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MILHDBK419A
where
= surface earth resistivity, (ohm  meters),
= 10 for a minimum value,
t = duration of shock (sec).
For a 30 millisecond or shorter duration, the maximum safe step voltage is 1000 volts, and for durations greater
than 3 seconds, it is 100 volts.
2.8.1.2 Step Voltages for Practical Electrodes. The expressions for step voltage estimates in homogeneous
soil for both flush and buried vertical rod electrodes and for buried grid electrodes are given in the following
paragraphs. It should be noted that step voltages depend upon electrode geometry as well as upon earth
resistivity and current magnitude.
2.8.1.2.1 Flush Vertical Rod. The potential on the earth at a distance x from the top of a single, isolated
flushdriven vertical rod is (23)
(256)
and the potential of the rod itself is
(257)
The step potential at the ground rod (where p is equal to a pace, or step, length from the rod) is therefore
(258)
249


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