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Grounding Electrodes. Definition of a Grounding Electrode. Article 100 – Grounding Electrode: A conducting object through which a direct connection to earth is established. 250.4 General Requirements for Grounding and Bonding. 250.4(A)(1) Electrical System Grounding
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Definition of a Grounding Electrode Article 100 – Grounding Electrode: A conducting object through which a direct connection to earth is established.
250.4General Requirements forGrounding and Bonding 250.4(A)(1) Electrical System Grounding Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operations. • Informational Note: An important consideration for limiting the imposed voltage is the routing of bonding and grounding electrode conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.
250.52 Grounding Electrodes (A) Electrodes Permitted for Grounding • Metal Underground Water Pipe • Metal Frame of Building or Structure • Concrete Encased Electrode • Ground Ring • Rod Electrodes and Pipe Electrodes • Other Listed Electrodes • Plate Electrode • Other Local Metal Underground Systems or Structures
250.52(A)(1)Metal Underground Water Pipe • Shall be in direct contact with a minimum 10 ft of earth • Shall include any well casing effectively bonded to the pipe • Shall be electrically continuous by bonding around insulating joints
250.52(A)(2)Metal Frame of Building or Structure Provides two conditions under which the metal frame of a building or structure can be judged suitable: • 10 ft or more of a single structural metal member in direct contact with the earth or in concrete directly contacting the earth • Use of hold-down bolts securing the structural steel to a concrete encased electrode that complies with 250.52(A)(3) and is located in the support footing or foundation.
250.52(A)(3)Concrete-Encased Electrode • A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either: • One or more bare or galvanized or other electrically conductive coated steel reinforced bars or rods not less than 13mm(1/2 in.) in diameter, installed in one continuous length, or if in multiple pieces connected together by the usual tie wire or other effective connection method • Bare copper conductor not smaller than 4 AWG • Encased by at least 50mm (2 in.) of concrete Electrode shall be: - at least 20 ft in length - reinforcing bars or rods not less than ½” in diameter - bare copper conductor not less than #4 AWG • Reinforcing bars may be bonded together by the usual steel tie wires
250.52(A)(4)Ground Ring A ground ring encircling the building or structure in direct contact with the earth, consisting of at least 6m (20 ft) of bare copper conductor not smaller than 2 AWG.
250.52(A)(5)Rod and Pipe Electrodes Electrodes of Pipe - 2.44m (8 ft) minimum length - 3/4 in. “trade size” minimum diameter - be of iron or steel and shall have the outer surface galvanized or otherwise metal-coated for corrosion protection Electrodes of Rod - 8 ft minimum length - 5/8 in. (0.625”) minimum diameter - 1/2 in. (0.500”) minimum diameter for stainless steel, and copper-coated or zinc-coated steel when listed
250.52(A)(7)Plate Electrodes • Plate electrodes shall expose not less than 0.186m2 (2 ft2) of surface to exterior soil • Electrodes of bare or conductively coated iron or steel plate shall be at least 6.4m (1/4 in.) thick • Solid, uncoated, non-ferrous electrodes shall be at least 1.5mm (.06 in.) thick
250.52(A)(8)Other Local Metal Underground Systems or Structures May include piping systems, underground tanks, and underground well casings not effectively grounded to metal water pipe.
250.53 Grounding Electrode System Installation Summary • If practicable, rod, pipe and plate electrodes shall be installed below permanent moisture level • Grounding electrodes shall be spaced not less than 1.83m (6 ft) apart where more than one electrode is specified • Informational Note: the paralleling efficiency of rods is increased by spacing them twice the length of the longest rod. • A bonding jumper shall be sized and spaced according to NEC • If a metal underground water pipe is used, it shall meet NEC requirements • A bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than 6 AWG copper wire. • Ground rings and plate electrodes shall be installed not less than 750mm (30 in.) below the surface of earth.
ANSI/NEMA GR 1-2007Zinc Coated Ground Rod Electrodes Diameter Ranges for Zinc Coated Steel Rods Finish Diameter Range, Inches Trade Sizes Minimum Maximum 5/8 0.625 0.640 3/4 0.750 0.765 Note: The above dimensions may vary if ground rod is certified to a U.S. nationally recognized standard.
ANSI/NEMA GR 1–2007Copper-Bonded Ground Rod Electrodes Diameter Ranges for Copper-Bonded Steel Rods Finish Diameter Range, Inches Trade Sizes Minimum Maximum 1/2 0.500 0.507 5/8 0.555 0.565 3/4 0.673 0.683 1 0.907 0.917
ANSI/NEMA GR 1–2007Copper-Bonded Ground Rod Electrodes Thread Specifications Trade Sizes Thread Length Thread B Profile (in. +1/8, -1/16) (UNC) 1/2 1-1/16 9/16-12 5/8 1-3/16 5/8-11 3/4 1-1/4 3/4-10 1 1-11/16 1-8
Properties of ANSI/NEMA GR-1 Zinc-Coated Ground Electrodes Zinc-Coated Steel • Straightness variation ≤ 0.25 inches in 5 ft • Steel core tensile strength ≥ 80k psi • Zinc deposit shall be minimum 0.0034 inches • Legibly marked with manufacturer’s identification • ASTM Standards - ASTM A 370 – Tensile strength ASTM A 123 or ASTM A 153 – Zinc coating ASTM E 376 – Coating thickness measurement
Properties of ANSI/NEMA GR-1 Copper-Bonded Ground Electrodes Copper-Bonded Steel • Straightness variation ≤ 0.25 inches in 5 ft • Steel core tensile strength ≥ 80k psi • Copper deposit shall be minimum 0.010 inches • Legibly marked with manufacturer and catalog or equivalent designation • ASTM Standards - ASTM A 370 – Tensile strength ASTM E 376 – Coating thickness measurement
Resistance of Electrodes • Water pipe – no NEC requirements • Building steel – no NEC requirements • Concrete encased – no NEC requirements • Ground ring – no NEC requirements • Rod, Pipe, and Plate – 25 ohms or less • A second rod is required if the first rod’s resistance is non-compliant • NEC Section 250.53 • Supplemental Electrodes for metal underground water pipe - Where of pipe, rod or plate – NEC 250.53(A) (2) is applicable
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