SPACING AND CLEARANCES FOR POWER TRANSMISSION LINES

From safety considerations, power conductors along the route of the transmission line should maintain requisite clearances to ground in open country, national highways, rivers, railways, tracks, telecommunication lines, other existing power lines.

The ground clearance for different voltages, which generally applicable are:

5.66 kV 6.5 m at  +650 C conductor temperature

6.132 kV 7.0 m at  +650 C conductor temperature

7.220 kV 7.5 m at  +800 C conductor temperature

The minimum clearances of conductor over rivers, which are not navigable, shall be kept 3.05 m over maximum flood level.

The minimum clearances between the conductors of a power line and telecommunication cable shall be:

132 kV 2.44 m
220 kV 2.74 m
400 kV         4.88 m

The minimum spacing between power lines shall be:

132 kV 2.75 m
220 kV 4.55 m
400 kV 6.00 m

The spacing of conductors is determined by considerations, which are partly electrical and partly mechanical. Usually conductors will swing synchronously (in phase) with the wind, but with long spans and small size of conductors, there is always possibility of the conductors swinging non- synchronously, and the size of the conductor and the maximum sag at the centre of the span are factors, which should be taken into account in determining the phase distance apart at which they should strung. As a rule of thumb, minimum horizontal spacing between conductors should not be less than 1% of the span length in order to minimize the risk of phases coming into contact with each other during swing.

There are number of empirical formula in use, deduced from spacing, which have successfully operated in practice:

NESC, USA formula
Horizontal spacing in cm,

Where A = 0.762 cm per kV line voltage
S = Sag in cm, and
L = Length of insulator string in cm
Swedish formula
Horizontal spacing in cm,

Where S = Sag in cm and
E = Line voltage in kV
French formula
Horizontal spacing in cm,

Where S = Sag in cm
L = Length of insulator string in cm
E = Line voltage in kV

Tower top clearance

Tower top clearance is the vertical clearance between earthwire and top conductor, which is governed by the angle of shielding. The shield angle varies from about 250 to 300,depending on the configuration of conductors. Tower top clearance shall be taken 1.5 and 2.25 m for 132 kV and 220 kV respectively for 00 swing.