Mid span clearance between earthwire and conductor

[nitin36537]

For calculating phase to phase vertical clearance between conductors we use following formula

cls = 0.75 * sqrt(max sag + insulator length)+ V/150

For 400 KV line this value is about 7.0m , however Indian code require this to be 8.0 m


For 400KV lines we provide 9.0 m as mid span clearance between earthwire and conductor but no such formula is there. How to calculate this value if specification do not provide any value for design?


OR Does any code provide any formula to calculate require mid span clearance between earthwire and conductor ??

Mid span clearance requirement depends on what factors?

Thanks

Nitin patel

 

[ergon]

The geometry of the earthwire and phases at the structure are determined from lightning investigations. Minimum phase to ground clearances must be satisfied at the structure. Generally the earthwire is sagged less than the conductor. This means that the shielding angle (when positive) at midspan is less than (better than) it is at the structure. A practical vertical tolerance needs to be given to account for the accuracy of the sagging. Usually the earthwire and the conductor have different diameter to mass ratios and will swing differently under identical winds. Therefore care needs to be exercised when using midspan separation formulas which are usually only relevant for identical conductors.

 

[towerengr]

Separation between the static wire and energized conductor is always an issue where icing is possible. As the conductor is energized ice may melt/ fall off but the static with no heating retains the ice. So, mid-span separation with differential ice loading (ice on static and no ice on conductor) deserves careful analysis. There is also the case of the conductor dropping the ice and bouncing up. But the 9 meter separation in a warm climate like India seems more than enough. Of course, this assumes that the shielding angle for lightning protection is taken care of separately.