EARTHING CURRENT AND ELECTRIC SHOCK HAZARD POTENTIAL DIFFERENCE 589- Use the method described in IEEE80 sub-section 14.4 even though it is more applicable to much
larger sites, but include the earthing rods. - If possible limit the maximum resistance to earth at the site to 5 ohms.
Step 3. Find the resistanceRepat the pole.
The following calculations are based on the methods given in IEEE80 Appendix C. The same
symbols and notation are generally used to avoid confusion with the reference. The design data and
constraints are:-
- Fault duration ts = 0.5 sec
- Resistivity of lower layer ρ = 100 to 1000 ohm-m
- Resistivity of upper layer ρs = 1000 to 5000 ohm-m
- Thickness of upper layer hs = 0.2 and 1.0 m
- Depth of burial of grid h = 0.5 and 1.0 m
- Site area A = 36 to 256 m^2
- Diameter of rods dr = 0.02 and 0.2 m
- Depth of each rod lr = 10and50m
- Number of meshes in each side of the grid Nmesh = 3to 8
- Spacing between the mesh nodes dsp = 2.0 m
The results of the calculations are shown in Table H.1c; Case C.3 is used for the worked
example. In this case the following additional information was used:-
- Number of outer peripheral rods Nrod1 = 4
- Number of inner rods Nrod2 = 0
- Number of rods on each side of grid N = 2
- Diameter of grid conductors dm = 0.01 m
Calculate the resistivity derating factor Cs{hs,K} from (13.3), in which us=√
( 1 +
(2mhs/ 0. 08 )^2 ). The number of termsmis taken to be 25 in order to obtain good convergence
of the factor. The reflection factorKis found fromρandρsto be−0.6667 per-unit.Cs{hs,K}is
found to be 0.8338 for this example.
The approximate grid resistanceRepowithout the earthing rods can be found from Figure H.1a,
which was derived from Figure B.1 of IEEE80 but applicable to small sizes of the meshRepo,is
approximately:-
Repo=ρ 51. 94
1000= 51 .94 ohms,which is too high and indicates the need for rods.
At this stage the 50 kg step and touch voltages can be calculated fromCs{hs,K},sinceρs
andtsare constants. The step voltageEstep50is: -
Estep50=( 1000 + 6 Csρs) 0. 116
√
ts=4267 volts.