 Flush Vertical Rod -Cont.      Custom Search   MIL-HDBK-419A
The step voltage near the lo-foot by l-inch (3.05 m x 2.54 cm) rod in 1000 ohm-cm soil is about 68 percent of
the voltage between the rod and a point approaching an infinite distance away. Step voltage near a ground rod
will be between 80 percent of the rod potential (for very short rods) and 50 percent (for very long rods).
The step voltage on the surface of the earth near an isolated lo-foot by l-inch (3.05 m x 2.54 cm) ground rod
= 1000 ohm-cm) carrying a lightning current of 20,000 amps could be fatal since the step voltage would be
(2-61)
which is 41 times higher than the safe step voltage derived above.
The resistance of the 10-foot by 1-inch (3.05 m x 2.54 cm) rod in 1000 ohm-cm soil is
(2-62)
Higher values of earth resistivity would cause the step voltage near the rod to be even higher than the
calculated 41,400 volts. For a three second duration shock condition, the requirement that the step voltage not
exceed 100 volts means that the single 10-foot by 1-inch (3.05 m x 2.54 cm) rod would produce an unsafe Step
voltage with a fault current greater than about 50 amperes, even in low resistivity (1000 ohm-cm) soil.
2-51  Integrated Publishing, Inc.