AT&T MERLIN LEGEND Release 2.0 Installation Section page 53

Installing the Control Unit
concrete-encased ground—must be an electrode, consisting of one of
the following:
—
—
—
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WARNING:
Do not use metal underground gas piping system—this is a
safety risk.
Central Office Trunk Protection
The telephone company is responsible for providing the following protection
of central office trunks at the entrance to the site:
carbon blocks or gas discharge tubes connected to an approved
ground
adequate bonding of the central office trunk protector ground and the
power company ground
2-14
Power and Grounding
at least 20 ft (6.1 m) of one or more steel reinforcing rods, each
being at least ½-inch (1.27 cm) in diameter
20 ft (6.1 m) of bare copper conductor not smaller than
#4 AWG, encased in 2 inches (5 cm) of concrete.
This electrode must be located within and near the bottom of a
concrete foundation or footing that is in direct contact with the
earth.
ground ring-consists of at least 20 ft (6.1 m) of bare copper
conductor not smaller than #2 AWG encircling the building.
The ground ring must be in direct contact with the earth and
buried at least 2.5 ft (77 cm) below the earth's surface.