5kV Insulation for Pilot Wire for Solkor Protection

In "Description" of Solkor Pilot Wire Current Differential Protection is written:

"The Solkor R/Rf relay has an insulation level of 5kV between pilot connections and the local ground to withstand voltages induced on the pilot cable due to coupling with the fault current and to withstand differential ground voltages caused by the flow of fault current. Experience has shown that 5kV insulation is usually adequate for most distribution feeders.

For higher voltage systems where feeders may be longer and fault levels higher, an additional external isolation transformer is available for use with the relay in Rf mode to increase the voltage withstand to 15kV." See:

http://www.energy.siemens.com/ecc_pool/REYROLLE_Protection/d91eca12-8c92-42d0-9e6d-110e9112180e/7PG21%20Solkor%20R%20Rf%20Catalogue%20Sheet.pdf

In my opinion this could be the maximum potential difference between the two grounding grids where each of terminals is located. Even if any neutral point of two sets of CT -in the both location-are not grounded the insulation of pilot wire will be stressed.

That usually what it called GPR-Ground Potential Rise of the Grounding Grid- during the ground fault will be the voltage the pilot wire insulation will withstand.

I encountered the same situation with Siemens Distance Protection pilot cable.

Thank you very much sir, for the valuable information.