Gas-filled surge arrestor

Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation

Patent

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Details

361124, 361129, 361130, 337 29, 337 32, H02

Patent

active

049125923

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to gas-filled surge arrestors or gas discharge tubes comprising at least two electrodes defining a spark gap enclosed in a gas-filled housing. Such surge arrestors are designed to have a normal breakdown voltage which is repeatable about a predetermined value. However if the interior of the surge arrestor becomes vented to atmosphere, for example by sustained current conduction and consequent physical damage, then the breakdown voltage will become very much higher than the normal predetermined value and hence the surge arrestor is largely ineffective in performing its desired protective function. As an example a gas-filled surge arrestor designed to have a normal breakdown voltage of about 150V may, when its interior is vented to atmosphere, have an erratic breakdown voltage between 2-4 kV.
Various proposals have been made to overcome this disadvantage. One such proposal makes use of what is known in the art as "narrow-gap technology" in which the gap between the electrodes is made so small that the breakdown voltage of the surge arrestor is very similar whether operating normally or when its interior is vented to atmosphere. However this is a solution which is fraught with technical difficulties and is also expensive to achieve. A further proposal is to provide an external air back-up gap connected in parallel with the electrodes defining the gas discharge gap within the housing of the surge arrestor. However again it is found that the breakdown voltage of such a back-up air gap is higher than is desirable and a repeatable back-up gap breakdown voltage is difficult to attain.
The present invention seeks to provide a gas-filled surge arrestor having an external air back-up gap which has both an acceptable value of breakdown voltage and a repeatable breakdown voltage.
According to the present invention a gas-filled surge arrestor is provided with an external air back-up gap comprising two spaced electrodes of which one has a sharp edge or corner adjacent the other electrode. The said one electrode may be coated with an insulating material at least over the region defining the sharp edge or corner.
According to a preferred form of the invention, said one electrode is positively urged towards the other electrode and is coated with an insulating material which prevents the two electrodes from being in direct contact and hence short-circuiting together. In this way the spacing between the two electrodes is defined to some extent by the insulation between them. However, the electrodes are so shaped and arranged in relation to each other that although the sharp edge of corner on said one electrode has a covering of insulating material it is also separated from the other electrode by an air gap. According to one embodiment of the invention, said one electrode is planar and defines at least one sharp corner and said other electrode defines a curved surface.
According to a further embodiment of the invention, said one electrode is made of a spring material so that it is urged towards said other electrode and the arrangement is such that if the surge arrestor becomes overheated the insulating material will soften or decompose, thereby allowing the two electrodes to come into contact and short circuit the gas discharge gap within the surge arrestor housing. In this way the structure of the back-up gap also functions as a fail-safe device for the surge arrestor.
The invention therefore also provides a gas-filled surge arrestor having an external air back-up gap connected in parallel with the electrodes defining the gas discharge gap within the housing of the arrestor wherein said back up gap is also constructed to function as a fail-safe device which short circuits the gas discharge electrodes in the event of overheating of the surge arrestor.
In the arrangements according to the present invention one of the electrodes of the back-up gap may also comprise one of the gas discharge electrodes of the surge arrestor.
The invention will now be further described, by way of example, with referenc

REFERENCES:
patent: 4034326 (1977-07-01), Hill et al.
patent: 4303959 (1981-12-01), Roberts et al.
patent: 4573100 (1986-02-01), Fasano
patent: 4649456 (1987-03-01), DeLuca et al.

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