Improvements in Safety Devices for Electrical Circuits.
Year: 1894

page_text: Date of Application, 31st Dec., 1894
Complete Specification Left, 28th Sept., 1895-Accepted, 2nd Nov., 1895
PROVISIONAL SPECIFICATION.
Improvements in Safety Devices for Electrical Circuits.
We, WALTER CLAUDE JOHNSON, of Victoria Works, Old Charlton, in the
County of Kent, and LOUIS JOHN STEELE, of 14 Clarendon Road, Holland Park
in the County of Middlesex, Electrical Engineers, do hereby declare the nature of
this invention to be as follows:-
When high tension currents of electricity are reduced, by means of transformers,
into low tension currents, there exists some danger of the high tension current
passing to the low tension circuit, should a fault exist in the system. In such a
case it is necessary that connection should automatically be made between the low
tension system and earth to enable the high tension current to pass directly to
earth. This is effected by this invention in the following manner :- -One pole of
the low tension system is electrically connected to a metallic part which may offer
a plane surface or be slightly convex.
Against the surface of this metallic disc is placed a thin sheet of some insulating
substance such as mica or glass (preferably mica) in which perforations are made,
in order that the metal at the back should be exposed in places.
The sheet of insulating substance might also consist of a suitable thickness of
some insulating varnish scraped away in places to lay the metal bare.
This arrangement of metallic disc and insulating piece is then brought up to
bear against a stretched metallic wire or thin narrow metal strip which is connected
to earth, the insulation acting as a separation between the metallic wire or strip and
the disc. It is preferable to place this disc in such a position as to prevent dust or
moisture from lodging between the metallic disc and the wire.
If, then, a fault exist in the system and the high tension current passes to the
low tension circuit, a spark will pass between the metallic disc and the stretched
wire, the latter will be fused and in the act of so doing will release some contrivance
which will make substantial metallic connection between the secondary low tension
system and earth.
By thus employing a thin sheet of insulating material of accurately predetermined
thickness for determining the distance between the metallic disc and the wire or
thin strip, the great difficulty is avoided of having to accurately readjust such
distance between the two parts every time the apparatus is re-set after action, in
order to insure that the sparking shall take place at a comparatively low tension,
such as 400 volts, as the sparking gap required for such a tension is only about the
two or three thousandth part of an inch.
The apparatus may be arranged in a variety of ways for carrying out the said
invention.
Thus the stretched wire may be made to directly hold back a metal contact piece
which, on being released by the fusing of the wire is moved either by gravity or
by spring action so as to close a contact connecting the low tension circuit with
earth.
According to one arrangement for this purpose the said metallic disc or plate
may itself be made to serve as the switch contact, for which purpose it is connected
to the said circuit and is pivotted to a support at one end, the said thin perforated
sheet of mica or other insulating material being fixed against its one face, and it is
made to bear with such mica sheet against the stretched wire, so that when this is

page_text: fused, the switch-contact is made to fall, either by gravity or by a spring, upon a
suitable earth contact.
According to another arrangement a separate switch contact is suspended from
the one end of the wire so that on the fusing of the latter, the contact falls or is
drawn by a spring on to the earth contact; or the metal disc with its mica sheet is
connected by a rod to a separate switch contact and is made to rest with the mica
sheet upon the stretched wire so that when this is fused, the contact drops on to
the earth contact.
In both the last named arrangements the switch contact may be made as a conical
or spherical plug, which fits into a corresponding conical or spherical hole in the
earth contact plate.
According to another arrangement, the stretched wire, instead of being made to
directly release the switch contact on its fusion, is made to release a suitable
catch, which in its turn releases the switch contact.
Dated this 31st day of December 1894.
ABEL & IMRAY,
Agents for the Applicants.
We, WALTER CLAUDE JOHNSON, of Victoria Works, Old Charlton, in the
County of Kent, and LOUIS JOHN STEELE, of 14 Clarendon Road, Holland Park,
in the County of Middlesex, Electrical Engineers, 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 :-
When high tension currents of electricity are reduced, by means of transformers
into low tension currents, there exists some danger of the high tension current
passing to the low tension circuit, should a fault exist in the system. In such a
case it is necessary that connection should automatically be made between the low
tension system and earth to enable the high tension current to pass directly to
earth. This is effected by this invention in the following manner:
One pole of the low tension system is electrically connected to a metallic part
which may offer a plane surface or be slightly convex.
Against the surface of this metallic disc is placed a thin sheet of some insulating
substance such as mica or glass (preferably mica) in which perforations are made,
in order that the metal at the back should be exposed in places. The sheet of
insulating substance might also consist of a suitable thickness of some insulating
varnish scraped away in places to lay the metal barc.
This arrangement of metallic disc and insulating piece is then brought up to bear
against a stretched metallic wire or thin narrow metal strip which is connected to
earth, the insulation acting as a separation between the metallic wire or strip and
the disc. It is preferable to place this disc in such a position as to prevent dust or
moisture from lodging between the metallic disc and the wire. If, then, a fault
exist in the system and the high tension current passes to the low tension circuit, a
spark will pass between the metallic disc and the stretched wire, the latter will be
fused and in the act of so doing will release some contrivance which will make
substantial metallic connection between the secondary low tension system and
earth.
By thus employing a thin sheet of insulating material of accurately predetermined
thickness for determining the distance between the metallic disc and the wire or
thin strip, advantage is gained of doing away with all doubtful adjustments by
accurately preadjusting such distance between the two parts and thus insuring
that the sparking shall take place at a comparatively low tension, such as 400 volts,

page_text: the sparking gap required for such a tension being only about the two or three
thousandth part of an inch.
The apparatus may be arranged in a variety of ways for carrying out the said
invention. Thus the stretched wire may be made to directly hold back a metal
contact piece which, on being released by the fusing of the wire is moved either
by gravity or by spring action so as to close a contact connecting the low tension
circuit with earth.
According to one arrangement for this purpose the said metallic disc or plate
may itself be made to serve as the switch contact, for which purpose it is connected
to the said circuit and is pivotted to a support at one end, the said thin perforated
sheet of mica or other insulating material being fixed against its one face, and it
is made to bear with such mica sheet against the stretched wire, so that when this
is fused, the switch contact is made to fall, either by gravity or by a spring, upon
a suitable earth contact.
According to another arrangement a separate switch contact is suspended from
one end of the wire so that on the fusing of the latter, the contact falls or is drawn
by a spring on to the earth contact; or the metal disc with its mica sheet is
connected by a rod to a separate switch contact and is made to rest with the mica
sheet upon the stretched wire so that when this is fused, the contact drops on to
the earth contact.
In both the last named arrangements the switch contact may be made as a
conical or spherical plug, which fits into a corresponding conical or spherical hole
in the carth contact plate.
According to another arrangement, the stretched wire, instead of being made to
directly release the switch contact on its fusion, is made to release a suitable
catch, which in its turn releases the switch contact.
Fig. 1 of the accompanying drawing shews a diagram of the above described
safety device.
A is a transformer to the low tension circuit a of which is electrically connected
a conducting plate B; to the face of this is fitted a thin sheet of insulating
material C, such as mica glass &c. of accurately determined thickness, having
holes c c so as to leave the plate B exposed at those points. The insulating sheet
is pressed close against a stretched wire D of fusible metal one end of which is
attached to a fixed point d, while the other end is fixed to a pivotted contact arm F
connected to earth at E and acted upon by a spring, which tends to cause the
arm to make contact at G, with the low tension circuit a. When by reason of a
fault the high tension current passes to the low tension circuit, sparks will pass
from plate B through the holes b to the wire D, and in fusing this, will cause the
contact arm F to establish the connection between the low tension circuit a and
earth.
Figs. 2, 3 and 4 shew by way of example one construction of the safety device
for practically carrying out the said invention. Figure 2 is a plan, Fig. 3 a
sectional elevation, and Fig. 4 a side view of the catch K.
B is the metal plate which is put in conducting connection in any suitable
manner with the low tension circuit to be protected, and which is pivotted to a
column H and acted upon by a spring I tending to press the plate downward.
C is the sheet of insulating material with holes c, which is held close against the
under face of the plate B by a spring J. D is a stretched wire, against which the
plate B is pressed, with the intervention of the sheet C, the wire being fixed at
one end to the post L and at the other end to a pivotted catch K engaging with a
stud m on a pivotted contact piece M so as to hold this in the position shewn, out
of contact with the contacts N and O, of which N is connected to earth while O is
connected to the low tension circuit.
The catch K is also connected to earth, and is acted upon by a helical spring P
which tends to withdraw it from engagement with the conducting contact piece M,
and this is acted upon by a spring which tends to turn it so as to make contact
with N and 0.

page_text: Thus on the high tension current leaking into the low tension circuit with
which plate B is connected, sparks passing through the holes of C will fuse the
wire D, thereby causing catch K to be turned so as to liberate M, which will be
turned by its spring so as to establish contacts with N and O and thus to connect
the low tension circuit with earth.
Fig. 5 shews a vertical section of a modified form of the sparking device; this
consists of a glass insulating tube R within which is a screwed metal stud S
connected to the low tension circuit, and which is adjusted so as to have its upper
surface at the exact sparking distance required from the fusible wire T stretched
over the top of the tube R.
It is to be understood that the constructions shown on the drawing only serve
as examples of the manner of carrying out the said invention, which, as before
stated can be carried out in a variety of ways.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. The use in safety devices for electrical circuits, of a pre-adjusted sparking
device consisting of the combination of a metallic conducting surface in connection
with the circuit to be protected and held at the required sparking distance from
a fusible metal wire or strip by means of a separating piece of insulating material,
the fusion of the wire being made to liberate a device for connecting the said
circuit with earth, substantially as described.
2. In a safety device for electrical circuit the combination of a metal plate
connected to the circuit to be protected, a sheet of insulating material of a thickness
equal to the sparking gap required, placed upon the said metal surface and
having one or more holes through which the metal surface is exposed, and a
stretched wire or strip of fusible metal laid upon the insulating sheet, such wire
being connected with earth and holding a device which serves to connect the said
circuit with earth, when the wire is fused, substantially as described.
3. In a safety device for electrical circuits, the combination of a metal plate
adjustably held in a tube of insulating material or in a hole or recess in block of
insulating material, and a wire or strip of fusible metal connected with earth, laid
over the said tube or block and holding a device which serves to connect the said
circuit with earth when the wire is fused, substantially as described.
Dated this 28th day of September 1895.
ABEL & IMRAY,
Agents for Applicants.