Transmission line insulator

There are three varieties in this category. They are pin-type insulator, suspension-type insulator and strain-type insulator. Usually the material used for insulation is hard porcelain which consists of kaoline, quartz and feldspar in the proportion 2:1:1. safety factor is an important parameter of an insulator which is defined as the ratio of the sparkover voltage to the working voltage. The dielectric strength and the mechanical strength should be very high for insulators when used in power transmission line. The insulators should not be porous and there should not be any internal impurity. These insulators should withstand change of temperature and should have large ratio of puncture strength of flashover voltage. In some special cases, annealed glass insulators and steatite (magnesium silicate) insulators are used. Pin type insulators are used for operation upto 25 kV. Here as a support to the insulator, a steel bolt or pin secured to a cross arm on the transmission pole is utilized. In high voltage transmission line, suspension type insulators are used and the transmission line here is suspended below the point of support by means of this suspension-type insulator. The main three varieties of suspension-type insulators are cemented cap-type insulator, Hewlett or interlinking type insulator and core and link type insulator. The strain-type insulator is used at line terminal and at dead end. Before installation insulators must undergo tests like dry flashover test, thirty second rain test, wet flashover test, impulse frequency flashover test, temperature cycle test, electromechanical test, puncture test, mechanical strength test, porosity test, high voltage test, proof load test and corrosion test.