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Theory of Operation
73
9.1.4 Emergency Transfer
To ensure that power remains connected to the load, the STS2/PDU automatically performs an emer-
gency 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 regard-
less 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 STS2/PDU senses the load current. If the load current exceeds the preset level deemed to repre-
sent 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 101 for details.
9.1.6 SCR Failure
The 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 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 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 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 touch screen display 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 STS2/PDU Color Graphical Display unit contains a touch screen color LCD to provide the opera-
tor interface to control and configure the operation of the unit and quickly diagnose problems. A pass-
word and an optional keylock, if installed, are provided for security.
Hardware
• The STS2/PDU operator interface in the Color Graphical Display unit is designed to provide all of
the information required to configure and monitor the unit’s operation.
• The control logic performs automatic operations with minimal operator interface.
• Each STS2/PDU unit is equipped with a color LCD touch screen.
• The STS2/PDU can have optional communication ports installed. See 6.0 - Options and 12.0 -
Communication Interfaces for more information on communication options.
• The RS-232 interface is used for configuration and status and event log monitoring.