A) OBJECTIVES
The objectives of this calculation are as follows :-
i) To design a common earthing system for a short circuit level of 50 kA for 1 second.
ii) To determine the number of earth pits for earthing and lightning protection.
iii) To determine the size of the main earthing grid conductor
iv) To determine the size of the down comers for lightning protection
v) To design the overall resistance of the earthing system to be less than one ohm.
B) REFERENCES
The following reference material has been used for this calculation.
i) Code of Practice for Earthing IS - 3043 - 1987
ii) IEEE Guide for Safety in AC Substation Grounding - IEEE - 80 - 1986
iii) Code of practice for the protection of buildings and allied structures against lightning IS - 2309 - 1989
iv) Report on soil investigation for the 2 X 55 MW CPP at DOLVI , MAHARASTRA .
v) Over all Earthing layout (99511-EL-0-03-D-092)
C) ASSUMPTIONS AND BASIS
The resistivity of the soil (30 ohm-m) is the average value assumed from the soil investigation report.
The CPP has many voltages/fault levels viz. 220KV/40KA/1sec , 6.6KV/31.5KA/1sec
3KV/26.3KA/1 sec ,and 415V/50KA/1sec.The ear thing system for CPP shall be common for all voltages and shall be as per IS 3043 - 1987.
D) EARTHING PHILOSOPHY
