Improvements in Safety Devices for High Voltage Circuits.
Year: 1889

page_text: Date of Application, 29th Apr., 1889
Complete Specification Left, 24th Jan., 1890-Accepted, 8th Mar., 1890
PROVISIONAL SPECIFICATION.
Improvements in Safety Devices for High Voltage Circuits.
We BERNARD MERVYN DRAKE and JOHN MARSHALL GORHAM, Electrical Engineers,
both of No. 2 Princes Mansions Victoria Street, Westminster S.W. in the City
of Westminster do hereby declare the nature of this invention to be as follows:-
This invention relates to a particular arrangement of the static device described
in Patent No. 15,484 by Philip Cardew Nov. 12, 1887. In order to provide a simple
and practical apparatus we dispense with the adjustable plate and auxiliary switch,
and rely upon the metal foil to carry sufficient current to sever the main fuses of
the transformer or battery.circuit. The apparatus then consists of two metal plates
separated the required distance by an insulating ring, the foil being laid in the
intervening space. The bottom plate on which the foil rests is connected to earth
and the top plate to the secondary or house circuit. to be protected. The bottom plate
may be flat and the top plate may have a projection formed on it to facilitate the
lifting of one end of the foil. The two plates are clamped together with insulated
screws and the foil may be conveniently kept in its place by some ebonito. pegs or
thin strips let into slots.
In order to fix and carry the apparatus we have metal pins attached one to the top
of the box and another of a different size attached to the bottom.
These enter sockets permanently connected up, one to the secondary or lamp
circuit & the other to earth. Or the box may be slid in between springs which will
carry it and make contact as above.
Dated this 27th day of April 1889
BERNARD MERVYN DRAKE.
JOHN MARSHALL GORHAM.
COMPLETE SPECIFICATION.
Improvements in Safety Devices for High Voltage Circuits.
We BERNARD MERVYN DRAKE and JOHN MARSHALL GORHAM Electrical Engineers
both of No. 2 Princes Mansions Victoria Street Westminster S.W. in the City of
Westminster, 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 a particular arrangement of the static device described in
Patent No. 15,484/'87 by Philip Cardew. In order to provide a simple and practical
apparatus we dispense with the adjustible plate and auxiliary switch and rely upon
the metal foil to sever the main fuses by carrying sufficient current through itself or
establishing an arc between the two plates. Where necessary this apparatus can be
arranged to ring a bell and drop an indicator by connecting these in the circuit in
the usual way.
The device in its most simple form consists of two metal plates separated a short
distance from each other by any insulating medium, a thin metal foil being laid on
the lower plate.
These plates are connected, one to the secondary circuit to be protected, and the
other to earth.
It is evident that the arrangement can be widely varied, as for instance by
suspending the foil vertically near a single plate, so that it is attracted and makes
contact with the metal plate when the difference of potential reaches the required

page_text: limit.
We prefer, however, the horizontal arrangement in which the foil rests by
its own weight on the lower plate and the force of gravity is overcome by the attraction
of the upper plate when the foil lifts.
In the drawings fig. 1 shows a section, fig. 2 an end view & fig. 3 a plan of the
apparatus as usually arranged.
A & B are the top and bottom metal plates separated from each other by an insulating
ring of ebonite D.
M. M. is the foil, preferably of aluminium, lying on the bottom plate B and retained
in position by the ebonite pins P. P.
G is a projection on the top plate A which is placed over the end of the foil M
which it is required to lift, the other end of the foil remaining in contact with the
lower plate B.
I. I. are insulated bolts for clamping together the two plates A & B.
The contact between the 2 plates and the two wires leading, the one to the secondary
circuit, and the other to earth, is effected by sliding the clamped plates between metal
springs S. S. which also act as carriers for the plates. For convenience one spring
is employed at the top and two at the bottom as shown in the drawing. The top
spring slides in a groove cut on the outside of the top plate which prevents lateral
movement and precludes the possibility of the apparatus being fixed with the wrong
side uppermost.
The top and bottom springs are fitted with terminals J. J. into which the wires
are clamped.
Fig. 4. shews a diagram of the connections, D is a high tension source of current,
such as a dynamo, F. F. are the fuses and T. is a transformer. The two plates of the
safety device A and B are connected, the one to S the secondary circuit, & the other
to E, earth.
If desired, a similar device may be connected to the other pole of the secondary
circuit. L. shews the lamps connected on the secondary circuit.
The action of the device is as follows:-
In the event of a difference of potential between the secondary circuit and earth
due to a leakage between the high & low tension circuits being established, which
exceeds the required limit, the metal foil is immediately attracted and wakes a metallic
contact with earth. This brings the house circuit to the potential of the earth
and the main fuses F. F. are severed by the increase of current in the primary
circuit.
If merely a static charge from the system has been communicated to the house
wires this is drawn off, the fuses remain intact, & the light is not cut off.
It is evident that the connections to the top & bottom plates may be made in any
convenient way such as by means of plain terminals attached to the top & bottom
plates direct, or by the method shewn in Figs. 5, 6, and 7, also the earth wire may
be connected to the top plate, & the secondary to the bottom plate, without affecting
the working of the apparatus.
Figs. 5. 6. 7 & 8 show an alternative arrangement in which the clamped plates.
rest on a circular metal case connected to earth at the terminal K, the contact to the
top plate being obtained by a metal plunger J to which the insulated wire leading to
the secondary circuit is connected.
A spring pressing on the underside of the metal cover Z maintains a good contact
between the metal plunger & the top plate.
Having now particularly described and ascertained the nature of our said
invention & in what manner the same is to be performed we declare that what we
claim is :-
(1.) The apparatus substantially as described for the purpose of automatically con-
necting the secondary circuit to earth in the event of a dangerous electro motive force
being developed between this circuit and earth

page_text: (2.) The use of two metal plates connected, the one with the secondary circuit, & the
other with earth with a light metal foil placed between them so that it can be attracted to
form a metallic circuit between the two, substantially as, & for the purpose described.
(3.) The use of a foil or thin plate suspended or pivotted so as to be attracted to
another plate by the static charge and to complete the circuit between the two, sub-
stantially as, and for the purpose, described.
Dated this Twenty-fourth day of January 1890.
BERNARD MERVYN DRAKE.
JOHN MARSHALL GORHAM.