Read the text. Insulators and Conductors

Insulators and Conductors

Line conductors are electrically insulated from each other as well as from the pole or tower by nonconductors which are called insulators. To determine whether or not an insulator can be used, both its mechanical strength and electrical properties must be considered. Two practical insulator materials are porcelain and glass. Both of these leave much to be desired. Porcelain can withstand heavy loading in compression, but tears apart easily under tension, that is, when pulled apart. In using a porcelain insulator, therefore care must be taken to make the forces acting on it compress and not pull apart. The same is generally true of glass. Although glass insulators are good for lower voltage application, porcelain insulators are much more widely used because they are more practical. Porcelain has two advantages over glass: 1. it can withstand greater differences in temperature that is, it will not crack when suspected to very high or very low temperature; 2. porcelain is not as brittle as glass and will not break as easily in handling or during installation. Polymer insulators are not so restricted and have the advantage also of being lighter in weight than porcelain or glass.

Pin-type insulators, in compression, supporting conductors may be classified as pin type and post type. The pin-type insulator is designed to be mounted on a pin which in turn is installed on the cross arm of the pole. The insulator is screwed on the pin and the electrical conductor is mounted on the insulator. Made of porcelain or glass, the pin insulator can weigh anywhere from ½ pound to 90 pounds. This type of insulator is applicable for rural and urban distribution circuit, and it is usually constructed as one solid piece of porcelain or glass. Lager, stronger pin – type insulators are used for high-voltage transmission lines. These pieces from what are called petticoats. They are designed to shed rain and sleet easily. Post – type insulators are somewhat similar to pin-type insulators. They are generally used for higher voltage applications with the height and number of petticoats being relater for the higher voltages. They may be mounted horizontally as well as vertically, although their strength is diminished when mounted horizontally. The insulator is made of one piece of porcelain and mounting bolt or bracket is an integral part of insulator.

The most common used insulators are the pin or post type and suspension (or hanging) type. A third type, the strain insulator, is a variation of suspension insulator and is design to sustain extraordinary pulls. Another type, the spool insulator, is used with secondary racks and on service fittings. The main advantage of the pin or post-type insulator is that it is cheaper. Also, the pin or post insulator requires a shorter pole to achieve the same conductor above the cross arm while the suspension insulator suspends it below cross arm.

Post-type insulators are also used to support conductors of a polyphase primary circuit without use of cross arm. This type of installation is employed where appearance or narrow rights-of-way considerations are important.