Fibrillation current is actually function of individual body weight

For 50 kgs body weight: fibrillation current (IB) = 0.116/ª ts (Limited to 0.03 – 3 sec. Range) 

Where ts = duration of current exposure in sec. 

Note = Above equation results = 116 mA for 1 sec. and 367 mA for 100 sec. For 70 kgs body weight: fibrillation current (IB) = 0.157/ª ts 

Note = Above equation results = 157 mA for 1 sec. and 496 mA for 100 sec. 

Above times are very - very important from the point view of clearing the fault. Above limit dictates that grounding should e such that current magnitude through human body should not increase the specified values. 

In order to ensure above following have been done. 

1. Current conductor have been burried in ground 

a. At the depth of 600 mm in switchyard. Depth 600 mm is normally selected because of freezing or drying out, the Resistivity of upper layers could vary with seasons, while the Resistivity of lower soil layers remains nearly constant. 

b. Horizontal grid conductors are more effective in reducing the danger of high step and touch voltages on the earth surface by creating equipotential surface during fault conditions. 

c. At the depth of 800 mm else where. Here depth is kept more because to care for under grounding services. Example laying of power cables, drainage etc.

2. 25-mm dia copper rod electrodes have been driven in soil. 

a. Upto 5 meters depth in 220 kV switchyard. 

b. Upto 3 meters elsewhere. Why only 5 meters and 3 meters depths have been selected is that the resistance is diminishes rapidly with the first few feet of driving, but less so at depths greater than 2 to 3 meters in soil of uniform resistivity. 

These lengths are adopted in selecting the ground electrodes.

3. 4-inch layer of gravel in 220 kV switchyard has been used. Purpose of using gravel is by doing steps 1,2 above tough and step potential are computed and compared with tolerable potential and found as given below.