General Order 128
Definitions of Terms as Used in These Rule
21.5     Grounded means connected to earth by a conductive path.
A.     Effectively Grounded (Effective Ground)
means permanently connected to earth through a ground connection or connections
of sufficiently low impedance and having sufficient current–carrying
capacity to prevent the building up of voltages which may result in undue
hazard to persons or to connected equipment.
If an impedance of less than 25 ohms is not obtained, the equivalent
of a ground conductor not less than #6 AWG copper connected to two corrosion
resisting rods, not less than 1/2” in diameter and 8 feet in length
and continuous throughout, driven to a minimum depth of 8 feet in the earth
at not less than 6 foot centers, will be considered an effective ground for
the purpose of these rules.