Power substations in an urban area are interconnected by high voltage underground power cables, resulting also in interconnections of their earthing grids through cable metallic screens/sheaths. Most modern power cables with XLPE insulated core have metallic screens which are practically insulated from ground. These screens can only be (and usually are) grounded at cable ends and therefore do not act like extra earth electrodes extending from a station.
Pages 10 / pdf / by Medic1, M. Majstrovic2 & P. Sarajcev1
2. Mathematical model
3. Calculation of the resulting screening factor
4. Illustrative example