Vertiv™ Liebert® STS2
SL-20610_REV12_09-21 3 Guide Specifications
Proprietary and Confidential © 2021 Vertiv Group Corp.
2. Load Current Inhibit (also called Ipeak or Peak Current Overload). The Liebert® STS2 shall sense
the load current and, if the load current exceeds an adjustable preset level deemed to represent a
load inrush or fault condition, the Liebert® STS2 shall disable the automatic transfer even if the
voltage on the selected source exceeds the transfer limits. The load current transfer inhibit shall be
user selectable between [(automatically) (manually)] reset after the current returns to normal to allow
for continued protection against a source failure.
3. Manual Transfer (Optimized Transfer Disabled). The Vertiv™ Liebert® STS2 shall allow manually
initiated transfers between the two sources, providing the alternate source has proper phase rotation
and is within acceptable voltage and frequency limits and phase tolerances with the preferred source.
Allowable phase differences between the sources for manually initiated transfers shall be adjustable
from the operator control panel. The Liebert® STS2 shall be capable of tolerating transfers up to 30
degrees out of phase for emergency conditions. The user-adjustable phase synchronization window
shall be limited to ±30 degrees. If the transfer is manually initiated, the Liebert® STS2 shall transfer
between the two sources without interruption of power to the load greater than 1/8 cycle or less
provided that both sources are available and synchronized within the user-adjustable phase
synchronization window. For sources where the two frequencies are not exactly the same (as would
be the case between a utility and standby generator source), manually initiated transfers shall be
delayed by the Liebert® STS2 until the two sources are within the user-adjustable phase
synchronization window.
4. Manual Transfer (Optimized Transfer Enabled). The Liebert® STS2 shall allow manually initiated
transfers between the two sources, providing the alternate source has proper phase rotation and is
within acceptable frequency limits, is within acceptable voltage and frequency limits at any phase
angle difference. When a manual transfer is initiated, the Liebert® STS2 will perform an optimized
transfer such that the flux linkage is balanced when transferring between sources. The transfer time
is typically less than one line-cycle and the load voltage is maintained within the ITIC voltage
classification.
5. Emergency Transfer (Optimized Transfer Disabled). In an effort to maintain power to the load, upon
loss of the source that the load is connected to, the Liebert® STS2 shall automatically transfer to the
other source within ¼ cycle typical, (depending on the load, phase difference between sources, and
type of source failure) overriding any retransfer time delays or other inhibits except load overcurrent
providing that the other source is available.
6. Emergency Transfer (Optimized Transfer Enabled). The Liebert® STS2 shall perform an optimized
transfer such that the flux linkage is balanced when transferring between sources, when any source
anomaly that exceeds certain voltage thresholds is detected. The transfer time is typically less than
one line-cycle and the load voltage is maintained within the ITIC voltage classification. By balancing
the flux linkage, the Liebert® STS2 minimizes any saturation current should a downstream
transformer be connected.
7. SCR Failure. The Liebert® STS2 shall continuously monitor the status of the SCR switching devices
for proper operation. In the event of a shorted SCR on the source powering the load, the Liebert®
STS2 shall automatically alarm the condition and trip open the other source isolation MCS. In the
event of a shorted SCR on the other source, the Liebert® STS2 shall automatically alarm the condition
and trip open the other source isolation MCS. In the event of an open SCR, the switch shall
automatically alarm the condition and transfer to the other source. All open and shorted SCR alarm
conditions shall be latched and require the system to be repaired and reset to restore normal
operation.
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