Improvements relating to Safety Devices for Electric Circuits.
Year: 1890

page_text: COMPLETE SPECIFICATION.
[Communicated from abroad by THE THOMSON-HOUSTON INTERNATIONAL ELECTRIC
COMPANY, of Boston, Massachusetts, United States of America, Manufacturers.]
Improvements relating to Safety Devices for Electric Circuits.
I, HENRY HARRIS LAKE, of the firm of Haseltine Lake & Co. Patent Agents,
Southampton Buildings, in the County of Middlesex, 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:-
My invention relates to safety devices for electric circuits.
My said invention is very advantageous for use in circuits of comparatively low
tension and is designed to prevent the accidentally charging of the same with a
current of much higher potential than that for which it is intended.
In the operation of electric circuits wherein currents of high tension are employed,
considerable danger arises from the fact that a local circuit conductor, or conducting
material in the vicinity of the said high tension circuit may become accidentally
connected therewith. Under this circuinstance a person whose body forms a ground
connection from the local circuit or conductor may receive a dangerous shock should
there be at the same time a leakage to earth in the main or high tension circuit.
This danger is present in greater or less degree whether the neighbouring conductor
be carrying current or not, and it is the object of my invention to provide for any
condition where from the location or nature of the neighbouring conductor it may
acquire accidentally, the potential of a high tension circuit and so become a source of
danger.
My invention, although having a wide application as above set forth is more
especially adapted for systems of electric distribution in which the local or
neighbouring circuit is the path of low tension currents derived or converted from
the high potential circuit, such for instance as the alternating current system of
distribution by induction coils or "transformers."
My present invention relates to the nse in the secondary or local circuit of a safety
device or low resistance shunt or connection to earth, so arranged that normally the
said shunt is interrupted, there being no connection to ground from the secondary or
local circuit until a connection is formed between the high tension circuit and the
secondary, whereby the high potential of the primary or main line is transferred to
the secondary or local circuit or apparatus. This interruption of the shunt is effected
by an insulating film normally separating or insulating two contacts or electrodes,
which insulating film will on the potential rising abnormally in the low tension
circuit, be punctured, pierced, disrupted or otherwise affected so as to establish electrical
connection between the said contacts by arcing and melting of the metal, transfer of
metal, impingement of one contact upon the other through spring or other pressure,
reduction of the film from an insulating to a conducting state, or by any other
convenient method whereby two contacts normally separated may have electric
connection established between them.
My invention further comprises the combination with the safety device, of a
condenser applied in the manner and for the purpose hereinafter set forth.
In the accompanying drawings
Fig. 1 is a diagram illustrating the application of my invention to the secondary
circuit of a transformer.
Fig. 2 shows the method of combining a condenser with my improved device.
Figs. 3 and 4 show modifications of any invention hereinafter described.
In Fig. 1, a and b indicate a high potential primary line, a branch from which
passes through a coil P forming the primary of an induction coil or transformer..

page_text: The secondary circuit S of this coil may be used to feed a circuit of lights L, L or for
other purposes, suitable safety-fuses f,f,f,f" being provided in both primary and
secondary circuits.
From a point in the secondary circuit, preferably near the middle of the coil S, a
connection is taken to a contact piece K, bearing on a thin layer or film F of oiled
paper or other suitable insulating material. An earth plate or connection G, is
provided on the other side of the film. I thus form, as it were, a film cut-out or
weak spot in the insulation between the earth or ground and the secondary
circuit.
circuit "
So long as no electrical connection exists between the primary coil or circuit P and
the secondary coil or circuit S, there will be no puncture or rupture of the film F.
If, however, such a connection, either temporary or permanent, is formed between the
bor primary coil P, and the secondary coil S, the film F is punctured by
the high potential current in endeavouring to leak to earth, provided that a leakage
from or static inductional capacity exists in the primary circuit a, b, sufficient to give
rise to the necessary discharge to earth in puncturing the film, as is generally the
case in a circuit of considerable length.
The insulating film F having been punctured a direct connection is established
between the contacts or electrodes K, G, one or each of which may be pressed by a
spring K towards the other in the manner indicated. The contact pieces K, G,
having thus come together the secondary coil S has thus a solid connection to earth
by a circuit of low resistance and no danger can exist in touching the wires of the
secondary circuit, even though connected with the primary circuit, when a person
may be at the same time grounded for he is already shunted by the connection
through the electrodes K, G, to earth.
In my present invention the connection of the secondary circuit to earth does not
take place until the current therein has reached the same or nearly the same potential
as the primary circuit when a puncturing of the film F takes place.
It is obvious that any other conductor or set of conductors can be similarly
protected in the manner above described the connection being made with the said
conductor at any convenient point.
It may be well in some cases to shunt the film as it were, or rather to place around
the film a condenser, as in Fig. 2 of small capacity, the condenser being represented
at C, und consisting of plates separated by insulating material in the ordinary way,
the plates opposed to each other being respectively connected one to the ground
circuit and the other with the connection to the secondary coil or conductor which
it is desired to protect. In this case a small amount of static induction or leakage
may occur between the primary and the secondary coils of the transformer without
puncturing the film F the condenser C, acting to remove the current which would
reach the secondary by such small or slight induction or leakage; but if a good
connection between the primary coil P, and the secondary coil S, be formed, then
the condenser C will be insufficient in capacity to lower the potential of the alternating
discharges which reach the secondary circuit and the insulating film will bo at
once punctured, making the connection to earth as before and shunting the
condenser.
:
A similar safety device may be connected with the conducting wires of main leads
in different parts of the circuit as indicated by the dotted lines 10, Fig. 1 in certain
instances to ensure greater security.
In the specifications of former British Letters Patent granted to mo viz. :
number 6063 and 60634 dated the 9th April 1889 I have shown the connection
of the safety device as leading from an independent conductor interposed between
the primary or main circuit and the secondary or local circuit and which conductor
assumes or partakes of the dangerous potential of the secondary or local circuit
whenever the latter becomes connected with the main or principal circuit carrying
the current of high tension by any discharge tending to pass from the high potential
primary to the secondary. This independent conductor is placed between the primary

page_text: and the secondary coils in cases where they are in proximity to one another or
wherever they are capable of having a connection formed between them.
The application of my present invention to this modification is shown in Fig. 3
where I indicates the interposed conductor between the primary and secondary.
In Fig. 4 I have shown a modified form of my invention in which each of the
wires 10, 10, connected to one of the terminals of the secondary circuit is separated
by an insulating film F, placed between a contact pieco K connected with the said
wire 10*, and a terminal or contact piece G connected to earth, the two contact
pieces G being likewise connected together by a suitable conductor from the two ends
of a spring G, connected to earth at E. A high potential in the secondary circuit
will puncture both insulating films F thereby grounding and also short-circuiting
the secondary circuit which will result in fuses f, f, or f',' being melted and
consequently cutting out the local circuit.
It is obvious that my present invention is applicable to any circuit or conductor
which is liable to dangerous connection with a high potential circuit carrying currents
of great tension.
Although I have specified oil paper as material for forming the insulating film,
yet I do not confine myself to this substance alone but can use other suitable materials
or combination of materials in different cases.
In some instances I employ material which is normally an insulator but which, by
the action of a current, will be reduced to a conducting state. For this purpose a thin
layer of oxide of lead may be applied as a paste to one of the contacts or electrodes.
This material is normally an insulator, but when the high potential current breaks its
way through the thin film it generates heat which reduces the substance to a
conducting state and forms, or aids in forming, the electrical connection between the
two electrodes.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, as communicated to me
by my foreign correspondents I declare that what I claim is :-
First. The herein described method of providing for the conduction to earth from
an electric circuit or conductor, of currents or excessive or abnormal potential whilst
permitting the use in the said circuit or conductor of currents of low or normal
potential.
Second. The combination, with a local or independent circuit or conductor liable to
accidental connection with a high tension conductor, of a safety device consisting of
a contact-piece or electrode connected with the said circuit or conductor and separated
by an insulating film from another contact piece or electrode connected to earth, as
and for the purpose described.
Third. In a system of electrical distribution in which local lines are supplied with
currents of low tension from a high potential line, a low resistance ground wire or
circuit normally interrupted by an insulating film interposed between two contacts
or electrodes as and for the purpose described.
Fourth. The combination, with a safety device, consisting of electrodes bearing one
upon the other, but normally separated by insulating material, of a condenser
substantially as, and for the purpose, above specified.
Fifth. The combination, with an electric converter, of a ground circuit from the
secondary thereof including electrodes between which is thin layer of insulating
medium, and a condenser in a shunt around the said electrodes, as, and for the purpose,
described.
Sixth. The combination, with an electric circuit, of two or more of my improved
safety devices substantially as herein before described with reference to Fig. 1 of the
accompanying drawings.
Seventh. The combination, with an electric circuit, of two or more safety devices
of the kind or class herein before described connected to earth and to each other so as
to form a short circuit when the thin insulating medium is punctured, for the purposes
above specified.

page_text: Eighth. The modification of my invention substantially as described with reference
to. Fig. 3 of the accompanying drawings, for the purpose specified.
Dated this 22nd day of March 1890.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, Agents for the Applicant.