Theory of Operation
63
9.2.2 Circuit Breakers
Three circuit breakers provide total isolation of the solid-state switching devices with an input
breaker for each source and a load isolation breaker. Two breakers provide for maintenance
bypassing of the solid-state switching devices to either input source.
All switches in the Liebert STS2 are plug-in types, which allows any switch to be replaced without de-
energizing the output bus. The breakers have an interlock in the plug-in base, which prevents the
switch from being withdrawn in the CLOSED position. The switch trips OPEN before it disconnects
from the plug-in base.
See 15.2.5 - Circuit Breakers for specifications and more information pertaining to the circuit
breakers. See Figure 14 to Figure 19 for circuit breaker locations. The circuit breaker schedule is
provided in Figure 20.
9.2.3 SCRs
Brick-type SCRs are used rated to carry the full 100% rated load. Because the unit is a fuseless design
with non-automatic circuit breakers, overcurrent protection is provided by upstream circuit breakers.
9.2.4 Logic Modules
The logic contains three separate redundant logic modules. The modules contain the logic necessary
to run the Liebert STS2 and control transfers and shunt trips under all circumstances, without any
outside assistance once set-up and started properly.
9.2.5 Audible Alarm
An audible alarm is installed on the control board. This alarm is triggered by an alarm condition
detected on the unit. The alarm can be turned off from the touch screen (Color Graphical Display
units) or by a push button (LED Display units). Turning off the audible alarm does not clear or reset
the condition that triggered the alarm.
9.2.6 RS-232 Port
An RS-232 serial port is provided to allow an external terminal to be connected as another user
interface. See 12.1 - Using the RS-232 Port on page76 for more information on using the RS-232
port.