Recognize a false negative in HiPot testing

摘要

A West Texas utility company received an alarm on one of its load tap changers. Field technicians found that one of the vacuum interrupters failed an ac high potential test. The tap changer was disassembled and the vacuum interrupter in question was tested again. Once removed, the vacuum interrupter passed an ac high potential test. The vacuum interrupter was reinstalled and placed back into service where it again passed an ac high potential test. The following explains why this utility company may have placed a failed vacuum interrupter back into service.