Pin Insulator

Pin insulators of porcelain or glass predominate as overhead line sus­pension or tension sets above 33 kV and they are also used for substation bus­bar high level strained connections. Almost any creepage distance may be achieved by arranging the required number of individual units in a string. Upper surface shed shapes are similar with the top surface having a smooth hard surface to prevent the accumulation of dirt and moisture and a slope greater than 5° to assist self-cleaning. The undersides have a considerable vari­ation in shape which depends upon aerodynamic and creepage distance requirements.   In the Standard, anti-fog and aerofoil disc pro­files, the Suspension insulator sets are rarely used in substation designs and sub­station busbar tension sets avoid the use of the anti-fog profiles because the deep ribs may not be naturally cleaned by rainfall when mounted nearly hori­zontally with short spans. Such substation short span applications do not require high strength cap and pin units and the insulators are often specified with 80 kN minimum failing electromechanical failing test load to meet a 3 X safety factor requirement. Overhead line cap and pin insulators are speci­fied with correspondingly higher failing loads of 125 and 190kN.

Long rod insulators are similar to porcelain solid core cylindrical post insula­tors except that the top and bottom fittings arc of the cap and pin type. Long rods are an alternative to the conventional cap and pin insulator sets with the possibility of providing longer creepage paths per unit length. Long rod insu­lators have not, however, exhibited for overhead line work any marked improvements in performance under heavy pollution conditions. In addition, the mechanical performance of porcelain under tension is such that brittle fracture could easily cause a complete failure of the whole unit leading to an outage condition. In contrast, cap and pin insulators using toughened glass or porcelain are designed so that they do not exhibit a brittle fracture character­istic. Cap and pin insulators are able to support the full tensile working load with the glass shed shattered or all the porcelain shed broken away. For these reasons cap and pin insulators are more often specified for overhead line work.

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