Calculation of earth electrode

[katelec]

We have Indoor LV substation area of 600sqmm. We need to derive the required no of earth pits to be placed.

The main earthing conductor is 120Sqmm cu, sleeved, soil resistivity is 100ohm/m, minimum earth rod size to be used is 25mm dia 1200mm long, required minimum earth resistance 1 ohm .

Anyone can please help me out how to derive the no of rods based on area, soil resistivity when using the 120sqmm cu, cable as main earthing conductor.

 

[jghrist]

IEEE Guide for Safety in AC Substation Grounding, IEEE Std 80-2000 gives formulas for resistance. For a combination of a grid and ground rods, the Schwarz's equations (IEEE 80 Section 14.3) are appropriate. They are from:

Schwarz, S. J., "Analytical expressions for resistance of grounding systems," AIEE Transactions on Power Apparatus and Systems, vol. PAS-88, pp. 910-919, June 1969.

 

[cuky2000]

Perhaps the easy way is to use grounding / earthing software to do this task. Manual calculation may be possible but it is time consuming and requires a good understanding of this subject.

Beware that rods is working in interconnected system and only will share a portion of the current injected in the earth. A starting point may be using the equations developed by S. Schwarz “Analytical expression for resistance of grounding systems,” AIEE Transactions on Power Apparatus and Systems, vol. 73, no. 13, part III-B, pp. 1011–1016, Aug. 1954. In addition, the IEEE Std 80-2000 contain portion of this subject on chapter 14.