IMPROVEMENTS IN ELECTRIC SAFETY PLUGS AND APPLIANCES IN CONNECTION " THED FUT
Year: 1883

page_text: LETTERS PATENT to Killingworth William Hedges of No 25 Queen Anne's
Gate Westminster in the county of Middlesex for an Invention of
IMPROVEMENTS IN ELECTRIC SAFETY PLUGS AND APPLIANCES IN CONNECTION
"
THED FUT
"
PROVISIONAL SPECIFICATION left by the said Killingworth William Hedges
at the Office of the Commissioners of Patents on the 9th February 1883.
KILLINGWORTH WILLIAM HEDGES of N° 25 Queen Anne's Gate Westminster in
the County of Middlesex "IMPROVEMENTS IN ELECTRIC SAFETY PLUGS AND
APPLIANCES IN CONNECTION THEREWITH
"
Electric safety plugs usually consist of wires or foils of metal which will fuse
under the heat caused by excess of electricity passing through them, the fusion
producing a breach of continuity and consequently throwing the circuit open.
According to one part of my present invention, I construct a safety plug of a
number of small wires or strips of lead, tin or other metal or alloy that melts at a
moderate temperature, these wires being laid side by side, but sufficiently far apart
to prevent sparking from one to the other, between plates of mica,, and having all
their ends attached by soldering to two metal terminals which can be connected in
the circuit. In like manner, when the safety plugs consist of strips of foil,
surrounded by mica, I connect the ends of the strips, to metal clamps forming
terminals for connecting to the circuit. The electric current divides itself into a
number of branches, one for each wire or strip and, should it become excessive the
wires or strips will fuse.
It is sometimes necessary to provide a by-pass for the current when the safety
plug gives way. For this purpose, employ either a safety plug of the kind
described above, or one consisting of pieces of metallic foil with intervening mica
plates, and suspend it vertically from one electrode, attaching the other electrode
to a clamp at its lower end, from which a wire projects downwards into a metal
cup containing mercury, but not touching the mercury. This cup is connected
through a resistance by a by-pass wire from the upper electrode. When the
current becomes so great that the safety plug gives way, then the clamp at its

page_text: lower end descends and its wire becoming immersed in the mercury, establishes a
circuit through the by-pass. In this by-pass circuit may be introduced a second
safety plug which will take the place of the first, in case of its fracture. Also,
either in this by-pass or in a subsidiary branch of it, may be situated an electric
bell which will act as an alarum giving notice of the fracture of the safety plug.
This bell may also be arranged so as to give warning of excessive increase of
current before actual rupture of the safety plug. The wire which dips in the
mercury may be fitted to slide vertically through the clamp, so that it may be kept
immersed maintaining the by-pass circuit while the clamp is raised to connect it to
a fresh safety plug. When the connection is made, the wire can be drawn up out
of the mercury, so as to open the by-pass circuit. Instead of employing a mercury
contact, as above described, the clamp may make contact with a spring, and in
order to ensure close contact it may carry the armature of an electromagnet
situated in the by-pass circuit so that, when contact is first made on the rupture of
the safety plug, the magnet becomes excited and draws the clamp into firm contact
with the spring. When this arrangement is adopted, the safety plug may be
subjected to the strain of a light spring acting-instead of the weight of the clamp,
and the apparatus may be arranged in any position. Part of the strain on the
plug may be relieved by extra clamps pressing on the mica or non-combustible
substance surrounding the fusible plug. In cases where local connection has to be
made from a cable or other main conductor carried along a channel formed in the
-ground, I mount the safety plug on a base of ceramic or other insulating material
having a section like the letter T, and as there are usually two cables or main
conductors in the channel, I insert this base with the vertical limb of its section
passing down between the two conductors and resting on the bottom of the
channel. On the upper face of the T, I fix the safety plug lying horizontally, one
end of it connected by springs or clips to one of the main conductors and the other
to the local branch conductor.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Killingworth William Hedges in the Great Seal Patent Office
on the 8th August 1883.
KILLINGWORTH WILLIAM HEDGES of N° 25 Queen Anne's Gate Westminster in
the County of Middlesex " IMPROVEMENTS IN ELECTRIC SAFETY PLUGS AND
APPLIANCES IN CONNECTION THEREWITH
"
Electric safety plugs usually consist of wires or foils of metal which will fuse
under the heat caused by excess of electricity passing through them, the fusion
producing a breach of continuity and consequently throwing the circuit open.
According to one part of my present invention, I construct a safety plug of a
number of small wires or strips of lead, tin or other suitable metal or alloy, these
wires being laid side by side, but sufficiently far apart to prevent sparking from
one to the other between plates of mica, and having all their ends attached by
soldering to two metal terminals which can be connected in the circuit.
Fig. 1 of the accompanying drawings shews part of a safety plug so arranged.
It consists of a number of fine wires W which at each end are gathered together
and soldered into a socket S connected to the main conductor. Between the two
sockets S, the wires are spread out and laid in parallel lines between two plates M
of insulating material, such as mica, which will not be fused when any of the wires
are fused. In order to keep the wires apart, they are threaded through holes in a
transverse strip of mica m. In like manner, when the safety plugs consist of
strips of foil, surrounded by mica, I connect the ends of the strips to metal clamps
forming terminals for connecting to the circuit.
Fig 1 shews part of a set of strips of foil thus arranged, separated by plates M
of mica or other suitable material except at the ends which are gathered together
into metal clamps C connected to the circuit. The electric current divides itself
into a number of branches, one for each wire or strip and, should it become
excessive, the wires or strips will fuse.
It is sometimes necessary to provide a by-pass for the current when the safety
plug gives way. For this purpose, I employ either a wire safety plug of the kind
described above, or one consisting of pieces of metallic foil with intervening mica
plates, and suspend it vertically from one electrode, attaching the other electrode to
a clamp at its lower end, from which a wire projects downwards into a metal cup
containing mercury, but not touching the mercury. This cup is connected through
a resistance by a by-pass wire from the upper electrode. When the current
becomes so great that the safety plug gives way, then the clamp at its lower end
descends and its wire, becoming immersed in the mercury, establishes a circuit
through the by-pass. In this by-pass circuit may be introduced a second safety
plug which will take the place of the first, in case of its fracture. Also, either in
this by-pass or in a subsidiary branch of it, may be situated an electric bell which
will act as an alarum, giving notice of the fracture of the safety plug. This bell
may also be arranged so as to give warning of excessive increase of current before
actual rupture of the safety plug. The wire which dips in the mercury may
fitted to slide vertically through the clamp, so that it may be kept immersed,
maintaining the by-pass circuit while the clamp is raised to connect it to a fresh
safety plug. When the connection is made, the wire can be drawn up out of the
mercury, so as to open the by-pass circuit.
be.
Fig. 2 shews this arrangement, P being the safety plug having at its lower end
a clamp in which is fitted a wire W, that is held out of contact with the mercury M
as long as the plug P remains entire, but when P gives way, the wire W descends

page_text: into the mercury and thus closes the by-pass circuit including the resistance R,
which may consist wholly or partially of a bell or other alarum. The wire W
can be slid downwards in the clamp to hold it in the mercury while the clamp is
raised for introducing a fresh plug.
Instead of employing a mercury contact, as above described, the clamp may
make contact with a spring, and in order to ensure close contact, it may carry the
armature of an electro-magnet situated in the by-pass circuit, so that, when contact
is first made on the rupture of the safety plug, the magnet becomes excited and
draws the clamp into firm contact with the spring. When this arrangement is
adopted, the safety plug may be subjected to the strain of a light spring acting
instead of the weight of the clamp, and the apparatus may be arranged in any
position. Part of the strain on the plug may be relieved by extra clamps pressing
on the mica or non-combustible substance surrounding the fusible plag.
Fig. 2 is a front view partly in section and Fig 3 is a side view of this arrange-
ment. The safety plug shewn in these Figures consists of several sheets of foil F
placed between plates M of mica. All the sheets of foil are pressed together at
--each end between metal clamps A and B, which are in connection with the circuit
wires L L respectively.
The clamp B is made with a central stem or guide 6 which passes down through
a metal tube C and through insulating material a lot into an iron plate E and is
strained by a spring D adjustable by a screw nut d. From the plate E project
upwards two iron cores cc which are coiled with insulated wire T so as to constitute
two limbs of an electro-magnet. To the clamp B an attached armatures G G which
are presented towards the poles of the electro-magnet, and a contact plate H which
is presented towards the mouth of the tube C. This mouth is split at several
places and bent over so as to present an electric abutment for the contact plate H.
The by-pass circuit consists of a conductor L' which connects from the clamp A
through the coils T to the tube C, and in the course of the conductor L' may be
inserted, as indicated at K, either a second safety plug arrangement like that
described, or an electrical bell to give an alarum when the by-pass circuit is closed.
When the safety plug is entire, as shewn in Fig. 2, the strips of foil F are subject
only to the strain of the spring D, the electro-magnet being inert, and the current
passes in the circuit through the plug and the conductors L L'. Should the current
become excessive, so as to fuse the foil F, then the clamp B is drawn by the spring
so that its plate H makes contact with the tube C. This closes the by-pass circuit
from L through L, the coils T, the tube C, the plate H and the clamp B, to L', and
the electro-magnet being thus excited, attracts the armatures G G so that H and C
are held firmly in contact.
When the apparatus is placed vertically, the spring D may be dispensed with,
the weight of the lower clamp and the parts attached to it being sufficient to cause
the descent when the safety plug gives way. The two terminals T' T may be
temporarily connected by a wire L so as to short circuit the by-pass by L³ to L'
through L³ when the plug has to be replaced.
In order to relieve the safety plug from excess of strain, due to the weight of
B and its attachments, or to the spring D, I fix on B two side springs N N which
press with frictional hold against the outer plates of mica M.
In the arrangement shewn diagrammatically in Fig. 4 the main line L L' passing
through the safety plug P has a by-pass through the coil of an electro-magnet M
and a resistance R, and a subsidiary by-pass through a bell B and resistance R¹,
the resistances being so adjusted that the bell does not sound unless the current in
the main circuit becomes so great as to endanger the plug P, in which case it operates
as an alarum. When the plug P gives way, the magnet M becomes sufficiently
powerful to attract a loaded armature A causing it to make contact so that the
principal part of the current passes through the branch O the core of the magnet M,
the armature A, and the branch C, a portion also passing through the bell B and
causing it to continue sounding.
It is often convenient to arrange safety plugs in duplicate so that when one gives

page_text: way, the other can at once be introduced into the circuit. Fig. 5 shews an arrange-
ment for this purpose. The current passes from L through the safety plug P the
metallic handle H and contact face C to L'. When P gives way, the handle H can
be turned on its hinge so as to make contact at C', in which case the current from L
passes through C¹, H and the plug P¹ to L'.
In cases where local connection has to be made from a cable or other main con-
ductor carried along a channel formed in the ground, I mount the safety plug on a
base of ceramic or other insulating material having a section like the letter T, and
as there are usually two cables or main conductors in the channel, I insert this base
with the vertical limb of its T section passing down between the two conductors
and resting on the bottom of the channel. On the upper face of the T, I fix the
safety plug lying horizontally, one end of it connected by springs or clips to one
of the main conductors and the other to the local branch conductor.
Fig. 6 is a section of a channel containing two conducting cables CC. Retween
these is inserted the vertical limb of the T shaped support S having on its upper
face the safety plug P.
The terminals to which the main conductors are connected may advantageously
be made as shewn in Fig. 7 with holes through them for circulation of air to keep
them cool, or with ribs, as shewn in Fig. 8, exposing large cooling surface.
Having thus described the nature of my invention and in what manner the same
is to be performed, I claim :-
First. Constructing electrical safety plugs of several fine wires or strips of foil
kept apart and enclosed by mica or other suitable insulating material, except at
their ends which are collected into sockets or clamps connected to the main
conductors, substantially as described with reference to Figs 1 and 1*.
Second. In combination with an electrical safety plug, a by-pass circuit through
mercury, substantially as described with reference to Fig. 2.
Third. In combination with an electrical safety plug, a by pass circuit and an
electro-magnet having its coil forming part of the by-pass, for which its attraction
of an armature forms effective contact, substantially as described with reference to
Figs. 2 and 3.
Fourth. In combination with an electrical safety plug, a by-pass circuit with a
subsidiary bell circuit giving an alarm when the plug is endangered and ruptured,
substantially as described with reference to Fig 4.
Fifth. The arrangement of electrical safety plug on a T frame in a conductor
channel, as herein described with reference to Fig. 6.
In Witness whereof I the said Killingworth William Hedges have hereunto set
my hand and seal this Thirtieth day of July in the year of our Lord One thousand
eight hundred and eighty-three.
KILLINGWORTH W. HEDGES (L.S.)