Insulators

You have known that an electrical insulator resists the flow of electricity. Application of a voltage dissimilarity across a good insulator results within negligible electrical current. Insulators made of glazed porcelain, tough glass and polymers are used for supporting the conductors. Porcelain insulators avoid the electrical current from energizing the power pole.

The principal categories of insulator are elaborates below:

- Pin insulators are manufactured for voltages up to 33 kV and are cheaper than the other categories. IS:1445 and 731 cover detailed specifications for these. The pins for the insulators are fixed in the holes given in the cross-arms and pole top brackets. The insulators are mounted over the tightened and pins. The cost of pin insulators increases extremely rapidly as the working voltage is increased. For high voltages these insulators are uneconomical. However, replacements are expensive.

- Disc insulators are made of glazed porcelain or tough glass. They are used as insulators on high voltage lines for suspension and dead ending. The line conductor is suspended below the point of support through means of the insulator or a string of insulators. A disc insulator consists of a single disc-shaped piece of porcelain, grooved on the under-surface to increase the surface leakage path among the metal cap at the top and the metal pin underneath. The cap is recessed so as to take the pin of another unit, and in this way a string of any required number of units could be built up. The cap is secured to the insulator through means of cement. Disc insulators are "ball and socket" or "tongue and clevis" category. A suspension clamp is used to support the conductor, if suspension configuration of the line is selected.

- Shackle insulators are used for distribution lines dead ending and supporting conductors laid in vertical creation. IS:1445-1977 covers shackle insulators for voltages below 1000 V. The two standard sizes listed within this specification are 90 mm dia x 75 mm height and 115 mm dia x 100 mm height. A shackle insulator is supported by either two straps and two MS bolts or one U clamp or D strap and two MS bolts as per IS:7935.

- Stay insulator/Guy strain insulators of egg category porcelain are used for insulating stay wire, guard wires, etc. wherever it is not proposed to earth them. As per IS: 5300, two strength sizes (ultimate tensile strength) are used: 44 kN and 88 kN, correspondingly, for LT and HT lines.

- Stays/Guys and staying arrangement: Guys of stranded steel wire are used on all terminals, angle and other like poles whereas the conductors have a tendency to pull the pole away from its true vertical position. A guy is fastened to the poles near the load middle point along with the help of pole clamps. The other end of the guy/stay is secured to a stay rod embedded in the ground. The stay rod should be situated as far away as probable.