AT&T System 25 - Manual (Page 43)

INSTALLATION
3.
4.
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 one of the following types of grounds.
●
Metal underground gas piping system—an electrically
continuous metal underground gas piping system that is
uninterrupted with insulating sections or joints and without
an outer nonconductive coating.
●
Other local metal underground systems or structures—local
underground structures such as tanks and piping systems.
●
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 electrode—expose a minimum of 2 square feet of
metallic surface to the exterior soil.
Concrete encased ground—defined to be an electrode, consisting of
at least 20 feet of one or more steel reinforcing rods at least 1/2 inch
in diameter, or 20 feet of bare copper conductor not smaller than #4
AWG encased in 2 inches 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 consisting of at least 20 feet of bare copper conductor
not smaller than
must be in direct
below the earth’s
Lightning Protection
#2 AWG encircling the building. The ground ring
contact with the earth and buried at least 2.5 feet
s u r f a c e .
System 25’s lightning protection plan involves five distinct but interdependent
items required at every installation:
● Primary protection in the form of voltage limiters (typically carbon
blocks or gas tubes) on all pairs that leave the building, whether
aerial or buried. These devices bypass surges to approved building
ground and limit potential differences between T/R pairs and building
ground to less than 1500 volts.
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