Grounding Electrode - a conductor(s) embedded in the earth for the purpose of dissipating the currents conducted to it via connecting grounding conductors. Where practical, the resistance to ground should be on the order of 1 ohm or less, although the NEC will allow a resistance up to 25 ohms (250-84).
A continuous underground metallic water piping system provides a very satisfactory grounding electrode if the pipe run is permanently made up without use of any plastic sections. The growing use of plastic pipe for plumbing, including sections used to make repairs, introduces a note of caution. Also consider whether the pipe is adequate to carry the available fault current. However, the size of the pipe and the extent of the buried sections should be considered if this is the sole grounding electrode. Hot water piping should NEVER be used as a grounding electrode because it is disconnected from the earth when the water heater is removed for its inevitable repair or replacement.
Ground Bus - a protective ground network for establishing a uniform potential in and about the structure. It is tied solidly to the grounding electrodes. It must be of adequate size for the installation and should run to various points in the building for convenient use.
Grounding Conductor - a conductor used to connect equipment frames or wiring system enclosures to the ground bus. Conductor material should be soft drawn or medium hard drawn copper wire or bar. Some of the commonly used types of equipment grounding conductors include metal conduit, metallic covered cable, metallic raceways, or bare or green colored insulated conductors within the conduit, armor or raceway (250-91b).
The NEC (250-80(a)) requires that the interior metal water piping system shall always be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to one or more of the grounding electrodes used. Section 250-80(b) requires that any other metallic piping that might become energized shall be bonded, also, in a manner permitted for water piping.
The NEC (250-83(a)) prohibits the use of metal underground gas piping systems as a grounding electrode.
