Vertiv™ Liebert® STS2
SL-20610_REV12_09-21 2 Guide Specifications
Proprietary and Confidential © 2021 Vertiv Group Corp.
• Electrostatic discharges (ESD) up to 10 kV at any point on the exterior of the unit
• Electromagnetic fields from portable transmitters that are not within 3 ft. (1m) of the unit
The Vertiv™ Liebert® STS2 shall comply with the immunity requirements of EN/IEC61000-6-2 Class A.
1.3 Definitions
• STS - Static Transfer Switch
• SCR - Silicon Controlled Rectifier
• MTBF - Mean Time Between Failure is the actual arithmetic average time between failures of the
critical AC output bus.
• Molded-case Switches (MCS) - A circuit breaker that has no automatic thermal overload trip
element but does have a magnetic trip element for short-circuit/fault protection. Short-circuit
and overload protection must be provided by an upstream overcurrent device.
1.4 System Description
1.4.1 Design Requirements
1. Voltage. Input/output voltage specifications of the Liebert® STS2 shall be [(200) (208) (220) (240)
(380) (400) (415) (480) (575) (600) ] volts, three phase, (60) (50) Hz, (3)-wire plus ground.
2. Output Load Capacity. Specified output load capacity of the Liebert® STS2 shall be (100) (250) (400)
(600) (800) (1000) A. The Liebert® STS2 shall be continuous rated to carry a full 100% load.
1.4.2 Modes of Operation
The Static Transfer Switch shall be a three-pole, double-throw, solid-state, automatic transfer switch that
is fed from two AC power sources. One source shall be designated as the preferred source, while the
other is the alternate source. Selection of which input source is preferred shall be user selectable from
the operator control panel. All transfers shall be a fast break-before-make with no overlap in conduction
from one source to the other.
The Static Transfer Switch is fuseless and consists of six pairs of Silicon Controlled Rectifiers (SCRs)
connected in an AC switch configuration. The SCRs are continuous rated to carry 100% of the Liebert®
STS2 rated load while operating within the Liebert® STS2 specifications. The use of fuses for protection
is not permitted due to possible fuse clearing in an out of phase transfer.
The Static Transfer Switch logic power shall automatically power up when connected to the power
source. The control panel shall be active as long as one input to the Liebert® STS2 is energized. The
Liebert® STS2 shall be supplied with factory default settings; mechanical trim pots shall not be used for
calibration or adjusting settings. All settings must be adjustable; the settings shall be
adjusted/configured from the LCD display.
1. Normal Mode. The unit is fed by two sources with the output connected to the load. In normal
operation, the load shall be connected to the preferred source as long as all phases of the preferred
source are within the acceptable limits. The transfer voltage limits shall default to ±10% of the nominal
input voltage for steady state conditions, with low voltage transfer limits having an inverse time
relationship that is within the IEEE Std. 446 computer voltage tolerance envelope. Upon failure of the
preferred source, the load shall be transferred to the alternate source. After the preferred source
returns to within the acceptable voltage limits for at least the preset adjustable retransfer time delay
(typically 3 seconds) and is in phase with the alternate source, the load shall be retransferred
automatically to the preferred source. The automatic retransfer to the preferred source can be
disabled if so selected by the user from the operator control panel. Provided the preferred source is
qualified, in the event the alternate source fails, the Liebert® STS2 will always transfer to the preferred
source, regardless of the automatic retransfer setting.
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