Lightning Arresters and Discharge Protectors.
Year: 1893

page_text: Date of Application, 18th Apr., 1893-Accepted, 20th May, 1893
COMPLETE SPECIFICATION.
Lightning Arresters and Discharge Protectors.
I, ELIHU THOMSON, residing at Swampscott, County of Essex, State of
Massachusetts, United States of America, 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 principal distinguishing feature of the present invention is the manner in
which a lightning or other high potential discharge is enabled to pass over a
discharge path interrupted by spark gaps, by inducing from such discharge in a
proper circuit a secondary discharge or discharges of greatly increased potential,
such as will leap a space between electrodes placed at a considerable distance apart,
or leap a succession of electrodes placed at such a distance apart that the normal
potential of the line is unable to maintain a continued arc thereat, thus avoiding
permanently grounding the line current, a difficulty which always has to be guarded
against, and which is especially liable to occur when the line current is of itself of
considerable potential. This secondary discharge gives rise to a series of sparks
jumping across the discharge space, which breaks down the dielectric so that the
main discharge will follow even though it might not otherwise, of itself form an
arc across the terminals.
The manner in which this result is secured will be understood from the drawing,
in which
Fig. 1 is a diagrammatic view of the invention.
Fig. 2 is a sectional view of a casing containing oil in which the coils are
immersed.
Fig. 3 is a diagram showing the mode of coupling up the secondary coils in
circuit, and
Figs. 4 and 5 illustrate different forms of arc disrupting devices which under
certain conditions it is desirable to use.
By E is represented a connection to earth or, it may be, to any part of an
electric apparatus to which it is desired a discharge shall run and be neutralized.
The discharge plates or terminals P, P¹, P", of which three are shown, are
separated a considerable distance apart, farther than has heretofore been the
custom, so that the line potential will not maintain an arc across them even though
a static discharge does leap the intervening spaces. The plate P is connected to
the earth plate E, P' is an intermediate plate, and P is connected to the line L to
be protected through the primary of an induction coil A, A', made of a few turns
of highly insulated wire. The secondary consists of fine wire coils S, S',
surrounding the primary turns, which coils respectively have one end connected to
plate P and to plate P, while the other ends of both coils are connected to the
intermediate plate P¹ through condensers C, C¹, of moderate capacity and high
insulation resistance, all as will be readily traced on Fig. 1. Shunting the gaps
between and connected to the plates P, P", is a condenser of somewhat larger
capacity as at K.
The action of the apparatus is as follows: Suppose line L, to which it is
connected, to be a high potential circuit, such as would maintain an arc across
the ordinary lightning arrester plates when unprovided with arc rupturing
appliances, but which will not maintain an arc across the gaps between the
plates P and P, since as has been explained these are of greater width than is
usual in such devices. Now let a static discharge pass through the line L tending
to go to earth, a quick impulse of current passes through the coarse wire primary
charging the condenser K; there will at once be an induction of great energy set

page_text: as a
up in the secondary fine wire coils S, S', which will charge the condensers C, C',
and cause thin sparks to leap across the gaps between plates P, P', and P', P".
These thin induced sparks will cause condenser K to discharge instantly as the
dielectric has been broken down by the sparks. A snap and somewhat considerable
discharge will appear between the plates, and for a moment this discharge will act
good connection to earth and discharge the line L of any static potential.
The distance, however, between the plates is so great that the normal potential of
the line cannot maintain arcs thereat and so become grounded. The number of
turns in the fine wire coils S, S¹ and the capacity of the condensers C, C and K
must be adjusted to give the spark as stated between the plates. This adjustment
once obtained need not be changed. The especial purpose of the small
condensers C, C¹, is to guard against any short-circuiting of the line current
between plates P and P through the secondary coils S, S'. If the resistance of
these coils be so high that any such leakage, in case the condensers be omitted, does
no harm, then the condensers may be dispensed with. It is obvious that the
condensers may be placed anywhere in the circuit of coils S, S', and need not
necessarily be in the position shown.
On account of the very high potential developed in the coils S, S¹, even when a
very moderate length of wire is used, it is well to immerse them in a good dielectric
fluid, like oil. For this purpose, as shown in Fig. 2, a small vat Q may be provided
containing oil, and the inner coarse coil forming the primary will then be put in a
glass tube around which the finer coils S, S' are wound, the whole being immersed
in the oil so as to be fairly free from static discharges or short-circuiting. I find
that a single layer of fine wire in the coils S, S' will often be sufficient. In case
more layers are used the windings should be carefully arranged so that the discharge
does not short-circuit between the layers.
In Fig. 3 the manner of coupling up the secondary coils in circuit with the
different arrester plates is shown separately. It is the same as in Fig. 1, but in this
figure the condenser K and the primary coils acting inductively on the coils, S, S',
are omitted for the sake of clearness. Here also the discharge plates are shown
tapered, thereby forming diverging discharge spaces between them, such as will
easily enable an are to become extended or lengthened and break.
Though in many cases the arrangement described will do away with the necessity
for special arc rupturing devices, still the invention does not preclude the application
of any of the usual means of blowing out or distorting an arc at the discharge
plates. For example, a magnet M, seen in Fig. 4, may be used, the poles of which
overlap the discharge gaps.
As shown in Fig. 5 an upward draft of air, or an air jet may be allowed to blow
on the spaces between the arrester plates, and in fact a portion of the current
itself may be made to set up a draft in properly constructed chimneys O, O', by
heating or resistance coils R R located therein, and through which the current is
passed. The addition of such appliances will be useful when dealing with very
high potentials in which case the discharge spaces, in order to properly extinguish
the are, would otherwise have to be made too great, and thus excessive potential
in the fine wire coils S, S', would be required in order to spark across them, and
thus start the discharge.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is:
1. The herein described means for breaking down the dielectric and enabling the
passage of an electric discharge over the spark gap of a lightning arrester, con-
sisting essentially of an induction device arranged to induce a magnified electric
strain across the spark gap or gaps greater than that of the discharge itself.
2. The combination of a protective spark gap or set of gaps, and means
controlled by the discharge current for breaking down the dielectric in the gap by

page_text: the momentary exhibition thereat of a potential or electric strain greater than that
which would be due to the discharge itself, as set forth.
11
1
T1
1000
ELIHU THOMSON.
Haseltine, Lake & Co.,
45, Southampton Buildings, London, Agents for the Applicant.