Cap and pin insulators: electrical puncture of porcelain under AC energization

For supporting the conductors of overhead power lines, many utilities use strings of porcelain cap and pin insulators. During recent years, sample testing has shown that some such insulators removed from the CEGB 275 kV and 400 kV Supergrid have cracks in the head of the porcelain shell. Post-mortem evidence suggests that the primary cause of failure was electrical puncture of the porcelain. Further, from considering the failure statistics and operational practice, it seems possible that such breakdown occurred under the stressing from the alternating voltage rather than that from lightning or switching surges. The porcelain of some transmission line insulators is electrically weak. The findings from laboratory tests are consistent with the statistics of service failures. The cause of failure is reasoned to be associated with incipient defects formed at the early stages in the manufacturing process. These defects possibly develop in size under certain service stressing conditions; in a string of insulators, some units are subjected to relatively large AC voltages for some considerable time. The standard, short-duration energization tests failed to identify this weak porcelain; however, a somewhat arbitrarily chosen 50 h, 85 kV (rms) test did do so.<>