Grounding Electrodes and Grounding Electrode Systems


(محمد سميري) #1

Grounding Electrodes and Grounding Electrode Systems

All grounding electrodes present at a building or structure must be bonded together to form a grounding electrode system, as required by NEC® Section 250.50. Doing so eliminates voltage gradients, and also improves reliability and grounding performance over time by creating redundancy. If one electrode is damaged, destroyed by corrosion, or removed, other electrodes are available to maintain the all-important connection from the service equipment to ground. Section 250.53(B) states that two or more grounding electrodes effectively bonded together are considered a single grounding electrode system.

Permitted Electrodes
The NEC recognizes the following seven types of grounding electrodes [250.52(A)]:

[ul]<LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Metal underground water pipe <LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Metal frame of a building or structure <LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Concrete-encased electrodes <LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Ground ring <LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Rod and pipe electrodes <LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l2 level1 lfo3; tab-stops: .5in”>Plate electrodes
[li]Other local metal underground systems or structures (piping, tanks, well casings, etc.)[/li][/ul]The Code provides detailed requirements for each type of electrode. For example, metal water piping must be in direct contact with the earth for at least 10 ft. Concrete-encased electrodes must be at least 20 ft of either steel reinforcing bars or bare copper conductors, installed near the bottom of a foundation or footing that is in direct contact with the earth. Rod or pipe electrodes must be installed with at least 8 ft of their length in contact with the soil.

Prohibited Electrodes
The NEC prohibits the following two types of metal objects as grounding electrodes [250.52(B)]:

[ul]<LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l0 level1 lfo1; tab-stops: .5in”>A metal underground gas piping system (because of explosion risk)
[li]Aluminum electrodes (because of corrosion risk)[/li][/ul]Connection to Grounding Electrode System
On ac premises wiring systems, a grounding electrode conductor must be connected to the grounded service conductor [250.24(A)]. This connection is often made at the grounding terminal bar in the service equipment enclosure. Section 250.64(F) permits the grounding electrode conductor to be connected to any convenient electrode in the grounding electrode system. The following rules also apply:
Size. Grounding electrode conductors are sized per Table 250.66 based on the size of the largest ungrounded service-entrance conductor. Minimum sizes are 8 AWG copper or 6 AWG aluminum or copper-clad aluminum.
Securing and Protection from Physical Damage. An exposed grounding electrode conductor must be securely fastened to the surface on which it is carried and protected against physical damage [250.64(B)]. When grounding electrode conductors are run in ferrous metal raceways or cable armor, this protective enclosure must be bonded to the conductor at each end [250.64(E)].
Material. Bare aluminum or copper-clad aluminum grounding electrode conductors cannot be installed in direct contact with masonry or earth or where subject to corrosive conditions. Where used outside, they cannot be terminated within 18 in. of the earth [250.64(A)].
Connection Methods. Grounding electrode conductors must be connected to grounding electrodes by exothermic welding or by listed lugs, connectors, clamps, or other means. Clamps for buried electrodes must be listed for concrete encasement or direct burial [250.70]. Connections must be accessible unless the grounding electrode is buried, encased in concrete, or encapsulated in fireproofing material [250.68].
Connections Between Grounding Electrodes
When multiple electrodes are bonded together to form a grounding electrode system, the following rules apply:
Spacing. Electrodes must be located at least 1.83 m (6 ft) apart [250.53(B)].
Bonding Jumpers. Bonding jumpers are sized and installed in accordance with the rules for grounding electrode conductors. Also, materials and connection methods for bonding jumpers are the same as for grounding electrode conductors [250.53©].

Supplemental versus Supplementary Grounding Electrodes
There are two different applications for grounding electrodes with confusingly similar names.

[ul]<LI class=MsoNormal style=“MARGIN: 0in 0in 0pt; mso-list: l1 level1 lfo2; tab-stops: .5in”>A supplemental grounding electrode is an additional electrode required when metal underground water piping is used as a grounding electrode [250.53(D)(2)], or when a single rod, pipe, or plate electrode has a resistance to ground of over 25 ohms [250.56]. A supplemental grounding electrode must be bonded to other electrode(s) and becomes part of the grounding electrode system.
[li]A supplementary grounding electrode is an additional electrode installed at the location of electrical equipment such as a machine tool or outdoor lighting pole. A supplementary grounding electrode need not be bonded to other electrode(s) [250.54]. However, they may not be used in lieu of connection to the equipment grounding conductor.[/li][/ul]Connection to Lightning Protection Systems
The NEC does not cover lightning protection systems, which are installed in accordance with NFPA 780, Standard for the Installation of Lightning Protection Systems. But lightning system ground terminals (ground rods) must be bonded to the grounding electrode system of a building or structure [250.106], and located at least 6 ft from any other grounding electrode [250.53(B)]. Section 250.60 prohibits the use of lightning protection ground terminals in place of the grounding electrodes required by 250.50.

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Key Definitions from Article 100
Bonding Jumper. A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.
Grounded Conductor. A system or circuit conductor that is intentionally grounded.
Grounding Electrode. A conducting object through which a direct connection to earth is established.
Grounding Electrode Conductor. A conductor used to connect the system grounded conductor or the equipment to a grounded electrode or to a point on the grounding electrode system.