An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-phase power) suspended by towers or poles. Since most of the insulation is provided by the surrounding air, overhead power lines are generally the least costly method of power transmission for large quantities of electric energy. Construction Towers for support of the lines are made of wood either grown or laminated, steel or aluminum (either lattice structures or tubular poles), concrete, and occasionally reinforced plastics. The bare wire conductors on the line are generally made of aluminum (either plain or reinforced with steel or composite materials such as carbon and glass fiber), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. A major goal of overhead power line design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line, and to provide reliable support for the conductors, resilience to storms, ice loads, earthquakes and other potential damage causes. Today overhead lines are routinely operated at voltages exceeding 765,000 volts between conductors. Classification of transmission lines By operating voltage Overhead power transmission lines are classified in the electrical power industry by the range of voltages: Low voltage (LV), less than 1000 volts, used for connection between a residential or small commercial customer and the utility. Medium voltage (MV; distribution), between 1000 volts (1 kV) and 69 kV, used for distribution in urban and rural areas. High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltages such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers. Extra high voltage (EHV; transmission) – from 345 kV, up to about 800 kV, used for long distance, very high power transmission. Ultra high voltage (UHV), often associated with ≥ ±800 kVDC and ≥ 1000 kVAC By length of the line The overhead transmission line is generally categorized into three classes, depending on the length of the line: Lines shorter than 50 km are generally referred to as short transmission lines. Lines between 50 km and 150 km are generally referred to as medium transmission lines. Lines longer than 150 km are considered long transmission lines. This categorization