Vertiv™ | Liebert® STS2/PDU
™
User Manual | Rev. 6 | 10/2017 72
Automatic Retransfer Inhibited
If the system makes five transfers from the preferred source to the alternate source in a five minute period,
automatic retransfer back to the preferred source is disabled.
To clear this alarm condition, you must assign the alternate source as the preferred source. Also, the
undervoltage detection thresholds may need adjustment. See Source Setpoints on page 102 for instructions on
setting the UV detection.
This alarm also is cleared if the unit transfers to the preferred source due to an alternate source AC voltage
failure (i.e., an emergency transfer.)
9.1.4 Emergency Transfer
To ensure that power remains connected to the load, the Liebert STS2/PDU 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/PDU senses the load current. If the load current exceeds the preset level deemed to represent
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 in Source Setpoints on page 102 for details.
9.1.6 SCR Failure
The Liebert STS2/PDU 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/PDU 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/PDU 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/PDU contains no master On/Off push button control. When connected to the input sources,
the unit’s logic power comes on automatically. The touchscreen is active as long as at least one input source is
energized and the Control Power Disconnect (located inside the static switch) is closed.
9.2 Detailed Component Description
9.2.1 Controls
Operator Interface
The Liebert STS2/PDU Color Graphical Display unit contains a color LCD touchscreen to provide the operator
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.
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