US2505668A - Lightning arrester - Google Patents

Description

April 25, 1950 E. GOMONET 2,505,668

LIGHTNING ARRESTER Filed Feb. 10, 1947 hum/roe L g lad??? 00 7' 5K 06 WE Patented Apr. 25, 1950 LIGHTNING ARRESTER Edouard Gomonet, Paris, France, assignor to Socit Anonyme pour les Applications de lElectricit et des Gaz Rares-Etablissements Claude- Paz et Silva, Paris, France Application February 10, 1947, Serial No. 727,552 In France February 25, 1946 My invention relates to overvoltage cutouts, hereafter named lightning arresters, which are used to protect electrical equipments against possible overvoltages.

It is known to use as lightning arrester an electric discharge apparatus with low pressure gaseous filling, so designed as to start before the overvoltage has risen to a dangerous value and then to divert to earth a portion of the electrical energy sufficient for the desired protection.

Whenever the voltage across the discharge apparatus reaches the starting voltage of this apparatus, the discharge starts in the form of a glow discharge, the glow covering then a small part of the cathode area. If the voltage rises, the discharge current grows up and the cathode glow spreads gradually over the entire cathode area. If the magnitude of the disturbance is sufficiently high, the discharge turns into an arc discharge; the discharge concentrates then on the cathode in a cathode spot the area of which is small but the temperature high. This hard working condition is liable to modify the characteristics of the lightning arrester, specially its starting voltage.

One object of the invention is to provide a simple, easy to manufacture and reliable lightning arrester.

Another object is to provide a, lightning arrester which keeps its characteristics constant, specially its starting voltage, which depends, inter alia, on the superficial condition of the electrodes.

Still another object is to provide a lightning arrester, of small overall dimensions and com prising no movable part, able to sustain strong currents without its starting voltage being altered.

According to the invention, each electrode of the lightning arrester comprises at least two zones, their relative places and differences, if any, in the superficial compositions and in their forms being such that the discharge starts in all cases on a first zone, named starting zone, without an arc discharge occurring there and that if the discharge turns into an arc discharge, the cathode spot be always localized on the second zone, named arc zone and only on this zone.

The starting zones are preferably of small radius of curvature and nearer the one to the other than any other part of the electrodes. Moreover, the surface of these zones must be as even as possible. In this manner, the starting of the discharge takes place between the desired zones.

Each "arc zone of one of the electrodes must be sufiiciently distant from the other electrode as Claims. 11. 250 2'z.s)

to avoid any discharge starting from this zone, notwithstanding the alterations produced in this zone by the arc discharges. To each arc zone" may be applied any known means, for instance emissive coating, heterogeneous surface, to facilitate the appearing or the maintaining of a cathode spot on this zone.

Moreover, it is advantageous that the relative positions of the different parts of the electrodes be such that the convection currents, due to the warmth produced by the discharge, or the electro-magnetic forces, or both of them, bring towards the "arc zone the arc discharge which would, on occasion, have started on another part of the electrode.

As is known, radioactive substances can be placed inside the lightning arrester in order to avoid starting delays.

In the accompanying drawing, given merely by way of example, Figures 1, 2, 3 represent, in axial section, different embodiments of the invention. In these three embodiments, the electrodes are enclosed in a glass enclosure R, filled with gas under low pressure.

Referring to Figure 1, which represents a lightning arrester intended to be used when its axis XX is substantially horizontal, in the enclosure R are placed two electrodes, each comprising a lead-in wire D and two cups A, C fixed on this wire. The edge T of each cup A. is the part of the corresponding electrode, which is the nearest to the other electrode; this edge T forms the "starting zone of the electrode and fixes the starting voltage. The edge U of each cup C is provided with 9, coating of emissive oxides and forms the arc zone. This lightning arrester comprises behind each electrode, a mica screen B which is intended to prevent the discharge from going around the electrodes and setting on the lead-in wires. The Working of this arrester is as follows: when the overvoltage occurs, a glow discharge starts; it begins on a part of edge T, which forms the "starting zone and spreads by degrees on the surface of the electrode working as a cathode, without deteriorating it if it has been suitably degassed. If the current through the arrester is sufficient, the glow covers the cups A and C and the section Y of lead-in wire which join them. If the current grows further, the discharge turns into an arc discharge; the cathode spot takes place on a point of the edge U of cup C on account of the emissive oxides there located, and which the glow discharge has heated up. The surface alterations, caused by the cathode spot, are confined to the arc zone and do not therefore influence the starting voltage. If, exceptionally, the cathode spot occurs on the cup A, it does not remain there because the convection currents shift upward the discharge.

Referring to Figure 2, which represents a lightning arrester the working of which is the same as above: each electrode comprises a conical part L ending in a, round shoulder E of small radius of curvature on which the starting takes place; cone L is joined, by a round shoulder of small radius of curvature, to a cylindricalrpaitta G intended to improve the cooling of cone L;

so does the metallic fiange 'I-Ifwhich-joinsthis cone to the lead-in wires J On, the surface 1.

of round shoulder F, which forms the arc zone/1' are imbedded silicon carbide grains, the object of which is to fix the cathode spot from which the arc originates, and to so prevent it from originat ing on the round shoulder E which forms the. starting zone.

Eachv electrode is supported by two lead-in wires J, K which are-joined together outside'the arrester; Mica screens I prevent the .arc,from going around the electrodes.

Referring to-Figure 3, each. electrode of the lightning'arrester comprises a heavy and polished metal wire N'and an helix M, single Or double coiled, made of a wire of refractory metal coated with earth alkaline oxides, the heavy wire and the helix being series, connected; the 1ead-in wire "is connected to the end of thehelixW not connected to the wireN. Each wire N comprises a section V located at such a distance from the other-that the startingof'thedischarge willoccur between these sections under the voltage: desired for the starting of the lightning arrester and shallalways occur between these sections; the arc zone of each electrode isformedby the helices M coated with emissive substance.

'The operation of this embodiment is as follows: when the overvoltage occurs, a glow discharge starts between the section V; as the .current builds up, the glow spreads and reaches the helix'M which it warms up jointly with the Joule effect caused by the flowing of, the current which delivered by the wires N; ultimately the, arc. strikes, its cathode ,spot taking place on the'helix M; The striking of the arc on the endiW of an helix to which the, current is brought, occurs easily, specially since, during the. glow discharge, 30

the ohmicvoltage drop, due to the resistance of helix M, causes a potential difference between the end W and the starting zone" of same electrode.

What I claim is:

1. A lightning arrester comprising an envelope containing a gaseous atmosphere and two electrodes sealed 'therein, each-electrode comprising a starting zone and an arc zone, the two zones differing in their surface composition, the surface composition of the starting zone being less liable: than that of the arc zone to promote the transformation of a glow discharge into an arc discharge} the distance between each electrode an'd 'the starting, zone of the other electrode being shorter than thedistance between said electrode and the arc zone of said other electrode, the discharge-,startingthus. in any case, on the starting zone and the cathode spot, if the discharge turns into any are discharge, localizing itself spontanously on the arc zone and only on this zone.

2. A lightning arrester according to 1, in which the surfaceof the arc zone comprises emissive oxides.

3'? A lightning arrester a'ccordingto 1; in which the arc zone surface comprises grains of; a

semi-conductor;

A lightning'arrest'er according to 1;,in which the arc zone"of each electrodeis constituted by a. wire of refractory metalhelically wound and'coated with thermoemissive. material," connected'in series with a rod which constitutes the startingzone of same electrode.

5. A lightningarrester according to 1, in which the'startin j zone of each electrode isplacedbe lowi at least a part of the arc zone of same electrode;

E O 'ARD. OMON T REFERENGES CITED;

The following' refe'r'ences are oi record inthe fileof this patent:

UNITED STATES, PATENTS

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