IG Em = LM Where, Km is the spacing factor for mesh voltage Ki is the correction factor for grid geometry LM is the effective length of the burried earthing conductor IG is the maximum grid currentД¬alculation of attainable step potentialвЂ¦ The attainable step potential is calculated by the following пЃІ. h is the depth of burried conductor LM is the length of the total buried conductorД¬alculation of tolerable touch potentialвЂ¦ Etouch = (1000+1.5xCsxпЃІs ).(0.116/пѓ–ts) Estep = (1000+6.0xCsxпЃІs ).(0.116/пѓ–ts) Where, ts is the duration of shock current & Cs is the surface layer derating factor and is given by: 0.09 (1- пЃІ/пЃІs) Cs = 1 вЂ“ 2hs+0.09 пЃІ is the soil resistivity of the substation пЃІs is the surface layer (gravel) resistivityД¬alculation of attainable touch potentialвЂ¦ The attainable touch (mesh) potential is calculated by the following expression пЃІ. + 1 + LM пѓ–(20xAe) 1+hпѓ–(20/Ae) Where, Ae is the area of the ground mat. (tc.пЃ¬r.пЃІr.104/Tcap) Ac2 = ln where, If is the symmetrical fault current in KA (rms), the maximum current that would flow in any section of the earthing system tc is the duration of fault current in sec.Д¬onductor Size CalculationвЂ¦ пЃ¬ris the co-efficient of linear expansion of the earthinging conductor пЃІr is the resistivity of the earthing conductor in пЃпЃ—- cm at reference temperature, Tr Tcap is the thermal capacity factor of the earthing conductor in Joules/ cm2/ 0C Tr is the reference temperature=200C K0 = Ta is the ambient temperature Tm is the maximum allowable temperature for jointsД¬alculation of resistance of the earthing grid 1 1 1 RG=пЃІ. Soil Resistivity Measurement Then, 4пЃ°aR пЃІa = _ 1+ 2a - a_ пѓ–(a2+4b2) пѓ–(a2+b2) where, пЃІa is the apparent resistivity of the soil in пЃ—-m R is the measured resistance in пЃ— a is the distance between adjacent electrodes in metres b is the depth of the electrodes in mД¬onductor Size CalculationвЂ¦ The area of the conductor of earthing mat is given by: If. The voltage between the two inner (potential, V) electrodes is then measured and divided by the current between the two outer (current, I) electrodes to give a value of resistance R. Soil Resistivity Measurement Wenner Four-Pin Method: Four probes are driven into the earth along a straight line, at equal distances вЂaвЂ™ apart, driven to a depth вЂbвЂ™. 60-100 mA Ventricular fibrillation Maximum Body Current: Ik = for t =.To limit the touch and step potential within tolerable limitsД .116 в€љt Human Safety Current RangeEffects on Humans.

Proper functioning of the protection system under fault condition.Safety of the living beings around the vicinty of the substation.

Earthing calculation ppt