Ground Clearance of Different Transmission Lines

According to Indian Electricity Rule 1956, Clause No. 77, the minimum ground clearance for a 400KV transmission line is 8.84 meters.

The same clause sets a minimum ground clearance of 5.2 meters for a high-voltage uninsulated electrical conductor such as a 33KV line, where the conductor is erected elsewhere than along or across a street.

For extra-high-voltage lines, clause 77 requires 5.2 meters plus 0.3 meters for every 33KV, or part of it, by which the line voltage exceeds 33KV.
Applying this rule to a 400KV transmission line,
400KV – 33KV = 367KV and 367KV/33KV ≈ 11.1, so any part of 33KV counting as a full step gives 12 steps
Now, 12 × 0.3 = 3.6 meters.
So, as per the rule, the ground clearance of a 400KV bottom conductor comes to 5.2 + 3.6 = 8.8 meters, and published Indian utility tables list 8.84 meters for 400KV lines.

Applying the same rule to a 220KV transmission line,
220KV – 33KV = 187KV and 187KV/33KV ≈ 5.7, so 6 steps apply
Now, 6 × 0.3 = 1.8 meters.
So, as per logic, the ground clearance of a 220KV bottom conductor works out to 5.2 + 1.8 = 7.0 meters. Applying the same rule to a 132KV transmission line,
132KV – 33KV = 99KV and 99KV/33KV = 3
Now, 3 × 0.3 = 0.9 meter.
So, as per logic, the ground clearance of a 132KV bottom conductor is 5.2 + 0.9 = 6.1 meters. The minimum clearance of a 66KV transmission line is also taken as 6.1 meters, because clause 77 fixes 6.1 meters as the floor along or across any street. In no case should the ground clearance be less than 6.1 meters across a street, so the ground clearance of a 33KV line must also be maintained at 6.1 meters across a street. Away from streets, the ground clearance of a 33KV bottom conductor is 5.2 meters.