Theory of Operation
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.)
9.1.4Emergency 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 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.
9.1.5Load 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 92 under the Source Set Points heading for details.
9.1.6SCR 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.
9.1.7On/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.
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