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14.2.2 Manual Transfer / Preferred Source Selection
The preferred source buttons allow you to change the preferred source. Transfers are only permit-
ted when the voltage and current are within their proper operating range and the sources are syn-
chronized. Transfer is inhibited if the deviation exceeds the parameter set in the Source Set Points.
NOTE
If one source will be de-energized for an extended period (hours or days), set
the unit to bypass mode for the remaining source. Without using bypass, if a
failure occurs in the switch components for remaining source, the STS2 does
not have a second source to switch to and the load would not receive power.
See 14.2.3 Maintenance Bypass on page 104 for bypass instructions.
To manually select the preferred source:
1. Ensure that the Transfer Inhibit (NOXF) LED is not lit.
If the Transfer Inhibit LED is lit:
• Check to see if the red alarm LED is lit.
• If the alarm LED is lit, check the alarm messages on a PC terminal.
• Correct alarm conditions before attempting a source transfer.
NOTE
When the remote source selection option (Remote Control) is activated, a
change of state (Remote Preferred Source Selection) may initiate a transfer if
the appropriate contacts are closed.
2. Verify that the LEDs are lit for both sources, indicating power is present and within acceptable
tolerances.
3. Verify that the LEDs for the SCRs (STS1 and STS2) are lit.
4. Verify that the input breakers CB1 and CB2 are closed.
• Verify that their respective LEDs are lit.
5. Push the Preferred Source button for the desired source. See Figure 51 on page 100.
The Preferred Source (LED_PREFx) LED turns on for that source.
When conditions are correct, the switch transfers the load to the alternate source by selecting
the alternate source as the preferred source.
This source is now monitored as the preferred source throughout the various configurations set
up through the PC terminal.
6. Check the LEDs to verify that the correct source is designated as the preferred source and that
the input source changes to the desired source and the output (OUT) LED remains lit.
14.2.3 Maintenance Bypass
The STS2 is equipped with two key-interlocked, bypass breakers, CB4 and CB5, to allow manual
bypass of the switch electronics for either input source in the event of switch failure or for mainte-
nance of the switch. Refer to the static transfer switch one-line diagram (Figure 3 on page 14).
While operating on maintenance bypass, all voltages are removed from the STS2 electronics to
facilitate safe repair, without de-energizing the load using a make-before-break switching sequence.
However, the unit can be bypassed using a make-before-break switching sequence only to the same
source to which that the switch is connected.
In the event of source or switch failure, the unit should be bypassed only to the current input source.
Bypass Procedures for Source 1
If transfer to Source 2 is not desired:
1. Follow the instructions to transfer the load to Source 1, if it is not the currently used source.
2. Remove the interlock key from CB5 and place it in CB4 interlock.
3. Open the alternate source input breaker CB2.
The CB2 LED is turned off.
104
Operating the LED Display STS2
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