As far as I know, there aren't any 'rules' for this - there are too many variables (soil type & year-round moisture content etc.).

With nice damp clay, you'll probably be OK with a 1m rod, but the dryer the soil, the deeper you'll have to go. You may even need to bang in several long rods to get down below 1Ω.

u can provide the earth mat & number of electrodes and using the following formula u can check the resistance value achieved.

vice versa ,You can also obtain the required length of the grid to achieve a resistance value of 1 ohm.

Resistance of mat R = (P/4*r )+ (P/L)

Where r -is the radius in meter,of circular space having same area as occupied by the grid.

L= Total length of conductor buried in meter(including earth electrode length).

P= Resistivity in ohm-m

"P= Resistivity in ohm-m"

If I'm interpreting your comment correctly, P depends on the nature of the soil - which is the big unknown.

thanks mini,

can you tell me the earth pit / rod calculation for transformer substation & generator building , where we use KA value?

Regards,

Baskar

Consider a Ufer ground also.

http://www.psihq.com/iread/ufergrnd.htm

G.

Thank you very much.Ver useful and important info.