Zs=Ze+(R1+R2)

[LalithUdawela]

For TT System , how find Ze for designing stage?(say before construct, we do estimate.)pls reply me..

 

[plugwash]

The system used in the UK is to decide what Ze is acceptable (IIRC regs here reccomend not more than 200 ohm because values higher than that are considered unstable. Then a rod is driven in and Ze is measured. If Ze is too high an extention is added, the extended rod driven in and Ze is measured again. This is repeated until the Ze is acceptablly low.

On a TT your Ze will almost certainly be too high to trip MCBs in any reasonable timeframe so at least under UK regs everything would have to be on some form of RCD.