Summary and Conclusions
Based on this metallurgical and fracture mechanics analysis of the failed insulator caps, the following conclusions can be drawn:
Damage to the glass insulators allowed flashover to occur between adjacent insulator caps on an insulator string.
These arc strikes led to localised melting of the malleable cast iron and a heat-affected zone (HAZ) developing near the strike.
Rapid cooling of the re-melted metal led to the formation of small quench cracks in the insulator caps.
These could easily extend in subsequent arc strikes to a critical size for fracture leading to the observed bursting of the caps under the electrical energy input.
Thus the main factors leading to this failure were the change from a relatively ductile ferritic structure to an as-cast pearlitic one, together with the formation of small quench cracks, during the first arc strike. Subsequent strikes led to crack growth to a critical size. An ancillary factor in the failure may be an increase in arc energy which might be associated with an increase in number of damaged adjacent glass insulators.
The obvious way of avoiding failures of this type, is to ensure that the glass insulators are not damaged by vandals. This is not easily controllable, however, and thought could be given to replacing the malleable cast iron with higher toughness spheroidal graphite (SG) iron.