AT&T System 75 Wiring Diagram page 239

CHAPTER 4. STATION WIRING
Table 4-B.
Primary *
(w/heat coil)
3B1A
(carbon)
3B1E-W
(wide gap
gas tube)
3C1S
(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-B
are to be used in domestic installations, they must
be UL Iisted/approved. Also any protectors to be
used in Canadian installations must be CSA
certified.
The maximum range for protection of out-of-building digital voice terminals
using a primary and a secondary (data link) protector is 5000 feet when using
24 awg wire and 4000 feet when using 26 awg wire. The secondary (data
link) protector used with the 7400-type digital voice terminal is an isolation
transformer and requires that the voice terminal be locally powered by an
external power supply or through the ac power cord provided with some of
the 7400-type voice terminals.
Except for the secondary (data link) protector, all the recommended
protectors are plug-in 5-pin untis. There is a variety of 25-, 50-, and 100-pair
protector panels that are equipped with 66-, or 110-type connecting blocks
and/or RJ21X connectors. The secondary (data link) protector is equipped
with 110-type connecting blocks and must be used with a primary protector.
4-42
Recommended Protectors
PROTECTORS
Primary Secondary
4B1C Data Link Protector
(carbon) (one or eight ckts)
4B1E-W
(wide gap
gas tube)
4C1S
Sneak Current
Protectors *
79A Fuse
SCP-1