Excess voltage arresters

Claim 1

An excess voltage arrester comprising a gas filled enclosure and an electrode structure housed within the enclosure which structure defines at least one discharge gap, said structure including (A) a first electrode, having an end portion in the form of a metal rod; and (B) a second electrode having a portion in the form of a tube, said portion of said second electrode coaxially surrounding said end portion of said first electrode, the arrester being constructed so that πr 1 2 (x+r 2 )>4/3πr 2 3 , where r 1 is the radius of said end portion of said first electrode, r 2 is the internal radius of said portion of said second electrode and x is tha axial overlap of the two portions; (C) and the material of which said end portion of the first electrode consists being such that (i) on overheating of the arrester due to the occurrence of a discharge in the arrester over a prolonged period, (ii) an end part of said end portion of the first electrode melts, (iii) the remainder of said first electrode remaining unmelted, (iv) said end part taking up under the action of surface tension a generally spherical form supported on said remainder, (v) the radius of said end part when molten being such that said end part will contact said portion of the second electrode regardless of the orientation of the arrester.