That's caused by....'surface creep'..... which will also depend on any dust/gunk / fluff sitting on the insulator.? ?The insulator can be cleaned up. The easy fix is to put a small piece of plastic sheet (with a hole in it). ..before the arced insulator is installed.? That will increase the leakage path by a bunch. ( plastic goes between the insulator and metal side wall)...so the hole in the plastic is just big enough....then the lip of the insulator sits on the plastic sheeting.? ? ?That or replace with a longer feedthrough HV connector, or use a feedthrough insulator ( 2 x piece type)...that is of the serrated type....which will also increase the leakage path by a bunch.?
