Basic Electrical Engineering

Types of overhead lines – Short transmission lines, Medium transmission lines, Long transmission lines, Guarding

Types of overhead lines

The transmission line has three parameters, resistance, inductance and capacitance distributed uniformly along the whole length of the line. The resistance and inductance forms series impedance. The capacitance existing between conductor for single phase line and a conductor, to neutralize the three-phase line, forms a shunt path through the length of the line. Therefore, capacitance effects introduce complications in transmission line calculations.

The overhead transmission lines are classified as

i. Short transmission lines

ii. Medium transmission lines

iii. Long transmission lines

i. Short transmission lines

When the length of an overhead transmission line is about 50 metres and line voltage about 20 KV is usually called as a short transmission line. Due to smaller length and low voltage, the capacitance effects are small. The total resistance and inductance are assumed to be at one point for calculation purpose.

ii. Medium transmission lines

When the length of an overhead transmission line is to a span of 50 m to 150 m and line voltage 20 KV to 100 KV. It is usually called as a medium transmission line. Due to sufficient length and voltage of the line, the capacitance effects are taken into account. For the purpose of calculation, the distributed capacitance of the line is divided in the form of condensers shunted across the line at one or more points.

iii. Long transmission lines

The length of an overhead transmission line is more than 150 metres and line voltage is 100 KV, it is called as a long transmission line.


Low, medium and high voltage conductors are caused by natural disasters, such as rain or storm which cause electrical accidents. This accident causes damage to life. If a live conductor is cut down, the earthed cradle part is used to hold the conductor and protects lives. This part is called as guarding. When the conductor touches the guarding, the circuit breaker automatically disconnects the supply.

Types of guarding

i. Cradle guarding

ii. Cage guarding

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