit differ
sufficiently from that of the earth, owing to a contact with the primary circuit, as to
cause danger to persons handling the leads, or wires, the insulated plate (H) in
the box will become charged to this potential through the terminal S, the upright T,
the fine wire and the flexible leads K2 K¹, the portion of the foil (g) under the
insulated plate H will become attracted by it, owing to the difference of potential
between them, and will rise into momentary contact with it, the other portion remaining
in contact with the base and so with the earth.
A momentary current will thus be produced which will start an arc, this will be,
however, immediately suppressed by the fusing of the thin wire holding up the
springs which being in consequence released short circuits the primary coil and causes
the cut outs C to act.
Figures 1, 2, 3 and 4 of the accompanying drawings show the arrangement in
which the metallic box and foil are employed, without the intervention of a relay
magnet, to break the circuit when the potential of the same becomes dangerously
high.
Figure 1 is a plan, Fig. 2 an elevation, partly sectional, of the metallic box taken
on the line ab of Fig. 1, and Fig. 3 an elevation of the arrangement for short circuit-
ing the primary circuit, taken on the line c, d, of Figure 1, the terminals being
removed to render the view clearer, and Fig. 4 a diagram showing the connections of
the apparatus to the transformer and primary and secondary circuits.
From these figures it will be seen that the base of slate A has suitably secured to it a
shallow cylindrical cast iron box B, provided with a groove or channel e in which the
strip of foil g lies, the position of this being visible through the glass window h. The
lid of the box is cast or formed with the upright D, into which the insulating annulus I
is screwed. Through this annulus passes the threaded spindle G, having at its lower
end the iron disc H, and at its upper end the thumb nuts J J' between which is

page_text: secured the washer K attached to the flexible connection K' (Fig. 1). The terminal E
is in metallic contact with the lid C and so with the box B and groove e.
The short circuiting arrangement (Fig. 3).consists of an insulating base A' mounted
on the slate base A and carrying the terminals 1. S. and P¹. The contact point p
in contact with the terminal P by means of the copper strip p³, while the point q is
connected to the terminal P¹ by the springs and copper strip st.
Normally the spring s presses the points p and q together but they are held apart
by the thin wire connected to the terminal S by the copper strip t and standard T,
the upper part of which is shown as broken away in Fig. 1. The spring s is provided
with a block of ebonite, or other insulating material, i at its end and to the screw j
thus insulated the wire fand flexible connection K2 are secured.
The ivory disc O attached to the spindle G and the graduated scale R allow of the
relative positions of the iron plate H and the foil g being adjusted.
When the Geissler tube is employed instead of the metallic box and foil as above
described the relay magnet becomes necessary as the current through the tube can
never be large, the connections in this case being arranged in either of the ways
shown by Figs. 5, 6 and 7.
Figure 5 shows a connection by which a dangerously high potential on the
secondary circuit finds its way through the terminal S, spring s, fine wire, maguet
coil m, and Geissler tube G to earth, causing a current which actuates the magnet so
closing the contacts b, b', and thus directing a current from the secondary coil of the
transformer through the terminal S, spring s, fine wiref, spring st, and contacts b,
S to sufficient to at once fuse the fine wire f, thus releasing the springs and causing
it, by closing the contacts a, a', to short circuit the terminals S and S, which will at
once cause the double pole cut-out C to act, cutting off the secondary circuit at both
terminals from the transformer.
'
Figures 6 and 7 show two arrangements in each of which the short circuiting and
subsequent cutting off is effected on the primary circuit. In Fig. 6 the terminal S of
the apparatus is connected to a point on the secondary circuit and should a dangerous
potential exist it will cause a current from S through the relay magnet m and the
Geissler tube G to earth. The magnet being actuated by this current closes the contacts b
and and thus gives rise to a current from the primary circuit to P, through
springs, fine wire f, spring s¹ and contacts b, b, to P1 and the other side of the
primary circuit. This causes the fine wire to be fused and the contacts a a¹ 'ɔ be
closed so short circuiting the primary coil and causing the cut outs C to act, thus dis-
connecting the transformer. The above explanation applies equally to the form of
apparatus shown in Fig. 7 with the exception that in the latter case the fine wire f is
fused by a current from the secondary instead of the primary circuit.
The apparatus constructed as hereinbefore described will, in one or other of its
forms, in general be found sufficient to ensure the safety of the whole of the
secondary circuit, since no two points on this can differ from each other in potential
by more than the electromotive force of the secondary coil, or say from 50 to 100
volts.
For greater security, however, it may be desirable to duplicate the arrangement,
in which case the Geissler tube may be made with a wire let into the centre as
well as one at each end, this wire being connected to earth, and the others each
to a relay magnet and thence to the two sides of the secondary circuit; the relay
magnets in either case being each arranged to close the circuit through the fuse
wire and actuate the contact making apparatus; or for the same purpose the lid
of the metallic box may be provided with two insulated plates, these being connected
as before described.
It will be understood that the apparatus herein before described may also be used
on circuits in which the difference in potential is liable to become dangerously
high although no transforming apparatus may be employed therewith, and it will
further be understood that in speaking of the potential of any point in an alternating
current circuit the mean difference of potential between this point and earth in each
alternation is meant.

page_text: 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
First. The herein before described method of constructing a safety device for use in
connection with electrical circuits whereon currents of dangerously high potential
may occur and substantially as shown in Figures 1, 2, 3 and 4 of the accompanying
drawings.
Second. The arrangement, in connection with a safety device, of a light piece of
metallic or other foil to be attracted, on a dangerous increase of potential, by a plate
of metal, at an adjustable distance from it, the said foil not being pivotted, suspended
or in any other way fixed, or controlled; except by being laid in a shallow groove, for
the purposes as hereinbefore described.
Third. The herein before described method of employing a Geissler tube as part of
a safety device for use in connection with electrical circuits whereon currents of
dangerously high potential may occur, and substantially as shown in Figures 5, 6
and 7 of the accompanying drawings.
Dated this 12th day of September 1888.
F. SEATON SNOWDON,
Agent for the Applicant.