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2.6.3 Automatic Transfers to Bypass, UPS System Faults
For specified UPS system faults, the control logic will initiate an automatic transfer to bypass fol-
lowed immediately by a shutdown and isolation of the UPS system. The output, battery (MBD) and
input circuit breakers are open. The bypass breaker will be closed if the bypass line is available. Note
that the bypass line is usually not available during Low Battery Shutdown. The circuit diagram is the
same as illustrated in Figure 28 for Emergency Module Off.
The following UPS system faults will initiate an automatic transfer to bypass:
• DC Overvoltage Shutdown
• Hardware Shutdown
• Inverter Fault
• Low-Battery Shutdown
• Output Overvoltage
• Overload Shutdown
• Overtemperature Timeout
• Rectifier Fuse Blown
• Reverse Power
Refer to Table 7 for more information about these alarm messages.
Some installations may include a Remote Emergency Power Off mode that can be initiated automati-
cally by a contact closure in the critical load equipment. Refer to 2.3.6 - Remote Emergency Power
Off.
2.6.4 Automatic Retransfers to UPS
In an automatic retransfer, the two circuit breakers (bypass and inverter output) are both closed
simultaneously for a short period of time (overlap).
The following critical bus conditions must be present to initiate an automatic retransfer of the critical
load from the bypass source to the UPS inverter:
1. Critical load was initially transferred to the bypass source due to a system overload only. A
manual retransfer from bypass is required if the transfer to bypass was caused by any condition
other than output overload.
2. Overload has since dropped below 100% of the rated load.
3. Both the Input and the Battery (MBD) circuit breakers have remained closed since the overload
transfer.
4. OK to Transfer signal received from the control logic for at least 10 seconds, within 5 minutes of
the system overload transfer. A manual retransfer from bypass is required for overloads lasting 5
minutes or more.
5. Cyclic-type overloads, which occur up to five (select range is 0 to 5) times in 60 minutes, are
automatically returned to the inverter for each event including the Nth overload.
NOTE
A load transfer to the bypass line will be completed whenever an automatic transfer to bypass
is initiated and the bypass line is available. If the OK to Transfer condition is present, the load
transfer will be uninterrupted. If the Static Switch Unable alarm message is present for any
reason, the automatic transfer will be interrupted for 40 to 120 milliseconds. Because of the
reliability of the UPS components, an interrupted load transfer is a very unlikely occurrence.
NOTE
If you do not want the UPS system to initiate any automatic retransfers, this can be set during
the initial commissioning or by calling your local Liebert Services representative.