In order to avoid the danger of transmission lines touching each other or hanging so low as to cause obstruction to people. Trees, vehicles, etc., it is necessary to provide supports which take the form of high poles, e.g wooden or metal structures called towers or pylons. For various types of pole and pylon. In rural distribution lines. Up to 33kv with spans of say 80m , wooden poles are cheaper to use than concrete PR steel supports. The 'A' type poles are used where greater strength is required and 'H' type poles are used where it is necessary to support switch gear and / or a pole mounting type transformer as well as the lines. The 'H: type pole is also used for tapping point to lines in other directions.
Reinforced concrete poles have higher initial cost, but a longer life span than wooden poled. They are used for high voltage and low voltage or the combination of both , with the high voltage usually 11kv or 6kv erected on cross arms high above the low voltage distribution lines. For the national grid lines of the order of 132kv and above galvanized or painted steel towers are essential because they have wide bases which are required to withstand the high mechanical stresses encountered from wind, weight and high pressure.
