Partial vacuum flashover of aerial lift documentation and failure analysis

Full understanding of different failure modes of extra high voltage (EHV) aerial lifts has been obtained in over 30 years of testing. Finding problems by flashover helped speed up the learning curve. One failure mode, the vacuum flashover is especially troublesome because of the lack of understanding of the problem. In 1983, a vacuum flashover occurred during a certification attempt on two different occasions. ANSI A92.2 states that vacuum flashover will be prevented by use of vacuum breakers. The actual preflashover conditions will be described and explained. Photographs and sketches will document the action that 15 witnesses saw during the certification test on an 800 kV class aerial lift. The actual vacuum flashover occurred during a daylight test at 467 kV starting a fire inside the insulating section. The test underway was a 600 kV 3 minute hold per ANSI A92.2. This fire burned itself out of oxygen, because of the sealed boom, and was not discovered until a retest was about to take place. A photograph was taken through the silica gel canister hole confirming the fire, which melted the all the hydraulic hoses. A full investigation proved the existence of the vacuum flashover and the pre-flashover photos and arcing condition added documentation to the vacuum flashover theory. The analysis places all the functions of this problem in proper order. The lack of understanding of this basic problem has caused fatalities and at least 11 or 12 vacuum flashovers of EHV aerial lifts in the world to date.