Improvements in or connected with Devices for Protecting Electric Circuits and Apparatus from Injury by Excessive Electrical Potential Due to Lightning or other Cause.
Year: 1897

page_text: Date of Application, 27th Nov., 1897-Accepted, 1st Jan., 1898
COMPLETE SPECIFICATION.
Improvements in or connected with Devices for Protecting Electric
Circuits and Apparatus from Injury by Excessive Electrical
Potential Due to Lightning or other Cause.
I, VALERE ALFRED FYNN, of 13 Bexley Road, Erith, in the County of Kent,
Electrician, 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: -
The object of this Invention is to provide a very sensitive device for preventing
the potential of that point of an electric circuit or apparatus to which the said
device is applied from rising above a certain definite value depending on the
particular conditions prevailing. The said apparatus is particularly suited for
use as a station lightning arrester for electrical power or light plants.
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It is well known that a spark gap between two opposed sharp points affords the
easiest path for such discharges as are here principally contemplated and that
the dynamo current is very apt to follow in the path of a spark bridging the air
space of the gap and thus creating a short circuit." blowing the fuses on the
switchboard or throwing the maximum cut-outs. This whilst protecting the
dynamos causes an interruption of the supply. By my Invention this last serious
drawback is obviated whilst retaining the sensitiveness incidental to the use of
the aforesaid small air gap principle.
I will refer in describing my Invention to the accompanying drawings and
diagrams, Figure 1 and Figure 2 show in front and side elevation partly in
section the arrester constructed according to this Invention. Figure 3 is a
diagram of the connections preferably used with the said arrester when used to
protect a direct current generator..
Figure 4 shews the connections when one pole of the generator is earthed, as
would be the case with a railway generator for instance.
The number and dimensions of the induction coils to be used with these arresters
and the number of arresters themselves and the dimensions of their air gaps are
determined by prevailing circumstances in a manner well understood by electrical
engineers.
The arrester illustrated is constructed as follows:-
A base S of insulating material such as slate or marble carries two blocks A
and B which may be of gun-metal provided each with a number of ordinary arc
lamp carbon pencils (1) filed to a point and longitudinally moveable in holes in
the blocks A and B. They can be secured in their adjusted position by set
screws K, the carbous at A pointing downwards and those at B pointing
upwards.
Lower down on the base is a double bracket 1) forming two bearings through
which passes the shaft E carrying the pulley F and the double lever C. This
lever and also the pulley F are secured to the shaft E so that they turn with it.
The double lever has two cross pieces at its top each of which cross pieces
carries a set of carbon pencils like those in the blocks A and B and adjustable
and fixable in the same manner.
Over the pulley F passes a chain secured to the pulley at 7 and carrying at

page_text: one end the weight W and at the other end the armature of an iron-clad magnet M
secured to the base as shewn and containing an exciting coil, the ends of which
are connected to the terminals 4 and 5 (see the diagram of connections). The
armature is guided to prevent any jambing by side pulls.
The spindle E is electrically and flexibly connected to the terminal 3 which
may be done by thin copper strips .
The arrester is shewn in its position of rest. The weight. W is so proportioned
that it will pull the bracket or lever C back into this position of rest out of any
other position into which a movement of the armature P might bring it.
The travel of the lever C is limited in one direction by the spring buffer Z
preferably made of insulating material, and in the other direction by the buffer X
(made preferably of rubber) against which the weight W hears. The buffer Z
can be regulated by the screw shewn so as to bring the carbon tips on the lever 0.
exactly opposite the carbon tips on the blocks A and B.
The terminals and 4 are connected through a resistance R. The terminal 1
in the block A is connected to the line or apparatus to be protected the terminals 2
and 5 are led to earth.
Two air gaps are now formed between the carbon pencils eiz. from those in A
to those in C, and from the others in C, to those in B. In parallel to the gap
between the carbons C to B is the resistance R in series with the coil exciting
magnet M.
A discharge occurring from the line to earth will, after passing the
gap between the carbons A to C. find two ways open for it, either the
difficult path of high self induction through the resistance and exciting coil, or
the easy path of the second gap between the carbons C, to B, and of course it
will follow the second, and far easier path. If however (and this is the most
probable and most frequent case) the gap between the carbons A to C, has been
made very small to secure sensitiveness, the discharge will bridge it sufficiently
to induce the line or generator current to follow if simultaneously the other pole
of the line or machine has (intentionally as in railway practice or accidentally on
the occurrence of a similar discharge through a lightning arrester) been per-
manently or momentarily electrically connected to earth. Having passed the
first gap the current from the generator or line will also have open to it the
aforesaid two paths but contrarily to the disruptive discharge will always select
what in this case is the easiest path for it, through the resistance Rand
exciting spool to earth. In so doing it will energise the magnet M which attract-
ing its armature with great force will cause the lever to swing out into some
such position as that shewn in dotted lines and thus instantaneously break the
are set up between the carbons A to C. Upon the are breaking, the current
exciting the magnet instantly dies away and the magnet losing its power allows
the lever C, to return to its original position in which it is again ready to discharge
the line.
The amount of current flowing through the are, resistance R, and exciting coil
to earth at such a moment will depend mainly on the resistance R, as the resist-
ance of the short are can be neglected. The exciting coil, according to circum-
stances, will have a negligeable, or appreciable, resistance but in any case much
inferior to that of R.
This condition will always prevail in the case where one terminal of the
generator is earthed (as in tramway practice). It will also be mostly the case
where the opposite has been accidentally earthed. If the short circuit be due to
such apparatus placed on opposite poles as in Figure 3 discharging simultaneously
two gaps, two resistances R and two exciting coils will be in the circuit.
The resistance R should be such that in the case of Figure 3 when two arresters
act simultaneously the current allowed to pass does not exceed say ten per cent.
of the rated full load of the machine to be protected. If through accident the
conditions of Figure 4 are brought about the current passing will amount roughly
to twenty per cent. of the full load current.
When the conditions of Figure 4 are permanent the resistance R may be

page_text: double the amount of that in the previous case so that the current again is
only ten per cent. These figures are of course in no way binding and I give
them only by way of example.
When used with alternate currents the resistance R should be non-inductive
and the self induction of the exciting coil diminished by the use of a condenser
in parallel to the gap between the carbons C. to B. as shewn in Figure 5. The
condenser is also useful with direct currents.
The ampere turns required to attract the armature 1 of the magnet can be
readily calculated by well known methods when travel and weight of Ware
given and the dimensions of the magnet itself are known'.
Having decided
what current should be allowed to flow the number of turns on the exciting spool
are made to suit. In choosing the wire for this spool and for the resistance R
it should be borne in mind that the current passing these lasts only for a very
short time and that they can both be much smaller than would otherwise be
required.
Having now particularly described and ascertained the nature of this
Invention and in what manner the same is to be performed I declare that what
I claim is:
1. In an apparatus for protecting electric circuits and apparatus from injury
by excessive electric potential the combination with two air gaps of a lever for
breaking the arc which may be set up at these gaps together with an exciting coil
and resistance in series therewith, controlling, through the medium of a magnet,
the movement of the said lever and connected in parallel to the second air gap,
substantially as hereinbefore explained.
2. In combination with an apparatus such as is hereinbefore described a re-
sistance in series with the exciting coil and in parallel with the second spark
gap substantially as herein before described.
3. The combination constituting the device described with reference to Figures 1
and 2 of the accompanying drawings for the purposes explained.
Dated this 25th day of November 1897.
JOHNSONS & WILLCOX,
47 Lincoln's Inn Fields, London, W.C., Agents.