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.