AT&T System 75, Wiring Diagram

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CHAPTER 4. STATION WIRING
Carbon block, or equivalent protection is required at both building entrances.
Also sneak current protection is required. Protection can be provided by a
4-type protector or a 3-type protector plus a separate sneak current
protector. The 4-type protector is equipped with a heat coil for sneak current
protection. The 4-type protector is the preferred device because it
eliminates the need to evaluate the interbuilding cable runs and its
vulnerability to power cross faults. When the 3-type protector is used, a
separate sneak current protector is required. The multi-pair protector units
and the off-premises cabling must be locally engineered. Connectorized
multi-pair protector units (female 25-pair connector out) are recommended.
The protector units can be ordered from the Premises Distribution Systems
Equipment and Supplies Catalog.
Table 4-A shows the recommended protectors.
Table 4-A. Recommended Protectors
PROTECTORS
Primary Sneak Current
Primary *
(w/heat coil) Protectors *
3B1A
4B1C 79A Fuse
(carbon)
(carbon)
3B1E-W
4B1E-W SCP-1
(wide gap
(wide gap
gas tube)
gas tube)
3C1S
4C1S
(solid state) (solid state)
* A sneak current protector is always required
when a 3-type primary protector is used.
Requirement:
If protectors other than those listed in Table 4-A
are to be used in domestic installations, they must
be UL listed/approved. Also any protectors to be
used in Canadian installations must be CSA
certified.
The maximum range for protection of out-of-building analog telephones (500-,
2500-, or 7100- types) connected to a TN742 or TN769 analog line circuit
pack through a primary protector is 6000 feet when using 24 awg wire and
3600 feet when using 26 awg wire.
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