Toughened glass insulators play two basic roles in overhead transmission lines, namely supporting wires and preventing current from returning to the ground. The operation life of an entire line (usually for 40 years) these two functions must be guaranteed. The insulators should not be subjected to various mechanical and electrical stresses caused by changes in the environment and electrical load conditions. The mechanical loads that fail to withstand the insulation, apart from the weight of the wires and metal accessories, are also required to accept the wind load in bad weather conditions. The impact load caused by improper load, snow load, conductor galloping and improper operation during transportation is analyzed. From the electrical point of view, the insulator is not only insulated from the wire and the ground, but also must withstand the overvoltage shock caused by the lightning and the switching operation. The partial overheating caused by the flashover of the voltage shock should not cause the explosion of the insulators' steel vitreous body. All external factors will affect the performance of insulators, so the design of insulators also requires higher requirements.