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Introduction to Liebert STS2 Operations
57
8.0 INTRODUCTION TO LIEBERT STS2 OPERATIONS
8.1 System Description
A Liebert STS2 provides an automatic, seamless transfer between input power sources to an output
load. The Liebert STS2 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 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 cycle.
If a problem is detected with a primary (Preferred) input source and defined trigger points are
reached, the Liebert STS2 automatically switches the input to the other (Alternate) input source. The
Liebert STS2 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 front panel contains either a Color Graphical Display on an LCD touch screen or an
LED display.
The Color Graphical Display provides a menu-driven operator interface, and a display of system infor-
mation, status information, a one-line diagram (Mimic) of the Liebert STS2, active alarms, and alarm
history information.
The menu selections provide control of the preferred source, manual transfer initiation. Buttons pro-
vide for audible alarm silence and reset.
Through the touch screen, 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.
In units not equipped with the touch screen, LEDs provide the user with the ability to monitor the
Liebert STS2 and quickly assess problems.
Preferred input source selection, alarm reset and alarm silencing are done through buttons on the
touch screen, or push buttons with the LED display.
Communication options can be installed in the Color Graphical Display (LCD) unit only. These
options provide external communication with the Liebert STS2 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 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 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 nec-
essary to run the Liebert STS2, and control all transfers.
Redundant fans are provided if a primary fan fails. The redundant fans turn on to prevent overheat-
ing inside of the unit.
Should the front panel interface fail, either touch screen or LEDs, the control logic continues to oper-
ate 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 avail-
able, or the DB9 (RS-232) connector located next to the LCD display behind the front door. See
Figure 28 on page 41 for details.