Vertiv™ | Liebert
®
STS2
™
100-1000A, 50/60Hz User Manual | Rev. 12 | 11/2017 58
Automatic Retransfer Inhibited
If the system makes five transfers from the preferred source to the alternate source in a five minute period,
automatic retransfers back to the preferred source is disabled.
To clear this alarm condition, you must assign the alternate source as the preferred source.
Also, the UV detection thresholds may need adjustment. See Source Set Points on page 87 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 volt failure
(i.e., an emergency transfer.)
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 without an interruption of power to the load longer than 1/4 cycle when utilizing the
standard transfer algorithm.When utilizing the Optimized transfer option the interruption does not exceed the
ITIC/CBEMA standard.
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 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 100 milliseconds 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 89 under the Source
Set Points heading for details. Only under open SCR condition would the Liebert STS2 automatically transfer
with an IPeak condition.
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 touchscreen 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.
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