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Operating the Liebert STS2 LED Display
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14.2.3 Maintenance Bypass
The Liebert STS2 is equipped with two key-interlocked, bypass breakers, CB4 and CB5, to allow man-
ual 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). While operating
on maintenance bypass, all voltages are removed from the Liebert 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.
4. Rotate and remove the interlock key.
5. Insert the key into the CB4 bypass breaker interlock.
6. Rotate interlock keys in the CB4 bypass breaker interlock to retract the interlock.
7. Close bypass breaker CB4.
The LED for CB4 turns on.
8. Open source input breaker CB1.
9. Rotate the interlock key and remove it to lockout the breaker.
The LED for CB1 turns off.
10. Open the output breakers CB3 and CB3A (if supplied) to remove power from static switch.
The LEDs for CB3 and CB3A (if supplied) turn off.
11. Verify that the output (OUT) LED is lit.
Power is now flowing directly from the source to the load, without passing through the static transfer
switch.
To return to the normal mode:
1. Verify that the LEDs for Source 1 are on, indicating that the source is supplying power and is
operating within acceptable tolerances.
2. Remove the key from CB5 and insert it in the CB1 breaker interlock.
3. Rotate key to retract interlock.
4. Close source input breaker CB1.
The LED for CB1 turns on.
5. Close the output breakers CB3 and CB3A (if supplied).
The LEDs for CB3 and CB3A are lit.
6. Open bypass breaker CB4. Rotate keys in CB4 interlock to extend interlock.
The LED for CB4 is turned off.
7. Remove proper key from CB4 interlock and insert it in CB2 breaker interlock.
8. Rotate the key to retract CB2 breaker interlock.
9. Close source input breaker CB2.
The LED for CB2 is turned on.
10. Verify that the output LED is lit.