Development of the Porcelain Insulator

Porcelain insulators have been manufactured and used for the transmission of high-tension electric power for forty years. The first designs were of the single-piece and multipart cemented pin type. Necessity for higher safety factors against flash-over and increase in operating voltages demanded a rapid increase in the size of the insulators. This reached an economic limit at the operating voltage of 66 kv. The suspension unit overcame this temporary check of increased operating voltage. Further study of the electrostatic capacitance of the various parts and consequent voltage distribution, made marked refinements in the pin type insulator possible. During this time the single-piece porcelain suspension unit took practically its present form. Early improvements were the provision of proper expansion joints and the separation of the lip of the cap from the porcelain hood. Gradual improvements have since been made resulting in a great increase in mechanical strength. These changes have been principally in hardware design. By experiment and analysis the shapes of the cap and pin have been determined to give a uniform distribution of load from the pin to the cap. Constant check tests by the quick pull and time loading methods have shown that the suspension insulator with properly designed hardware and a suitable coating on the cap to prevent the cement from adhering to the metal, has a high strength associated with electrical reliability. Ceramic research and exact manufacturing control has made possible the production of non-absorbent, thoroughly vitrified porcelain of consistent strength.