Many times I have been warned by linesmen to be careful. Glass suspensions can explode without warning they said. A friend of mine had his car window explode on a sunny summers day, so I knew there was some truth to what the linesmen were saying. I found this today and thought it was worth sharing.
5. Spontaneous Shattering
This failure mode applies only to glass disc insulators and is due to sudden release of internal stresses induced in the shell during the toughening process. The incidence of the problem is linked to inferior quality control such as the presence of inclusions (hard particles that have not fully melted in the furnace) or impurities in the tensile zone of the glass body. Since the average size of these impurities is less than 100 microns, there is no practical method to detect them other than to screen out glass shells containing inclusions through thermal shocks prior to assembly of fittings. Even though the thermal shock test is carried out twice during production, there is a very low probability that some shells containing inclusions survive and shatter subsequently during service. The time when this finally happens can be triggered by a variety of environmental factors or even occur with no special external circumstance. Statistics reveal that typical annual rates of spontaneous shattering are on the order of up to 1 per 10,000 units and will depend on the manufacturer.