Page 14 of 20
P403-3222 Rev. K 4/18
For each multiple increase in current either the maximum calculated protective ground resistance must be
divided by the multiple or a new calculation must be made, to reduce the protective resistance accordingly.
For multiples of the 1,000 Amp. fault current given above:
Fault Current
Multiplier
Maximum Resistance
2,000
2
50 milliohms
5,000
5
20 milliohms
10,000
10
10 milliohms
Results of calculations illustrating the body current level, below which 99.5% of the affected workers will
not experience heart fibrillation, is shown in Chart 1 and Graph 1. This chart is based upon C.F. Dalziel’s
research and widely used industry values. Calculations were made using equation 1, with the variable being
time duration of current flow. Chart 2 and Graph 2 illustrate the maximum allowable resistance of the pro-
tective ground, based upon the constraint that the maximum voltage drop across the man will never exceed
100 volts, at fault current level below the selected value. Equations 3 and 4 were used, the variable being
the fault current available. These charts and graphs are presented for information only.
The appropriate department of the using utility MUST define the method and level of protection they wish
to use. The values for the variables can then be defined for that utility. See the sample calculation on page
16.