None of the simulations, however, indicated that the
electric stress is high enough to cause puncture in the porcelain
or glass insulators. The most critical parts in the porcelain were
stressed at 14.3 kV/mm, which is approximately 36% of the
minimum theoretical impulse puncture strength. The ratio is
lower for the glass insulator, where the largest recorded stress
was 15 kV/mm, reaching 9% of the minimum puncture
withstand strength. Relying only on simulation results,
puncture shouldn’t occur at all in either type of insulator. The
following reasons may be proposed as to why a number of
porcelain insulators in previous experiments punctured: