Failsafe overvoltage protector

Abstract

 A secondary or back-up protection device, for use in protecting telephone apparatus against lightning or other surges, comprises a pair of electrodes (22, 24) abutting one another. At least one of the electrodes has its abutting surface portion coated by an oxide layer (26). The oxide layer insulates the two electrodes from one another until its breakdown voltage is reached, whereupon the layer is punctured and conduction between the electrodes is permitted. Preferably both electrodes have oxide layers on their abutting surfaces. Generally the device will be used in combination with a gas tube device which is the primary protector. Conveniently the secondary device electrodes then comprise a pair of washers mounted on an insulated spigot (20) protruding from one of the electrodes of the gas tube device.

Description

[0001] The invention relates to overvoltage protection devices particularly for use in protecting communications equipment, for example telephone equipment, against hazardous voltages due to lightning or induced power surges in interconnecting cables.

[0002] It is common practice to protect such equipment by a primary protector, for example a gas tube or carbon block device, which will operate repeatedly to shunt surge energy away from the equipment. However, over a period of time such devices may cease to operate correctly. In particular a gas tube may leak and admit air to replace the gas. As a result the breakdown voltage of the gap increases, perhaps to a level too high for the equipment to tolerate.

[0003] It is desirable then to provide a secondary protection device in parallel with the primary protector. The secondary protection will have a breakdown voltage slightly higher than that of the primary device so it will only operate if the primary protector fails to operate. It is preferable for the secondary protector to operate in air. For telephony and similar equipment, its breakdown voltage will usually need to be about, or less than 1,000 volts. A normal spark gap designed to operate at this voltage in air would have a gap of 0.005 inches or less, which would be difficult to set and maintain. It would also be susceptible to contamination.

[0004] A voltage sensitive switch which does not employ a spark gap and which will operate at less than 1,000 volts has been disclosed in U.S. patent specification number 3,412,220 issued November 19th, 1968. That switch comprises a piece of aluminum foil having a double layer of oxide formed on one surface. The first oxide layer is porous and the second layer is dense. A conductive film provided over the second layer and electrodes are adhered to the film and foil, respectively, by epoxy resin. The entire switch is then encapsulated in resin.

[0005] Such a switch is not entirely satisfactory for use as a secondary protector because its operating voltage is too low and the provision of two layers and use of epoxy resins complicates manufacture with consequent increased costs, a serious disadvantage for a device which is to be mass produced.

[0006] An object of the present invention is to overcome these problems in providing a secondary overvoltage protection device which is cheap and simple to construct yet reliable in operation.

[0007] According to the present invention an overvoltage protection device, for protecting telephone or other communications equipment against lightning or power surges, comprises a pair of electrodes having respective surface portions disposed in abutting relationship. At least one of the electrodes has an oxide layer formed on its surface portion. The oxide layer serves to insulate the electrodes from one another and has a dielectric strength equivalent to a required breakdown voltage for the device. When this breakdown voltage is exceeded, the oxide layer breaks down and permits conduction therethrough between the pair of electrodes.

[0008] Unlike a conventional spark gap, the electrodes of the present device are in contact with each other. A significant advantage of this feature is that ingress of dirt or other contaminants is prevented and the dielectric thickness is determined solely by the oxide thickness, which can be accurately controlled during manufacture.

[0009] Preferably both electrodes have surface oxide layers which between them define the required breakdown voltage. It has been found in practice that, for a required breakdown voltage, two thin films will give a more easily determinable dielectric strength than a single equivalent thick film.

[0010] A particularly compact and cheap embodiment of the invention comprises a pair of washers, each having an annular surface coated with oxide. The washers may readily be combined with a conventional protector, such as a gas-filled tube device, conveniently by fitting them upon an insulated spigot extending from one electrode of the gas-filled tube.

[0011] An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a sectional side view of an overvoltage protection device embodying primary and secondary voltage-dependent switches; and

Figure 2 is a graph showing relative performances of secondary switches having one and two insulating oxide layers.

[0012] Figure 1 shows an overvoltage protection device comprising two switching means connected in parallel. The primary switching means is a gas-filled tube device of generally known construction and is formed by a pair of coaxial cylindrical metal electrodes 10 and 12 sealed in a ceramic tube 14, which is filled with inert gas at sub-atmospheric pressure. The innermost end faces 16 and 18 of the electrodes 10 and 12, respectively, are spaced apart by a small distance, approximately 0.020 inches, to form a spark gap.

[0013] The outer end of electrode 10 has a reduced-diameter axial extension forming a spigot 20.

[0014] The secondary or back-up switching means comprises a pair of metal electrodes in the form of aluminum washers 22 and 24 respectively. Each of the washers 22 and 24 has an annular surface 26 coated with an insulating layer of aluminum oxide. Each layer 26 is formed by anodizing the chemically cleaned surface area of the washer using conventional techniques, for example hard anodizing by immersion in a bath of sulphuric acid electrolyte at low temperature, for a prescribed period of time. Each layer extends across the entire annular surface and at least part of the way across the inner and outer edge surfaces. The enclosed corners are rounded to prevent electric stress concentration.

[0015] The washers 22 and 24 are arranged with their coated surfaces in contact with each other, but are not bonded, and are supported by the spigot 20. A sleeve 28 of insulating material, such as surrounds the spigot 20. The washers 22 and 24 are a friction fit on the sleeve 28. The innermost washer 22 abuts a flanged end part 30 of the electrode 10, which projects radially across the end of the ceramic tube 14. The outermost washer 24 is retained by an inwardly-turned annular flange 32 of a metal cylindrical collar 34 which surrounds the end part of the ceramic tube 14 around the electrode 10.

[0016] The combination of gas-filled tube and oxide-coated washers is housed in a metal casing 36 which is closed at one end. The closed end is formed externally as a hexagon and houses a compression spring 38. The spring 38 acts between the interior end at the casing 36 and a fusible disc 40, made for example of bismuth/tin, disposed between the free end of the spring 38 and the opposed outer end face 42 of electrode 12 projecting from the gas-filled tube. A retainer member 44, in the form of a circular base 46 with a plurality of axially-extending spring arms 48 spaced around its periphery, is located with its base 46 between the spring 38 and the disc 40. The spring contact arms 48 extend in the space between the outside of the gas-filled tube and the interior of the casing 36. The arrangement is such that the casing 36 presses the arms into contact with the metal collar 34. Inturned ends 50 of the arms extend beyond the end of the collar and serve to retain the sub-assembly of gas-filled tube and washers in the casing 36.

[0017] In use, electrical connections are made to the spigot 20 and casing 36, which, by way of spring 38, retainer base 46 and fusible disc 40 connects to the electrode 12 of the gas-filled tube device. The secondary protection means electrodes are connected in parallel with those in the gas-filled tube. Thus, washer 22 is connected to electrode 10 by virtue of its abutment with the flanged surface 30. Washer 24 is connected to electrode 12 by way of the collar 34, spring-retainer arms 48 and disc 40.

[0018] It should be noted that the secondary device will only operate a limited number of times, possibly only once if the current is high enough, before "failing" short-circuit. This is quite intentional since its operation only takes place when the gas tube has failed. Once the secondary device has failed, the fact that it has done so can readily be detected and the faulty protection device, including gas tube, can be replaced.

[0019] The breakdown voltage range of the secondary protection device is arranged to be slightly higher than that of the gas-tube device, but significantly less than the breakdown voltage of the spark gap between surfaces 16 and 18, should the tube 14 become filled with air instead of gas.

[0020] A typical operating voltage range for the secondary protection device is about 700 to 1,000 volts. This was obtained using two oxide coated layers, each about 0.0005 inches thick. Other breakdown voltages can be predicted by interpolating from the following table of breakdown voltages against thickness for single oxide layers:-

[0021] As mentioned previously, the breakdown voltages of a number of devices show less variation if two oxide layers are used instead of one. Figure 2 is a graph plotting number of samples against breakdown voltage to illustrate this effect.

[0022] In Figure 2, curve A represents single oxide layer devices and curve B represents two separate layer devices, one layer on each electrode.

Claims

1. An overvoltage protection device, for telephone or other communications apparatus, comprising:-

(i) a pair of electrodes having respective surface portions disposed in abutting relationship.

(ii) an insulating oxide layer on said portion of at least one of said electrodes.

(iii) said oxide serving to insulate said electrodes from each other and having a predetermined dielectric strength equivalent to a required breakdown voltage for the device.

2. A device as defined in claim 1, wherein each said surface portion has an oxide layer, said layers being in contact with each other and serving between them to define said breakdown voltage of the device.

3. A device as defined in claim 1, wherein the said layer comprises a single surface film formed by anodization.

4. A device as defined in claim 3, wherein each said electrode comprises a washer, the said surface portion comprising an annular area of said washer.

5. A device as defined in claim 4, wherein said washers are supported by an insulated coaxial spigot.

6. A device as defined in claim 5, wherein said spigot comprises a sleeve of insulating material extending between said washers.

7. A device as defined in claim 1, wherein said pair of metal electrodes comprise part of a combination protection device including a further pair of electrodes operable in parallel with the first pair of electrodes.

8. A device as defined in claim 7, wherein one of said further pair of electrodes has a spigot, the first pair of metal electrodes comprising a pair of washers mounted in juxtaposed relationship about said spigot, said surface portion comprising an annular surface of said washer.

9. A device as defined in claim 8, wherein said further pair of electrodes are mounted in an insulating sleeve in a metal casing, said casing connecting together electrically, the other of said further pair of electrodes and one of said first electrodes, the other at said first electrodes being connected to the spigot-bearing electrode.

10. A device as defined in claim 9, wherein both juxtaposed annular surfaces of said washers have oxide layers, said layers defining between them the breakdown voltage between said washers.

11. A device as defined in claim 9, wherein said further pair of electrodes are sealed to said insulating sleeve and said sleeve is filled with gas at sub-atmospheric pressure.

Drawing