Evaluation of porcelain and toughened glass suspension insulators removed from service

Porcelain and toughened glass suspension type insulators that were removed after 25-30 years of service were evaluated in the laboratory with the objectives of understanding failure mechanisms and assessing the condition of similar insulators in the field. The remaining electrical and mechanical strength was highly variable for porcelain and for some they were significantly reduced from the rated value. Scanning electron microscopy and electric field calculations provided an insight into the failure mechanism responsible for electrical strength reduction in porcelain. Toughened glass insulators with shells shattered intentionally in the laboratory (stubs) always flashed over externally and were therefore puncture proof. Multiple explanations are provided to explain this phenomenon based on internal examination of the stubs. The variation in the tensile load to cause failure for toughened glass insulators was smaller when compared to porcelain and was close to the rated value.