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ABB Zenith
ZTSCT / ZBTSCT Operation & Maintenance Manual
Sequence of Operation
(cont’d)
Closed Transition
ABB Zenith Closed Transition Transfer Switches are designed
to Transfer the Load between two available sources with
out interrupting power to the Load (Make-Before-Break).
Paralleling of the two sources occurs within a predefined
window of synchronization and lasts less than 100 millisec-
onds. The initial Source is then disconnected.
Initiating a Closed Transition
When a Test is initiated, the controller initiates the Time
Delay Source 2 Start (Engine Start Timer “P”) cycle. Upon
completion of the (P) time delay, an Engine start Signal
is sent to Source 2. When Source 2 voltage and frequency
reach the preset “Restore” values, the time delay to open
Source 1 timer (W) begins its timing cycle. After the (W)
time delay, the ATS closes into Source 2 only after the
controller ensures the proper phase relationship between
the two Sources. After the ATS closes into Source 2, the
SE limit switch becomes activated. The controller initi-
ates a transfer signal through the SCR-NO which opens
the ATS out of Source 1. The SNO limit switch activates.
The ATS has now closed into Source 2 position without
interrupting the load.
Re-Transfer with Closed Transition
Deactivating the Test Switch initiates the re-transfer. When
Source 1 voltage and frequency reach the preset “Restore”
values, the time delay to open Source 2 timer (T) begins
its timing cycle. After the (T) time delay, the ATS closes
into Source 1 only after the controller ensures the proper
phase relationship between the two Sources. After the ATS
closes into Source1, the SN limit switch becomes activated.
The controller initiates a transfer signal through the
SCR-EO which opens the ATS out of Source 2. The SEO
limit switch activates. The ATS has now closed back into
Source 1 position without interrupting the load.
The ATS defaults to an open transition transfer when
Source 1 fails. Closed transition transfer is not possible
with only one source available. If the optional Transition
Mode Selector (TMS) is available, one can select an Open
Transition Transfer or a Closed Transition Transfer when
both Sources are available.
If while in Closed Transition Mode, the ATS fails to open
the source it is attempting to “transfer out of ” within
100 milliseconds, an Alarm will sound and the source that
the ATS just closed into will be opened leaving the ATS in
its initial source while disabling all other transfer operations
until the problem is corrected and the “Fail to Open
Lockout Reset” has been pressed.
A signal will be provided (STE) to Shunt Trip the Generator
Circuit Breaker if the Transfer Switch remains closed into
both Sources for more than 325 milliseconds. The load
will then be fed by the Source 1. The maximum time that
the two Sources would be parallel, under these conditions,
is less than 500 milliseconds. Before operating the Closed
Transition Transfer Switch, the condition must be cor-
rected and the Shunt Trip reset. Refer to procedure for
servicing the Switch after Shunt Tripping occures.
The unit is Factory set to accomplish transfer within
5 electrical degrees. Requires an Isochronous Governor
with an operating frequency of 60+/- 0.2 Hz. Requires a
Shunt Trip Breaker on the Generator set with a response
time not exceeding 50 milliseconds.
