US2562692A - Overvoltage protective device - Google Patents

Description

y 1951 J. A. BIGWOOD OVERVOLTAGE PROTECTIVE DEVICE Filed Juno 4, 194a Avail-V all I I I FIG. 2

FIG. 3 FIG. 4 FIG. 5

Patented July 31, 1951 2,562,692 OVERVOL'IAGE PROTECTIVE DEVICE John A. Bigwood, Stamford, Conn, alslgnor to Air Reduction Company, poration of New York Incorporated, a cor- Application June 4, 1948, Serial No. 31,113 2 Claims. (01. 200-1135) This invention relatesto devices for protecting electrical apparatus from injury by abnormally great electric currents, such for example as accidentally produced violent surges of current, lightning discharges, and the like. More particularly the invention relates to improvements in overvoltage protective devices and lightning arresters oi the kind in which a pair of electrodes are mounted in a glass tube or other container filled with an ionizable gas, at least one of the electrodes being thermally responsive or thermostatic so that it is movable upon being heated, and portions of the two electrodes being normally in spaced relation to form a discharge gap between them.

Devices 01' this kind can be used, for example, to protect electrical equipment in a grounded electric circuit by connecting one electrode of the protective device to the high potential side of the circuit and grounding the other electrode. An overvoltage in the circuit that might produce a current large enough to injure the electrical equipment will, it high enough to equal or exceed the breakdown voltage of the discharge gap in the protective device, ionize the gas at the gap so that current will pass across the gap. This produces heat which will cause relative movement of the electrodes towards each other until they make contact and thereby short circuit the gap and connect the high potential side of the circuit directly to the ground thereby protecting the electrical equipment in the circuit. When the overvoltage condition is corrected the electrodes will cool oil and return to normal position.

Considerable difllculty has been experienced in manufacturing a protective device of the kind above described in which the moving electrode or electrodes will return exactly to normal position after each operation of the device. This is due to distortion of the electrodes which nearly always results after repeated operation of the device and sometimes after only one or a few operations of it. The electrode distortion changes the size of the original or normal discharge gap between the electrodes, making it smaller if the distortion is in the direction of movement 01 the electrodes toward closed-gap position and making it larger if the distortion is in the opposite direction. Since the breakdown voltage of the device varies with the size of the discharge gap between the electrodes, any changes in the size of the gap during successive operations of the device will cause corresponding variations in the breakdown voltage and this is objectionable, of

r or thermostatic course, because the breakdown voltage should remain substantially constant at all times to insure reliable operation of the device.

An overvoltage protective device of the kind described above is disclosed in the patent to L. B. Brach No. 2,124,364. In the device of that patent it was sought to overcome the above-mentioned difficulty resulting from electrode distortion by the use of stop means for limiting the outward return movement of the electrodes durmg their cooling so that they will not move tarther apart than the distance across the original or normal discharge gap. It is obvious that such stop means, while operative to prevent widening of the normal discharge gap of the kind which elec trode distortion tends to produce when the distortion is in the opposite direction to the direction of movement or the electrodes towards is not effective to prevent a lessening in the size of the normal discharge gap of the kind which electrode distortion tends to produce when the distortion is in the direction of movement of the electrodes towards closedgap position.

According to the present invention entirely different means are provided to preserve the approximate size of the normal discharge gap and such means is eii'ective no matter in what direction the electrode distortion takes place, i. e. no matter whether the distortion is in a direction to cause lengthening of the normal discharge gap or in a direction to cause shortening of it. At least one of two cooperating electrodes is bimetallic or otherwise made thermally responsive so as to be movable upon being heated to produce relative movement between the electrodes. Parts 01' the two electrodes are arranged for relative parallel motion and to form between them a discharge gap which, in the normal position of the electrodes, is the smallest gap that exists between the electrodes, and con tinues to be so without substantial change in size throughout a predetermined range of relative movement of the electrodes. Thus, even it the electrodes fail to return all the way to their original position upon cooling, or even if they move apart too far during cooling, the size of the normal discharge gap will not be materially aifected. However, when the relative movement of the electrodes during heating is extensive enough, parts of the electrodes, or electric contacts carried by the electrodes, are brought into engagement to short circuit the discharge gap.

The accompanying drawing illustrates the invention as applied to several different types of overvoltage protective device or lightning arrester.

In the drawing:

Figure l is a longitudinal section through one type of protective device embodyingthe invention, the device shown in this figure being of the so-called double-end" type;

Fig. 2 is a longitudinal section taken on the line 2-2 of Fig. 1;

Fig. 3 is a longitudinal section through another type of protective device embodying the invention, the device shown in this figure being of the so-called single-end type;

Fig. 4 is a horizontal section taken on the line 4-4 of Fig. 3; and

Fig. 5 is a longitudinal section through a double-end type of protective device operating on the same principle as the single-end device illustrated in Figs. 3 and 4.

Referring first to Figs. 1 and 2, the protective device therein illustrated comprises a gas-tight container or envelope I which is preferably a sealed glass tube having at opposite ends re-entrant portions 2 and 2 commonly known as presses. In press 2 are sealed lead-in wires 3 and 4 electrically connected to an electrode 5. Since both lead-in wires are connected to the electrode they constitute in effect only a single electric conductor, but by using two of them and making them heavy enough they serve as supports for the electrode. If desired, however, a single lead-in'wire may be employed and if it is not heavy enough to support the electrode the electrode may have a special support which is heavy enough for the purpose and the lead-in wire may be electrically connected with this support or to the electrode itself. Similarly, in press 2' are sealed lead-in wires 6 and 1 electrically connected to an electrode 8. The envelope I is evacuated and then filled with an ionizable gas such for example as argon or helium.

In the form of the device shown in Figs. 1 and 2 each electrode is in the nature of a coiled thermostatic strip, preferably a bi-metallic strip, the two strips being coiled in opposite directions as clearly shown in Fig. 1. The electrodes are arranged so that the coiled portions of the electrodes are located side by side and partly overlap. Thus one edge 9 of the electrode 5 is in confronting spaced relation with one edge ID of the electrode 8. These parts of the two electrodes form between them a discharge gap H which, in the normal position of the electrodes, is the smallest gap between them. When a voltage is applied to the two electrodes which equals or exceeds the break-down voltage of the device the gas at the gap is ionized and current passes from one electrode to the other across the gap. This produces sufiicient heat to cause the coiled portions of the electrodes to straighten out or move toward an uncoiled condition. When the electrodes have moved far enough in this manner the end of the electrode 5 will make contact with a strip of metal l2 projecting laterally from the electrode 8, and the end of the electrode 8 will make contact with a similar strip of metal 13 projecting laterally from the electrode 5. The discharge gap will then be short circuited and a metallic conducting path will be established from the high potential side of the circuit to the ground as above described. When the overvoltage condition that caused the electrodes to uncoil is corrected either by the short circuiting of the gap or in some other way the electrodes will cool and return to their coiled condition. It will be noted that during relative movement of the electrodes the parts of the electrodes between which the discharge gap H is formed, 1. e. the confronting edges 9 and 10 of the electrodes, have relative parallel motion such that the discharge gap ll does not materially change in size and continues to be the smallest gap between the electrodes throughout practically the entire range of relative movement of the electrodes until the ends of the electrodes closely approach the contact strips I2 and I3. It will therefore be seen that even if the electrodes do not return entire- 15/ to their original position upon cooling, or move beyond their original position during cooling, it will not materially affect the size of the normal discharge gap II.

While both electrodes 5 and 8 have been described as being bi-metallic or otherwise thermally responsive, only one of them need be so, in which case the other one will not move. However, the bi-metallic movable electrode will move during heating until its end makes contact with the laterally projecting strip on the stationary electrode and the gap will be short circuited. The lateral projecting strip on the movable electrode in this case has no function so may be omitted.

Figs. 3 and 4 illustrate a modified form of protective device embodying the invention. In this case the protective device is of the single-end type, both electrodes projecting into the gas filled envelope at the same end thereof. The glass envelope I has a press M at one end which supports the two electrodes 5' and 8' as by means of the two lead-in wires 3 and 4' electrically connected to the electrode 5 and the two lead-in wires 6' and l'electrically connected to the electrode 8'. The two electrodes project into the envelope, as shown, with their distal ends in confronting spaced relation. At least one of the electrodes is bi-metallic or otherwise made thermally responsive. A pair of electric contacts 15 and [6 may be provided on the confronting faces of the distal ends of the electrodes. One of the electrodes, say the electrode 5', has a metal strip II secured to it and this strip projects transversely toward and a short distance past the other electrode 8'. The strip I1 is arranged so that its inner face l8 forms with the adjacent edge IQ of the electrode 8' a discharge gap 20 (Fig. 4) which in the normal position of the electrodes is the smallest gap which exists between the electrodes. Therefore, when a potential is impressed on the electrodes which exceeds the breakdown voltage of the device the gas at the gap 20 will be ionized and current will fiow across this gap. The heat produced will cause relative movement between the electrodes due to one or both of them being thermally responsive. The electrodes are positioned so that the relative movement produced when they are heated causes the distal ends of the electrodes to relatively approach each other. The transversely extending strip I1 is directed'so ,that its inner face 18 and the edge IQ of the electrode 8' have relative parallel motion and so that the discharge gap 20 does not materially change in size and continues to be the smallest gap between the electrodes during a predetermined amount of relative movement of their distal ends. When the distal ends of the electrodes have moved far enough the electric contacts l5 and I6 are brought together thereby short circuiting the discharge gap and causing the electrodes to cool and return to their normal position.

Fig. illustrates a double-end protective device operating on the same principle as the singleend device shown in Figs. 3 and 4. In this case the glass envelope I is similar to the glass envelope of the device shown in Figs. 1 and 2, and the electrodes 5" and 8" project into the envelope from its opposite ends so that their distal ends are in confronting spaced relation, one of the electrodes having attached to it a transverse strip of metal l1 related to the other electrode and functioning in the same manner as the transverse strip 11 of the protective device of Figs. 3 and 4.

It will now be seen that in all forms of the protective device at least one of the electrodes is thermally responsive so that it is movable upon being heated to produce relative movement between the electrodes, and that parts of the two electrodes are arranged for relative parallel motion and they form between them a discharge gap which is the smallest gap existing between the electrodes when they are in their normal position and this gap does not materially change in size throughout a predetermined range of relative movement of the electrodes. This range of relative movement of the electrodes is chosen or predetermined with due regard to the maximum amount of normal electrode distortion that would be expected to take place in the ordinary operation of the device. Therefore, even if the electrodes do not resume their original or normal position after an operation of the protective device due to any amount of electrode distortion within said maximum amount, the size of the normal discharge gap will not be materially affected and the breakdown voltage of the device will therefore remain substantially the same even after a number of operations of the device.

I claim:

1. An overvoltage protective device comprising a sealed envelope containing an ionizable gas, an electrode in the envelope comprising a bimetallic strip which is stationarily mounted at one end but can flex upon being heated, a second electrode in the envelope relative tcwhich the first electrode moves when it flexes, said second electrode having a surface which is parallel to and in overlapping spaced relation withan edge portion of said bimetallic electrode and extends in the direction of movement of such edge portion and forms therewith a discharge gap which in the normal position of the electrodes is the smallest gap existing between the electrodes and continues to be so without substantial change in size throughout a predetermined range of relative movement of the electrodes, parts of the two electrodes being arranged to make contact after a predetermined maximum relative movement of the electrodes from normal position said overlapping relation between said surface on the second electrode and said edge portion of the bi-metallic electrode existing throughout a portion only of said bi-metallic electrode which is spaced from the stationary end thereof whereby current supplied to the stationary end of said iii-metallic electrode passes through a portion of such electrode between its stationary end and the gap-forming edge portion thereof.

2. An overvoltage protective device comprising a sealed envelope containing an ionizable gas, an electrode in the envelope comprising a bimetallic strip which is stationarily mounted at one end but can flex upon being heated, a second electrode in the envelope relative to which the first electrode moves when it flexes, said bimetallic electrode having a fiat surface which moves in its own plane when the bi-metallic electrode flexes and that is in overlapping spaced relation with an adjacent part on the second electrode and forms with such part a discharge gap which in the normal position of the electrodes is the smallest gap existing between the electrodes and continues to be so without substantial change in size throughout a predetermined range of relative movement of the electrodes, parts of the two electrodes being arranged to make contact after a predetermined maximum relative movement of the electrodes from normal position said overlapping relation between said surface on the bimetallic electrode and said part on the second electrode existing throughout a portion only of said bi-metallic electrode which is spaced from the stationary end thereof whereby current supplied to the stationary end of said bi-metallic electrode passes through a portion of such electrode between its stationary end and its gapforming surface.

JOHN A. BIGWOOD.

REFERENCES CITED The following'references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,228,210 Hays, Jr. Jan. 7, 1941 2,327,557 Rath Aug. 24, 1943 2,329,134 Peters Sept. '7, 1943 FOREIGN PATENTS Number Country Date 539,465 Great Britain Sept. 11, 1941

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