GB2173942A

GB2173942A - Surge voltage arrestors

Info

Publication number: GB2173942A
Application number: GB08506313A
Authority: GB
Inventors: William Arthur Levett
Original assignee: DUBILIER BESWICK DIV
Current assignee: DUBILIER BESWICK DIV
Priority date: 1985-03-12
Filing date: 1985-03-12
Publication date: 1986-10-22
Also published as: GB8506313D0

Abstract

A surge voltage arrestor comprises two spaced electrodes 1, 2 defining a discharge path or a spark gap S and which are fitted in opposite ends of an insulating sleeve or cylinder 3, the interior of the surge arrestor being gas filled. The inner wall of the insulating sleeve or cylinder carries two ignition strips 4 each of which extends from a respective one of the electrodes but is insulated from the other electrode by means of a gap 5. According to the invention, the peripheral region of each of the electrodes is insulated on its surface, as shown at 6. The insulation may be provided by a coating or sleeve of any appropriate insulating material. This insulation may also extend over the base region of the electrodes as shown at 6a and furthermore could extend partly over the facing surfaces of the electrodes defining the spark gap S. <IMAGE>

Description

SPECIFICATION Surge voltage arrestors The present invention relates to surge voltage arrestors including at least two electrodes defining one or more discharge paths or spark gaps, 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 or disposed at opposite ends of a housing of insulating material and frequently include some form of ignition system carried by the inner wall of the housing.

More specifically, such surge arrestors are generally gas filled and comprise two or more electrodes separated by an insulating sleeve or cylinder which encloses a gas at low pressure.

In use, as a protective device, the surge arrestor becomes conductive when a predetermined ignition voltage is exceeded and then serves to divert a current from another device or component where that current may cause damage. The voltage at which any given surge arrestor strikes or becomes conductive in order to provide this protective effect is governed by the response voltage of the surge arrestor.

In practice, the entry into the conductive state or ignition of a surge arrestor at the ignition voltage is always subject to a delay which is caused by the necessity to provide free charge carriers for the ionisation of the gas discharge path to produce ignition. When a voltage which changes with time is connected both to the surge arrestor and to a device which it is designed to protect, such a delay in ignition also involves a higher response voltage which may endanger the device which is to be protected. Generally speaking, the response voltage of surge arrestors rises when a voltage connected to the discharge path increases with time.

It is known to reduce the ignition delay and hence the response of a surge arrestor to a surge voltage by the provision of some form of ignition system in the form of stripes, bands or other areas of electrically conductive material arranged on the inner wall of the insulating sleeve or cylinder forming the housing of the device. Such ignition systems provide free electrons at high concentration by field electron emission produced by voltage change. The ignition system must always incorporate a gap in order to ensure electrical insulation between the electrodes in the nonoperative mode of the surge voltage arrestor.

A common cause of failure in such surge voltage arrestors is due to the deposition of a conductive layer of electrode material on the inner wall of the insulating sleeve or cylinder during the condition of arc discharge of the device, thus bridging or eliminating the insulating gap and thereby creating a short circuit between the electrodes. This is particularly caused by wandering of the arc across the face of the electrodes and on to the peripheral areas of the electrodes adjacent the insulating sleeve or cylinder. Such deposition of a conductive layer may also influence the ignition system.

It is an object of the present invention to provide an improved construction of surge voltage arrestor in which this undesirable condition is substantially reduced or avoided and more specifically in which the deposition of electrode material on the inner wall of the insulating sleeve or cylinder is largely avoided.

According to the present invention parts of the surface areas of the electrodes of the surge voltage arrestor are insulated such that the insulation will restrict the random movement of the arc under discharge conditions.

More specifically by a suitable design of electrode geometry the gap in the ignition system may be shielded from deposition of electrode material and short circuit conditions can be avoided. In this way the current carrying capacity of the gas discharge device or surge arrestor is enhanced.

The invention will now be further described, by way of example, with reference to the accompanying drawing, in which Figures 1 and 2 are cross-sections through embodiments of surge voltage arrestor according to this invention.

Referring to Fig. 1, a surge voltage arrestor comprises two spaced electrodes 1,2 defining a discharge path or spark gap S and which are fitted in opposite ends of an insulating sleeve or cylinder 3, the interior of the surge arrestor being gas filled. The inner wall of the insulating sleeve or cylinder carries two ignition strips 4 each of which extends from a respective one of the electrodes but is insulated from the other electrode by means of a gap 5. In practice each ignition strip 4 extends beyond the spark gap S to terminate at a position opposite but spaced from the periphery of the other electrode. According to the invention, the peripheral region of each of the electrodes is insulated on its surface, as shown at 6. The insulation may be provided by a coating or sleeve of any appropriate insulating material.This insulation may also extend over the base region of the electrodes as shown at 6a.

By means of the application of this insulating material it has been found possible to exercise a degree of control over undesirable wandering of an arc struck between the electrodes. In particular the arc is not allowed to wander on to the cylindrical side areas of the electrodes which, being very close to the inner surface of the sleeve 3, would cause the deposition of electrode material on to this surface, which in turn gives rise to insulation problems, in the absence of an insulating sur face according to the invention.

Fig. 2 shows a similar arrangement appiied to a three electrode surge voltage arrestor of the same basic design as that in Fig. 1.

As mentioned above, the insulating surface can be obtained either by a coating on a selected part or parts of the electrodes or by fitting an insulating sleeve on to the electrode or electrodes.

If desired, the insulating layer could extend partly over the facing surfaces of the electrodes defining the spark gap S.

Claims

1. A surge voltage arrestor including two spaced electrodes defining a discharge path or spark gap between them which is situated within a housing of insulating material and an ignition system carried by the inner wall of the insulating housing, wherein parts of the surface areas of the electrodes of the surge voltage arrestor are insulated such that said insulation on the electrodes will restrict the random movement of the arc between the electrodes under discharge conditions.

2. A surge voltage arrestor including two spaced electrodes defining a discharge path or spark gap between them and spaced apart by a tubular member of insulating material forming a housing enclosing a gas at low pressure, and an ignition system carried by the inner wall of the insulating housing, wherein parts of the surface areas of the electrodes of the surge voltage arrestor are insulated such that the insulation on the electrodes will restrict the random movement of the arc between the electrodes under discharge conditions.

3. A surge voltage arrestor as claimed in claim 1 or 2, in which the insulation of the electrodes is such that a gap provided in the ignition system is shielded from deposition of electrode material under discharge conditions.

4. A surge voltage arrestor as claimed in any preceding claim, in which the ignition system comprises one or more strips, bands or other areas of electrically conductive material arranged on the inner wall of the insulating housing.

5. A surge voltage arrestor as claimed in any preceding claim, in which the insulation extends over the peripheral region of each electrode.

6. A surge voltage arrestor as claimed in claim 5, in which the insulation also extends over the base region of each electrode.

7. A surge voltage arrestor as claimed in claim 5 or 6, in which the insulation also extends partly over the facing surfaces of the electrodes which define the spark gap.

8. A surge voltage arrestor as claimed in any preceding claim, in which the insulation on the electrodes is in the form of a coating of insulating material.

9. A surge voltage arrestor as claimed in any of claims 1 to 7 in which the insulation is in the form of a sleeve of insulating material.

10. A surge voltage arrestor as claimed in any preceding claim comprising more than two electrodes and more than one spark gap.

11. A surge voltage arrestor substantially as hereinbefore described with reference to the accompanying drawings.