Results of these investigations show that the two 150-megohm resistors are a source of thermal, mechanical, and electrical stress in this insulation system. This stress, in conjunction with moisture degradation during power-off conditions, is the major driving force for producing treeing in this design. In the post mortem, freshly produced units were subjected to a 48-hour 95-percent relative humidity soak at 85 oC, followed by one week at 75 oC under rated load. These units showed treeing in the vicinity of the resistors in less than 72 hours. Removal of the resistors from the insulation system was tried. Units were produced without resistors, and follow-up with humidity, temperature-cycle, and accelerated temperature tests showed the design to be free of treeing problems.