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 MIL-HDBK-419A
2.8.2 Beating of Electrodes. It is necessary to use enough material in an earth electrode to prevent excessive
2.8.2.1 Steady State Current. The presence of fault current in the earth electrode subsystem must be limited
to a value which will not raise the temperature of the soil above the boiling point of water. The tolerable
steady state ac current into an earth electrode is (2-3).
(2-68)
where
= earth resistivity,
= electrode resistance to earth, and
= permissible temperature rise
For
= 60C, the permissible steady state current is limited by
ohm-cm. Since the voltage at the earth electrode is equal to the product I R, the corresponding
when
voltage limits are
and
2.8.2.2 Transient Current. The permissible transient current density for a temperature rise that does not
exceed 60C is found from the transient temperature time expression (2-3)
(2-69)
2-57
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