AT&T System 75 Wiring Diagram page 169

CHAPTER 3. EQUIPMENT ROOM DESIGN
Installing Coupled Bonding Conductor (CBC) Grounding
A CBC must be tie-wrapped to all trunk cables. The CBC connects the
cabinet single-point ground block to the approved ground located
nearest the telephone company owned protector block at the building
entrance facility (Figure 3-59).
Note: The CBC is a conductor that is connected to ground and
run adjacent to the pairs in an inside wiring cable. The mutual
coupling between the bonding conductor and the cable pairs
reduces potential differences in terminating equipment. The
conductor consists of a No. 10 AWG wire that is tie-wrapped to the
inside wiring cable.
APPROVED GROUNDS
An approved ground is an acceptable medium for grounding the
building entrance protector, entrance cable shield, or single-point
ground of electronic PBX equipment. If more than one 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.
Metal Frame of the Building — the metal frame of the building where it
is effectively grounded by one 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
inch in diameter, in direct contact with the earth for at least 10 feet. 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 is connected. 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 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
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