LA rating

The LA Rating is selected on the formula:

Highest System Voltage (HSV) / Co-efficient of Earthing (Ground Fault Factor)

At EHV Levels, like 220kV & 400kV, the system will be solidly earthed and for solid earthing, the co-efficient of earthing is 0.8.

So, for 220kV Nominal System Voltage the HSV is 245kV. And, the LA rating will be: 245 x 0.8 = 196kV. The nearest standard LA available in the market is 198kV.

Similarly, for 400kV, the HSV is 420kV; and, the LA rating will be: 420 x 0.8 = 336kV; the nearest standard LA available in the market is 360kV.