GB2153138A

GB2153138A - Surge voltage arrestors

Info

Publication number: GB2153138A
Application number: GB08401444A
Authority: GB
Inventors: Alistair Mark Hunt
Original assignee: Kenneth E Beswick Ltd
Current assignee: Kenneth E Beswick Ltd
Priority date: 1984-01-19
Filing date: 1984-01-19
Publication date: 1985-08-14
Also published as: GB8401444D0

Abstract

The invention provides a surge voltage arrestor including one or more ignition electrodes I comprising particles 4 of conducting material (which may be granular or alternatively flake-like) dispersed in a dielectric matrix 5 and in which the thickness or height of the electrode (as opposed to its length or width) contains a plurality of said particles of conducting material which are both capacitively coupled together across the thickness of the electrode and capacitively coupled to other particles in the lengthwise and/or widthwise direction of the electrode. The electrode I is applied to the inner surface of the wall of the housing 3 of the surge arrestor in the form of a strip or blob. <IMAGE>

Description

SPECIFICATION Surge voltage arrestors The present invention relates to surge voltage arrestors including two electrodes defining a discharge path or spark gap, such as are employed to provide protection for electrical and electronic equipment against damage due to sudden high voltage surges, which may be of very short duration.

Such devices generally have the electrodes located in a housing of insulating material and frequently include one or more ignition electrodes carried by the inner wall of the housing. As is well understood in the art, the function of the ignition electrodes is to aid the rapid initiation of a discharge between the electrodes when a voltage surge occurs across the electrodes.

It is known to form the ignition electrode or electrodes as one or more strips or areas of electrically conductive material on the inner insulating wall of the housing, generally connected to one of the discharge electrodes and extending towards the other. More specifically the ignition electrodes may be formed as a graphite coating, for example a simple pencil stripe, or as a metallic paint or metal film or layer.

In practice such ignition electrodes generally form a single layer of capacitively coupled discrete particles or islands of conductive material having capacitance between each other and at least one of the discharge electrodes which they closely approach or touch at one end.

As indicated above, the effect of an ignition electrode is capacitive and its action is such that in the case of a slowly rising voltage applied to the spark gap. breakdown occurs at a voltage predicted by the Paschen curve.

However, when the applied voltage rises rapidly, (for example at 1 KV per jLs) then the ionisation delay of the device causes an increase in the breakdown voltage of the spark gap. For example, if the ionisation delay is only 1 ys and the voltage rises at 1 KV per ys, then the breakdown voltage is increased from, say, 250V for a slowly rising voltage to 1,000V for a rapidly rising voltage.The delay is also inversely proportional to the overvoltage, that is to say the difference between the applied voltage and the D.C. spark-over voltage, the effect of which is to increase the electric field intensity. (V/mm) The effect of an ignition electrode, such as a strip of conductive material, is to provide capacitive coupling between one discharge electrode of the device and a point close to the other discharge electrode of the spark gap, so that the electric field strength in that small region between the strip and the other electrode is increased when the applied voltage changes with respect to time (dv/dt). Obviously, the "strip of conductive material" must not be a continuous conductor, or it would wholly or partly short-circuit the spark gap, according to its length.

Hence, an increase in capacitance between the ends of the ignition strip improves the response of the spark gap to rapidly rising voltages, as for example those caused by large short duration transients or lightning.

A fundamental limitation of the presently used pencil stripe or film type ignition electrodes is that they have negligible thickness and are therefore virtually two-dimensional so that their capacitance, and hence efficiency, are severely restricted. Moreover, it is not practical to increase the capacitance of the strip beyond a certain point by making it wider, due to the limited internal perimeter of the housing or casing of the device. A width of about 20-25% of the diameter of a cylindrical casing is close to the maximum effective width, due to the wrap-round effect which can occur.

It is an object of the present invention to provide an improved form of ignition electrode for surge voltage arrestors in which a significant increase in the capacitance of the electrode can be obtained and hence more rapid ignition of the surge voltage arrestors embodying such an ignition electrode.

From one aspect the invention provides a surge voltage arrestor including one or more ignition electrodes comprising particles of c9n- ducting material dispersed in a dielectric matrix and in which the thickness or height of the electrode (as opposed to its length or width) contains a plurality of said particles of conducting material which are both capacitively coupled together across the thickness of the electrode and capacitively coupled to other particles in the lengthwise and/or widthwise direction of the electrode.

From another aspect the invention provides a surge voltage arrestor comprising at least two spaced electrodes defining a discharge path or spark gap located within a housing of insulating material and including at least one ignition electrode carried by the inner wall of the housing for aiding the rapid initiation of a discharge between said two spaced electrodes when a voltage surge occurs across said electrodes, wherein said at least one ignition electrode comprises particles of conducting material dispersed in a dielectric material and which is formed such that a plurality of capacitively coupled particles occur through the thickness as well as through the length and width of the electrode.

In one form of the invention the particles are of generally granular or roughly spherical form. Alternatively, the particles may be generally flat or flake-like. In either case, the particles preferably have dimensions of the order of a few or several microns.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view, greatly enlarged, illustrating the character of known forms of ignition electrode, Figure 2 is a diagrammatic sectional view, greatly enlarged, through one form of ignition electrode according to this invention, Figure 3 is a diagrammatic sectional view, greatly enlarged, through another form of ignition electrode according to this invention, Figure 4 is a sectional view of an enlarged scale of one embodiment of surge voltage arrestor incorporating ignition electrodes according to this invention; and Figure 5 is a sectional view on an enlarged scale of a further embodiment of surge voltage arrestor according to the present invention.

Figure 1 shows diagrammatically and on a greatly enlarged scale how the conducting particles occur in a known form of ignition strip, whether they be formed of separated areas of graphite resultingfrom drawing a lead pencil stripe or of metallic particles resultingfrom the application of a metallic paint or layer. In either case the particles 1 adhering to the wall 2 of the insulating housing 3 (for example of ceramic material) of a surge arrestor are deposited on the wall as a single layer so that the capacitive coupling between adjacent particles is only an edge effect, as indicated by the electric field lines E. Hence the capacitance of the ignition electrode is small.

However it is known that the capacitive coupling between electrodes is directly proportional to their areas and the present invention provides higher capacitance by utilising capacitively coupled areas of the particles instead of edges.

Thus as shown in Figure 2, an ignition electrode I comprises metallic particles 4 dispersed in a dielectric matrix 5 so that there is capacitance between opposing faces of adjacent particles, which are assumed to be approximately spherical in shape. The electric field lines E between the particles are illustrated diagrammatically in the left-hand part of the figure.

In a second embodiment shown in Figure 3, the capacitance effect is further enhanced by using particles which are flat platelets or "flakes" 6 dispersed in the dielectric matrix 5. These can be natural platelets such as colloidal dispersions of graphite, or manufactured flakes such as the flake metal powders made for their covering power in metallic inks and paints. Effectively, this increases the ca pacitive area per unit volume, as indicated by the electric field lines E diagrammatically shown at the left-hand side of Figure 3.

According to the size and type of surge arrestor, known types of ignition electrode are from 4 to 25 times as long as they are wide, and their thickness is negligible in the case of pencil-marks or rub-lines. In the case of known strips or shaped areas applied by other surfacecoating processes, such as brushing, spraying, roll-coating etc, the thickness of the ignition electrode is finite but is incidental or accidental to the parameters which are sought for the electrode. In other words, the method of application is not chosen to provide a specified thickness but is chosen for other reasons, and it merely produces whatever thickness is characteristic of that method.

However in the case of the present invention, it is necessary that the thickness of the material forming the ignition electrode shall be significant, and defined. When the dispersed particles are granular or approximately spherical, it is preferred for the thickness to be not less than 4 particle diameters. If, as is usually the case, the metal powder employed consists of a range of particle sizes, it is preferred that less than 1% of the particles by weight should be smaller than 1/10 of the diameter of the longest particles, that there should be an upper cutoff of particle diameter (by sieving, etc), and that the strip film should be not less than 3 times the mesh size of the longer particles.

In the case of flake particles, commerciallyavailable flake has a "diameter" to thickness ratio of the order of 10:1, and a electrode thickness of at least 20 times the flake thickness is preferred.

(The flakes are usually prepared by milling size-seiected spherical particles, in a controlled process, so that there is only a limited range of particle-sizes in one particular grade).

The upper limit to the thickness of ignition electrodes according to this invention is determined by factors such as cost, convenience of application or drying and dimensional clearances.

According to one manner of forming an ignition electrode according to this invention, a quantity of a metallic or metallic oxide powder, for example manganese or manganese oxide powder, having a particle size in the order of 6 microns, is dispersed in an insulat ing matrix which may be glass frit, and is applied to the inner ceramic wall of the hous ing of a surge arrestor in the form of a stripe or finite thickness or a blob.

More specifically, the method of carrying out this process is to mix the metallic or metallic oxide powder with a matrix forming material which, for example, may be sodium silicate or glass frit. One purpose of the matrix is to adhere the particles to the surface of the ceramic wall and to isolate the particles from one another so that no continuous path of electrical contact of active material exists. A further purpose of the matrix is to minimise the tendency to sparking between adjacent particles which is characteristic of the known forms of ignition electrodes.

The proportions of powders to matrix form ing material is dependent upon the desired performance of the system. A binder which may consist of polyvinylacetate (PVA) or similar material is added to the powder and matrix mix and the resultant mixture is applied to the inner ceramic wall in the form of a coating or blob. The coated or blobbed ceramic is then sintered in order to burn off the binder and to leave a metallic or metallic oxide matrix adhering to the inner wall of the ceramic body.

Figure 4 shows a cross-section through one embodiment of surge voltage arrestor incorporating two ignition electrodes according to the present invention. The device comprises a ceramic tube 10 in the opposite ends of which are respectively located two discharge electrodes 11,12 defining a spark gap S between the adjacent faces of annular portions 1 3 of the electrodes.Two strip-like ignition electrodes 11 and 12 according to the invention are provided on the inner wall of the ceramic tube 1 0. The ignition electrode 11 is capacitively connected to the discharge electrode 11 and extends beyond the spark gap S to terminate opposite the annular portion 1 3 of electrode 1 2. Similarly the ignition electrode 12 is capacitively connectedto the electrode 1 2 and extends beyond the spark gap S to terminate opposite the annular portion 1 3 of the electrode 11.

Figure 5 shows a further embodiment of surge voltage arrestor incorporating ignition electrodes according to this invention. The device is of a generally similar type of construction to that shown in Figure 4 but is a three-electrode device and to this end incorporates a central electrode 1 4 located between the end electrodes 11 and 1 2. Two similar ceramic tubes 10 are provided to locate the electrodes and enclose two spark gaps S1 and S2 respectively formed between the annular portions 1 3 of the electrodes 11,14 and 1 2,1 4. Also, in this embodiment, besides the ignition strips 11 and 12 associated with the electrodes 11 and 1 2. the device includes two further ignition strips 13 and 14 both extending from the central electrode 14, respectively towards the annular portions 1 3 of the electrodes 11 and 12.

It will be obvious that the surge voltage arrestor devices shown have been given soleiy by way of example and that the devices may employ various other configurations, either overall, or for the discharge electrodes and/or the ignition electrodes.

Claims

1. A sure voltage arrestor including one or more ignition electrodes comprising particles of conducting material dispersed in a dielectric matrix and in which the thickness or height of the electrode (as opposed to its length or width) contains a plurality of said particles of conducting material which are both capacitively coupled together across the thickness of the electrode and capacitively coupled to other particles in the lengthwise and/or widthwise direction of the electrode.

2. A surge voltage arrestor comprising at least two spaced electrodes defining a discharge path or spark gap located within a housing of insulating material and including at least one ignition electrode carried by the inner wall of the housing for aiding the rapid initiation of a discharge between said two spaced electrodes when a voltage surge occurs across said electrodes, wherein said at least one ignition electrode comprises particles of conducting material dispersed in a dielectric material and which is formed such that a plurality of capacitively coupled particles occur through the thickness as well as through the length and width of the electrode.

3. A surge voltage arrestor as claimed in claim 1 or 2, in which the particles are of generally granular or roughly spherical form.

4. A surge voltage arrestor as claimed in claim 1 or 2, in which the particles are generally flat or flake-like.

5. A surge voltage arrestor as claimed in any preceding claim, in which the particles have dimensions of the order of a few or several microns.

6. A surge voltage arrestor as claimed in claim 3, 4 or 5, in which the particles are metallic particles.

7. A surge voltage arrestor as claimed in claim 4, in which the particles are natural platelets such as colloidal dispersions of graphite.

8. A surge voltage arrestor as claimed in claim 3, in which when the particles are granular or approximately spherical, the thickness of the material forming the ignition electrode is not less than four particle diameters.

9. A surge voltage arrestor as claimed in claim 4, in which when the particles are flat or flake particles. the electrode thickness is at least 20 times the flat particle or flake thickness.

1 0. A surge arrestor having an ignition electrode substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.

11. A method of formingan ignition electrode for a surge arrestor according to any preceding claim, in which a quantity of a metallic or metallic oxide powder having a particle size in the order of a few microns is dispersed in an insulating matrix. and is applied to the inner ceramic wall of the housing of the surge arrestor in the form of a strip of finite thickness or a blob.

1 2. A method as claimed in claimll, in which the metallic or metallic oxide powder is mixed with a matrix forming material, such as sodium silicate or glass frit in order to adhere the particles to the surface of the wail of the housing and to isolate the particles from one another.

1 3. A method as claimed in claim 11 or 12, in which a binder, which may consist of polyvinylacetate or similar material is added to the powder and matrix mix and the resultant mixture is applied to the inner wall of the housing in the form of a coating or blob and is then sintered in order to burn off the binder and to leave a metallic or metallic oxide matrix adhering to the inner wall of the housing.

1 4. Methods of forming an ignition electrode in a surge arrestor substantially as hereinbefore described.