GE Zenith Controls 21■
ZTSCT / ZBTSCT Operation and Maintenance Manual (71R-5000A)
Testing (cont’d)
Closed Transition
GE 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 milliseconds. 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 initiates 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.
Retransfer with Closeed 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 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 corrected 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.
NOTICEA periodic test of the transfer switch under load conditions is recommended to insure proper operation.
(See National Electric Code articles 700 and 701)