number of ground rods to be used, based on local
conditions, earth resistivity data, and on the size
and type of the electrical installation. Copper
clad steel rods will be specified for normal
conditions. Zinc coated steel or stainless steel
rods will be used where low soil resistivities are
encountered and galvanic corrosion may occur between
adjacent underground metallic masses and the copper
clad rods. Stainless steel rods have a longer life
than the zinc coated steel, but use of stainless
steel must be justified based on the higher cost.
Rods 15.9 mm (5/8 inch) in diameter and 2.4 m (8
feet) in length are generally acceptable; however in
rocky soils 19.1 mm (3/4 inch) rods will be
specified. In high resistivity soils, 3.1 m (10
feet) or sectional rods may be used to obtain the
required resistance to ground; however where rock is
encountered, additional rods, a counterpoise, or
ground grid may be necessary.
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The resistance to ground shall be measured using the fall-of-potential
method described in IEEE Std 81. The maximum resistance of a [driven
ground rod] [pole butt electrode] shall not exceed 25 ohms under normally
dry conditions. Whenever the required ground resistance is not met,
additional electrodes shall be provided [interconnected with grounding
conductors] [as indicated], to achieve the specified ground resistance.
The additional electrodes shall be [up to three, [2.4] [3] m [8] [10] feet
long rods spaced a minimum of 3 m 10 feet apart] [a single extension-type
rod, [15.9] [19.1] mm [5/8] [3/4] inch] in diameter, up to 9.1 m 30 feet
long, [driven perpendicular to grade] [coupled and driven with the first
rod]. In high ground resistance, UL listed chemically charged ground rods
may be used. If the resultant resistance exceeds 25 ohms measured not less
than 48 hours after rainfall, the Contracting Officer shall be notified
immediately. Connections below grade shall be fusion welded. Connections
above grade shall be fusion welded or shall use UL 467 approved connectors.
3.15.2
Items to be Grounded
Ground conductors, metallic conduits, junction boxes, and
noncurrent-carrying metallic parts of equipment shall be grounded.
Connections above grade shall be made with solderless connectors, and those
below grade shall be made by a fusion-welding process.
3.15.3
Lighting Pole
One ground rod shall be provided at each pole. Bases of metal or concrete
lighting poles shall be connected to ground rods by means of No. 8 AWG bare
copper wire. Lighting fixture brackets on wood and concrete poles shall be
grounded to a No. 6 AWG bare copper grounding conductor connected to the
ground rod.
3.15.4
Handhole
In each handhole, at a convenient point close to the wall, a ground rod
shall be driven into the earth before the floor is poured, and
approximately 100 mm 4 inches of the ground rod shall extend above the
floor after pouring. When precast concrete units are used, the top of the
ground rod may be below the floor, and a No. 1/0 AWG copper ground
SECTION 16528A
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