Emerson Liebert Static Transfer Switch - Manual (Page 69)

Theory of Operation
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9.1.4 Emergency Transfer
To ensure that power remains connected to the load, the Liebert STS2 automatically performs an
emergency transfer from one input source to the other when an outage occurs on the existing source.
The automatic transfer takes place if the other source’s voltage is within acceptable limits, and
regardless of the phase difference between the two sources.
Emergency transfers occur with a loss of power on the output of less than 4 ms.
Emergency transfers from the alternate source to the preferred source are not affected by the Auto
Retransfer Enabled/Disabled or Retransfer Delay user settings.
9.1.5 Load Current Transfer Inhibit
The Liebert STS2 senses the load current. If the load current exceeds the preset level deemed to rep-
resent a load inrush or fault condition, the unit’s logic disables manual and automatic transfers, even
if the selected source’s input voltage is outside the acceptable limits. This event triggers an I-Peak
alarm. If so configured, the Load Current Transfer Inhibit is automatically reset after the current and
voltage return to within normal parameters. This reset provides continued protection against a
source failure. The Load Current Transfer Inhibit also can be programmed to require a manual reset.
See step 14. on page 92 under the Source Set Points heading for details.
9.1.6 SCR Failure
The Liebert STS2 continuously monitors the status of the SCR switching devices for proper operation.
In the event of a shorted SCR on the input source powering the load, the Liebert STS2 automatically
triggers an alarm and trips open the other source isolation breaker. In the event of a shorted SCR on
the other input source, the Liebert STS2 automatically triggers an alarm and trips open the other
source isolation breaker.
In the event of an open SCR, the switch automatically triggers an alarm and transfers the load to the
other source. All open and shorted SCR alarm conditions are latched and require the system to be
repaired and reset to restore normal operation.
9.1.7 On/Off Sequence
The Liebert STS2 contains no master On/Off push button control. When connected to the input
sources, the unit’s logic power comes on automatically. The touch screen display (if so equipped) is
active as long as at least one input source is energized and the Control Power Disconnect (located
inside the static switch) is on.
9.2 Detailed Component Description
9.2.1 Controls
Operator Interface
The Liebert STS2 Color Graphical Display unit contains a touch screen color LCD to provide the oper-
ator interface to control and configure the operation of the unit and quickly diagnose problems. A
password and an optional keylock, if installed, are provided for security.
The LED Display unit includes push buttons for source transfer, alarm reset and audible alarm
silence.