In this article, we will discuss the minimum ground clearance of different transmission lines. There should be a minimum distance between the bottom conductor of a transmission line and the ground for different voltage levels as per the Indian electricity Rule 1956, Clause No.77.
As per the IE rule, for a 400 kV transmission line, there should be a minimum clearance of 8.84 meters between the bottom conductor and the ground. For a 33 kV transmission line, the ground clearance must be 5.2 meters as per Indian electricity Rule.
Section 77(4) in The Indian Electricity Rules, 1956
For extra-high voltage lines the clearance above ground shall not be less than 5.2 metres plus 0.3 metre for every 33,000 volts or part thereof by which the voltage of the line exceeds 33,000 volts: Provided that the minimum clearance along or across any street shall not be less than 6.1 metres.
We can calculate the minimum clearance for the transmission line of different voltages using this IE rule.
The minimum ground clearance for a 33 kV transmission line is 5.2 meters. Now, if we calculate ground clearance of 400 kV, the additional voltage above 33 kV is;
= 400 -33
= 367 kV
Now, we calculate the number of times the voltage 367 kV is higher than 33 kV.
= 367/33 = 11 times
For every increment of 33 kV, we need to increase the ground clearance by 0.3 meters. Thus, 400 kV line ground clearance is;
= 5.2 + 11 x 0.3
= 5.2 + 3.3
= 8.5 Meters
Ground Clearance for 132 kV Transmission line
Calculate the difference between the 132 kV and 33 kV
= 132-33
= 99 kV
Now, divide 99 kV with 33 kV.
= 99/33
=3
Thus, the ground clearance for 132 kV is;
= 5.2 + 0.3 x 3
= 5.2 + 0.9
= 6.1 Meters
Ground Clearance for 220 kV Transmission line
Calculate the difference between the 220 kV and 33 kV
= 220-33
= 187 kV
Now, divide 187 kV with 33 kV.
= 187/33
=5.66
Thus, the ground clearance for 220 kV is;
= 5.2 + 0.3 x 5.66
= 5.2 + 1.7
= 6.9 Meters
The minimum clearance of the 66KV transmission line is also taken as 6.1 meters. Actually, in any case, the ground clearance should not be less than 6.1 meters across a street. So, ground clearance of the 33kV line should also be maintained at 6.1 meters across a street. Ground clearance of 33KV bottom conductor would be 5.2 meters above cultivated land.
Ground Clearance for 132 kV Transmission line
Calculate the difference between the 132 kV and 33 kV
= 132-33
= 99 kV
Now, divide 99 kV with 33 kV.
= 99/33
=3
Thus, the ground clearance for 132 kV is;
= 5.2 + 0.3 x 3
= 5.2 + 0.9
= 6.1 Meters
Ground Clearance for 132 kV Transmission line
Calculate the difference between the 132 kV and 33 kV
= 132-33
= 99 kV
Now, divide 99 kV with 33 kV.
= 99/33
=3
Thus, the ground clearance for 132 kV is;
= 5.2 + 0.3 x 3
= 5.2 + 0.9
= 6.1 Meters
Indian Electricity Rule, 1956,Section 77
77. Clearance above ground of the lowest conductor:
(1) No conductor of an overhead line, including service lines, erected across a street shall
at any part thereof be at a height of less than-
- For low and medium voltage lines: 5.8 meters
- For high voltage lines:6.1meters
(2) No conductor of an overhead line, including service lines, erected along any street shall at any part thereof be at a height less than-
- For low and medium voltage lines: 5.5 meters
- For high voltage lines: 5.8 meters
(3) No conductor of an overhead line including service lines. erected elsewhere than along or across any street shall be at a height less than-
- For low, medium, and high voltages lines up to and including 11,000 volts. if bare: 4.6 metres
- For low, medium and high voltage lines up to and including 11,000 volts, if insulated: 4.0 metres
- For high voltage lines above 11,000 volts: 5.2 meters
(4) For extra-high voltage lines, the clearance above ground shall not be less than 5.2 meters plus 0.3 meters for every 33,000 volts or part thereof by which the voltage of the line exceeds 33,000 volts: Provided that the minimum clearance along or across any street shall not be less than 6.1meters.