Liebert Static Transfer Switch 2, Installation, Operation & Maintenance Manual, Page: 69 / 150

Theory of Operation 57
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 neces-
sary to run the 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 page 70 for more information on using the RS-232
port.