Specifications

100 – 1000 Amps
SL-20610 (R 11/02) 3
settings. All setting must be adjustable; the settings shall be adjusted/configured from the LCD display.
A. 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. Upon failure of the preferred source, the load shall be transferred to the
Liebert Static Transfer Switch 2 Guide Specifications
alternate source until such time as the preferred source returns to within the acceptable limits. Transfer
voltage limits shall be +/- 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. 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. When the
automatic retransfer is disabled, emergency transfers from the alternate source to the preferred source
shall not be disabled upon alternate source failure.
B. Load Current Inhibit. The STS 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 STS shall disable the
automatic transfer even if the voltage on the selected source exceeds the transfer limits. The load current
transfer inhibit shall be [(automatically) (manually)] reset after the current returns to normal to allow for
continued protection against a source failure.
C. Manual Transfer. The STS shall allow manually initiated transfers between the two sources,
providing the alternate source is within acceptable voltage 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 STS shall be capable of tolerating transfers up to 180
degrees out of phase for emergency conditions. However, the user-adjustable phase synchronization
window shall be limited to +/- 30 degrees. If the transfer is manually initiated, the STS shall transfer
between the two sources without interruption of power to the load greater than 1 millisecond 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 STS until the
two sources are within the user-adjustable phase synchronization window.
D. Emergency Transfer. In an effort to maintain power to the load, upon loss of the source that the load
is connected to, the STS shall automatically transfer to the other source in less than 1/4 cycle, overriding
any retransfer time delays or other inhibits except load overcurrent providing that the other source is
available. If one source is shorted upstream which causes an under voltage condition on that source, the
STS will sense the under voltage and transfer to the alternate source.
E. SCR Failure. The STS 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 STS shall automatically
alarm the condition and trip open the other source isolation breaker. In the event of a shorted SCR on the
other source, the STS shall automatically alarm the condition and trip open the other source isolation
breaker. In the event of an open SCR, the switch shall automatically alarm the condition, transfer to the
other source and trip open the previous source isolation breaker. All open and shorted SCR alarm
conditions shall be latched and require the system to be repaired and reset to restore normal operation.
F. Maintenance Bypass. The STS shall be furnished with key-interlocked maintenance bypass breakers
that allow the STS power, controls and monitoring electronics to be bypassed to either input source for
maintenance without interruption of power to the load. The packaging of the STS shall have all
electronics isolated from the input, output, and bypass connections to allow safe servicing of any