AT&T System 75, Wiring Diagram

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Miscellaneous Wiring Installation
Installing Emergency Transfer Units and Associated
Telephones
Requirements
Emergency transfer capability is provided by a transfer unit mounted on
the wall next to the trunk/auxiliary cross-connect field.
Only 500- or 2500-type telephones can be used for emergency transfer.
For R1V2/V3 switches, 500-and 2500-type telephones can also be used
as normal extensions. For R1V1 systems, 500-type telephones are
used when the CO has only rotary dialing service and they cannot be
used as normal extensions. Emergency transfer capability may be
provided on CO and Wide Area Telecommunications Service (WATS)
trunks.
The following transfer units are available:
●
●
574-5 power transfer unit—Each unit serves up to five power
failure transfer telephones. The unit provides automatic ground
start.
Z1A emergency transfer unit—Each unit serves up to six power
failure transfer telephones. A ground-start switch is required at
each preselected telephone when ground start trunks are used.
At the cross-connect field, the transfer units are connected to a yellow
terminal row/connecting block in the trunk/auxiliary field. The units are
powered (-48 V dc) from the EM TRANS RELAY PWR terminals. There
are seven EM TRANS RELAY PWR terminal pairs which allows powering
of up to seven transfer units.
Installing the 574-5 Power Transfer Unit
The 574-5 power transfer unit has connection for five emergency
transfer telephones and is compatible with both loop start and ground
start trunks. Ten slide switches in the center of the unit (Figure 4-25) are
set for the trunk type (two per trunk). The switches are set at the factory
for ground start trunks.
4-45