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AC Power and Ground
Issue 10 June 2005 43
Approved Grounds
An approved ground is the closest acceptable medium for grounding the building entrance
protector, entrance cable shield, or single-point ground of electronic telephony equipment. If
more than 1 type of approved ground is available on the premises, the grounds must be bonded
together as required in Section 250-81 of the National Electrical Code.
Grounded Building Steel — The metal frame of the building where it is effectively grounded by
1 of the following grounds: acceptable metallic water pipe, concrete encased ground, or a
ground ring.
Acceptable Water Pipe — A metal underground water pipe, at least 1/2-in. (1.3 cm) in
diameter, in direct contact with the earth for at least 10 ft (3 m). The pipe must be electrically
continuous (or made electrically continuous by bonding around insulated joints, plastic pipe, or
plastic water meters) to the point where the protector ground wire connects. A metallic
underground water pipe must be supplemented by the metal frame of the building, a
concrete-encased ground, or a ground ring. If these grounds are not available, the water pipe
ground can be supplemented by 1 of the following types of grounds:
● Other local metal underground systems or structures — Local underground structures
such as tanks and piping systems
● Rod and pipe electrodes — A 5/8-in. (1.6 cm) solid rod or 3/4-in. (2 cm) conduit or pipe
electrode driven to a minimum depth of 8 ft (2.4 m)
● Plate electrodes — Must have a minimum of 2 ft
2
(0.185 m
2
) of metallic surface exposed
to the exterior soil
Concrete Encased Ground — An electrode encased by at least 2 in. (5.1 cm) of concrete and
located within and near the bottom of a concrete foundation or footing in direct contact with the
earth. The electrode must be at least 20 feet (6.1 m) of 1 or more steel reinforcing bars or rods
1/2-in. (1.3 cm) in diameter, or at least 20 ft (6.1 m) of bare, solid copper, 4 AWG (26 mm
2
) wire.
Ground Ring — A buried ground that encircles a building or structure at a depth of at least 2.5
ft (0.76 m) below the earth’s surface. The ground ring must be at least 20 ft (6.1 m) of 2 AWG
(35 mm
2
), bare, copper wire.
Approved Floor Grounds
!
CAUTION:
CAUTION: If the approved ground is inside a dedicated equipment room, then these
connections must be made by a qualified electrician.
Floor grounds are those grounds on each floor of a high-rise building that are suitable for
connection to the ground terminal in the riser closet and to the cabinet single-point ground
terminal. Approved floor grounds may include:
● Building steel
● The grounding conductor for the secondary side of the power transformer feeding the floor
● Metallic water pipes