For supporting extra-high-voltage or EHV overhead transmission lines, steel towers are used. These are also called lattice towers. Electric Transmission towers are constructed with GI (galvanized iron) angles of different sizes. The angles are assembled together with the help of nut and bolts. The height of the towers is generally from 20 to 40 m but in some special cases, the height may be more than this range. Each of the towers has four legs. The towers are generally so designed that it can withstand tensile stresses from all sides. The tension -withstand-capacity of a transmission tower depends on the cross-section of its base. The distance between two adjacent legs of a tower is one-third (1/3) to one-seventh (1/7) of its total height. The cross-sectional area of the base of a transmission tower depends on the position and height of the tower.
As per standard, there are four types of towers available.
The base of this tower is comparatively smaller. For a single circuit tower, the base is rectangular. The tower is installed in such a way that the wider side of the rectangular base remains perpendicular to the alignment of the line. The base of a double circuit tower is square. The allowed angle of diversion of the line for this tower is up to 2 degrees.
Small Angle Tower
The base of this type of tower is square and wider than that of a tangent tower. These towers are used for the diversion angle of 2 to 15 degrees.
Medium Angle Tower
The base of this tower is also square and wider than that of a small angle tower. These towers are used for the diversion angle of 15 to 30 degrees.
Large Angle Tower
A large angle tower is also called a dead-end tower or terminal tower. The base of this type of tower is most wide and square. The angle of diversion for this type of tower is 30 to 60 degrees.
A tangent tower is designed and constructed that on the event of snapping one conductor from one side of the tower, it can withstand the unbalance tensile stress due to this snapping. A small angle tower is similarly designed. A medium angle tower is so designed and constructed that when two conductors from one side are snapped, the tower is still able to withstand the unbalanced tensile stress acting on it. A large angle tower is designed and constructed that on the snapping all the conductors from one side, it can still withstand the tensile stress imposing on the tower.
For Extra High Voltage lines, where a large distance between the phase conductors is to keep, a special type of tower is used. These towers are called corset towers.
The legs of a tower, some main parts of cross arms and the stubs used inside the ground are made of angles of minimum 6 mm thick. The angles used for other parts of the tower are of a minimum 5 mm thick. The dimensions of the angle are decided by the tower designers. If the tower angles are not galvanized, these must be painted with red oxide.
The cross arms are used to keep the conductors and insulator-strings at a safe distance from the tower body. The cross arms are made by assembling GI angles of suitable sizes. The cross arms are so designed that during the sudden collapse of the conductor, it can withstand sudden torque developed. A cross arm is provided with necessary arrangements for fixing suspension and tension insulator strings. For that, a cross arm is provided with the holes for fixing u bolts and fitted with a gusset plate with holes. For an angle tower, flexible hangers are fitted with the cross arms for that purpose. For connecting the earth wire fittings, the pic of the tower is fitted with a bent plate with holes.