Lucent Technologies MERLIN LEGEND Release 5.0 Pocket Reference page 30

The following is a list of approved grounds, starting with the most preferred:
Building steel
Acceptable water pipe, must be a metal, underground water pipe at least
½-in. (30.4 cm) in diameter, and in direct contact with the earth for at
least 10 ft. (3 m).
— It must be electrically continuous so that the protector ground is
connected. (Check for insulated joints, plastic pipe, and plastic water
meters that might interrupt electrical continuity.)
— A metallic underground water pipe must be supplemented by the
metal frame of the building, a concrete-encased ground, or a ground
ring.
— Other local metal underground systems or local underground
structures such as tanks and piping systems
— Rod and pipe electrodes, a 5/8-in. (1.6-cm) solid rod or ¾-in.
(1.9-cm) conduit or pipe electrode driven to a minimum depth of 8 ft.
(244 cm)
— Plate electrode, a minimum of 2 square ft. (61 square cm) of metallic
surface exposed to the exterior soil
Concrete-encased ground, which must be an electrode, consisting of
one of the following:
— At least 20 ft. (6.1 m) of one or more steel reinforcing rods, each
being at least 0.5-in. (1.27 cm) in diameter
— 20 ft. (6.1 m) of bare copper conductor not smaller than #4 AWG,
encased in 2 in. (5 cm) of concrete. This electrode must be located
within and near the bottom of a concrete foundation or roofing that is
in direct contact with the earth.
— Ground ring, consisting 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.
!
WARNING:
Do not use a metal underground gas piping system. This is a safety risk.
For most power surges, the following standard grounding requirements
provide adequate lightning and surge protection:
Properly wired/grounded/bonded outside line protectors
Properly wired/grounded AC outlet
Properly grounded single-point ground bar
Properly wired connection between single-point ground and power
supplies
Additional Power Surge Protection
The 391A1 power supply has built-in AC line protection. This built-in
protection handles almost all situations.
Occasionally, additional protection may be needed if the customer is
located in a heavy lightning area. A 147A surge protector can be connected
to the system to limit surges from the AC lines and outside lines. One 147A
protector provides protection for four outside lines. Up to three 146A
protectors can be added to the 147A to provide protection for a maximum of
16 outside lines. For more than 16 lines, additional 147A protectors are
required.
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