It ensures the connection of all metal supply lines entering a building, including power and communication cables, to the lightning protection and earthing system by direct junctions across disconnection spark gaps, or arrestors in the case of live conductors. This recommendation is in addition to any made grounding electrodes, such as the lightning ground ring. However, in general, a low earth resistance is recommended. This effectively minimizes voltage differentials on the ground plane which exceed a value that will produce noise or interference to communications circuits. The resistance to earth of the earth electrode subsystem should not exceed 10 ohms at fixed permanent facilities.
Deep wells achieve low dc resistance, but have very small benefit in reducing ac impedance.
The objective of the earth electrode subsystem is to reduce ac and dc potentials between and within equipment. If deep wells are utilized as a part of the earth electrode subsystem grounding net, the other portion of the facility ground network shall be connected to them. Equipment grounding conductors are bonded to the system grounded conductor to provide a low impedance path for fault current that will facilitate the operation of overcurrent devices under ground-fault conditions. Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 10 ft.