<p>A porcelain shell 32 encloses resistance blocks 22 forming an impedance path between end terminals 16 and 34. The lower end is sealed by a membrane 38 with O-ring 42 held in place by cap 40. An ablative liner 52 generates increased pressure if internal arcing occurs and thus blows off the cap 40. Blocks 22 with spring 24 are retained by spider 62 engaging inturned edge 56 of liner 52. Alternatives described are for the spider 62 to engage openings in the end of the liner 52 or a groove in the shell 32. Electrical continuity is maintained through conducting blocks 44 and 46 and steel membrane 38. </p>
