Abridgment of development of the porcelain insulator

Porcelain insulators have been manufactured and used for the transmission of high-voltage electric power for forty years. The first designs were of the single piece and multipart cemented pin type. Necessity of higher safety factors against flashover 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 of 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, to have 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. This has centered largely about the proper firing of the clay. Recent experiments upon the properties of the combination of porcelain and glaze has eliminated surface stress and consequently assured stronger, longer lived porcelain. Still greater uniformity has been gained by glazing the sanded surfaces. The elimination of the abutting joint and the proper design of the cemented joint has stopped expansion troubles. Proper use of Portland cement has resulted in insulators able to withstand drastic temperature changes without harm. A recent improvement in the pin type insulator is the metal threaded pin hole. This has lessened manufacturing and construction difficulties and in addition due to the exact fit of the insulator on the pin, overcomes hidden corona and the consequent radio interference.