AT&T System 75 Wiring Diagram page 170

Equipment Room Hardware And Cabling Installation
Rod and pipe electrodes—a 5/8 inch (solid rod) or 3/4 inch
(conduit or pipe) electrode driven to a minimum depth of 8 feet
Plate electrodes—expose a minimum of 2 square feet of metallic
surface to the exterior soil.
Concrete Encased Ground — an electrode encased by at least 2
inches 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 of one or more steel reinforcing bars or rods 1/2
inch in diameter, or at least 20 feet of bare, solid copper, No. 4 AWG
wire.
Ground Ring — a buried ground that encircles a building or structure at
a depth of at least 2-1/2 feet below the earth's surface. The ground ring
must be at least 20 feet of No. 2 AWG, bare, copper wire.
APPROVED FLOOR GROUNDS
Approved floor grounds are those grounds on the floor of a high-rise
building suitable for connection to the ground terminal in the riser closet
and to the PBX equipment single-point ground terminal. Such grounds
may be one of the following:
Building steel
The grounding conductor for the secondary side of the power
transformer feeding the floor
Metallic water pipes
Power feed metallic conduit supplying panel boards on the floor
A grounding point specifically provided in the building for the
purpose.
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