ABB Zenith
25
ZTSCT / ZBTSCT Operation & Maintenance Manual
Sequence of Operation
(cont’d)
Bypass-Isolation Operation
An automatic transfer switch equipped with a bypass-
isolation switch provides the ability to withdraw the ATS
for testing and/or maintenance purposes without interrupting
the served load.
Operation of the unit is quick and convenient requiring
only one operator and less than one minute to complete.
Instructions are mounted on the front of each isolation
switch door along with a mimic panel providing indication
of power source availability and ATS/bypass switch positions.
The bypass switch is normally open on both sources with
the ATS feeding the system load. During operation, the
bypass is closed paralleling the ATS contacts which then
allows withdrawal of the ATS to the “TEST” or “ISOLATE”
positions. Mechanical and electrical interlocks are included
to prevent cross-servicing or bypassing to a dead source.
In the “TEST” position, the ATS is disconnected from the
load (now fed through the bypass) but control power is
present to allow complete operational testing through
the control panel of the transfer switch
In the “ISOLATE” position, the ATS is completely withdrawn
and may be removed from the enclosure for maintenance
if desired.
After the isolation operation, if the bypass is closed on
Source 1 and if this source fails, an auxiliary contact on
the bypass control will automatically start the engine-
generator set. When the second source is available, the
manual handle of the bypass may be operated to transfer
the load to the available source. Interlocks prevent both
this transfer, if the ATS is in the circuit and connected to
the opposite source or reconnection of the ATS unless the
positions match.
While the bypass switch is out of the AUTO
position/mode, the ATS is INHIBITED from
automatic operation. Make certain the ATS is left
in automatic after completion of any service.
WARNING
4
1
e
r
u
g
i
F
3
1
e
r
u
g
i
F
BYPASS
SOURCE 1
SOURCE 1
AVAILABLE
BYPASS
SOURCE 2
BYPASS
SOURCE 1
SOURCE 2
AVAILABLE
SOURCE 1
AVAILABLE
