AT&T System 25 - Manual (Page 102)

INSTALLATION
6. Check that all cables are labeled.
INSTALL EMERGENCY TRANSFER UNITS
(ETUs)
The 106 Emergency Transfer Unit (ETU) (Figure 2-32) supports up to five
Power Failure Transfer (PFT) sets and a DID make-busy function. Up to four
ETUs can be supported for a maximum of 20 PFT sets. The sets can be
connected to selected loop-start or ground start trunks. PFT sets must be
FCC registered single-line sets.
If ground start trunks are used, a ground
start button must be provided at each PFT set.
IMPORTANT: Only 2500MMGB, 2500MMGT, or 500MM sets may be
used as PFT sets. Only loop-start and ground start trunks can be used
for PFT service (no DID or tie trunks). Also, voice terminals that share
a cable with an ADU may not be used as PFT sets.
If power fails (switch has lost power or a major fault has occurred), a contact
closure is provided to the Central Office (CO) over a dedicated pair of wires.
The CO then makes all DID trunks busy. When power is restored, the
closure is removed and the CO restores DID service. External alarm contacts
are provided on the front of the ETU for use as required.
Note: It is recommended that customers with DID service make
provisions with their CO to provide this arrangement.
The ETUs are mounted on the cross-connect backboard. Connections are
through 25-pair receptacle-ended (CO and SIP) and plug-ended (switch line
and trunk) connectors.
Modular jacks are provided for the -48 V control
signal from the CPU/Memory and for additional ETUs. Screw terminals are
provided for the connection of external alarms.
Figure 2-33 shows a single-line voice terminal that has been connected as a
Power Failure Transfer station. In normal operation, the CPU/Memory CP
supplies -48 Vdc to the ETU. The voice terminal is connected through the
ETU to the station port CP and can support all calling activities. The trunk
connection through the ETU to the trunk port supports normal trunk calls.
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