Vertiv™ | Liebert® STS2/PDU
™
User Manual | Rev. 6 | 10/2017 68
8.0 INTRODUCTION TO LIEBERT STS2/PDU
OPERATION
8.1 System Description
A Liebert STS2/PDU provides an automatic, seamless transfer between input power sources to an output load.
The Liebert STS2/PDU monitors the input sources, to ensure each source is operating within set parameters, as
well as ensure that each source is within acceptable tolerances for a transfer between sources to be completed.
The Liebert STS2/PDU contains two static transfer switches in one enclosure to allow the unit to transfer its load
between the two input sources without disruption to the output for longer than 1/4 of a cycle.
If a problem is detected with a primary (Preferred) input source and defined trigger points are reached, the
Liebert STS2/PDU automatically switches the input to the other (Alternate) input source. The Liebert STS2/PDU
continues to monitor both sources and can automatically transfer the load back to the preferred source, if so
configured, when that source returns to acceptable operating parameters.
The system control logic automatically monitors the input sources and power output to ensure that they are
operating within acceptable tolerances. The system logic manages the transfer between sources.
The Liebert STS2/PDU front panel contains a Color Graphical Display on an LCD touchscreen. The Color
Graphical Display provides a menu-driven operator interface and a display of system information, status
information, a one-line diagram (Mimic) of the Liebert STS2/PDU, active alarms and alarm history information.
The menu selections provide control of the preferred source, manual transfer initiation. Buttons provide for
audible alarm silence and reset.
Through the touchscreen, specific voltage, current and environmental parameters can be configured to trigger
events that can sound an alarm, write to a history log and either transfer sources or inhibit a transfer.
Preferred input source selection, alarm reset and alarm silencing are done through buttons on the touchscreen.
Communication options provide external communication with the Liebert STS2/PDU through a modem, a
network card, a programmable relay board, a contact isolator or an RS-422 port to Liebert’s SiteScan.
8.1.1 Redundancy
Redundancy within the Liebert STS2/PDU prevents one component from being a single point of failure, should a
problem occur. Redundant circuits and components are used to eliminate single points of failure.
Since the Liebert STS2/PDU has two input sources, no single component failure can cause a loss of power to the
load. Redundant power supplies are provided which feed a DC dual-bus used throughout the unit to prevent any
single-point power supply failure modes.
Three separate redundant control logic modules are built into the system. Each contains the logic necessary to
run the Liebert STS2/PDU and control all transfers.
Redundant fans are provided if a primary fan fails. The redundant fans turn on to prevent overheating inside of
the unit.
Should the front panel interface fail, the control logic continues to operate and the unit would transfer should a
problem be detected on the preferred source. Information on the monitoring parameters would be available
through the remote communication options, if available, or the DB9 (RS-232) connector located next to the LCD
behind the front door. See Figure 54 for details.
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