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104 Operation
3.4.2 Automatic Transfers to Bypass
The UPS system will initiate an automatic load transfer to the bypass line if an overload condition exceeds the
current versus time curve of system overload capacity, or capacity of the modules on-line, or if specified UPS
system faults occur. Note that when the UPS system is operating in the non-redundant mode, an automatic trans-
fer to bypass will be initiated when any UPS module is taken off line, either automatically or manually. Load On
Bypass is illustrated in Figure 48.
The Overload Transfer and Output Undervoltage alarm messages will initiate an automatic transfer to bypass and
the Load On Bypass message will be displayed. The status report screens will include the Automatic Transfer to
Bypass message. Other UPS system faults will initiate an automatic transfer to bypass followed immediately by
the shutdown and isolation of the UPS modules. Refer to 3.4.5 - Automatic Emergency Modules Off.
In an automatic transfer to bypass the UPS Output and System Bypass circuit breakers do not overlap as they do
in a manual transfer. The static bypass switch is fired (closed), the UPS Output circuit breaker is opened, and the
System Bypass circuit breaker is closed.
Bypass power is supplied to the critical load through the static bypass switch during the short time while both cir-
cuit breakers are open.
3.4.3 Automatic Retransfers to UPS
Automatic Retransfer to UPS is an option that you can select from the SCC System Configuration screen. If you
do not want the UPS system to initiate any automatic retransfers, set Max Auto-Rexfer Attempts to zero (0).
In an automatic retransfer, the two motorized circuit breakers (System Bypass and UPS 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 system:
1. The number of Auto-Rexfer Attempts selected must be greater than zero (0). If zero (0) is selected, no
automatic retransfers will occur.
2. Critical load was initially transferred to the bypass source due to a system overload only. (A manually
initiated retransfer from bypass is required if the critical load exceeded the capacity of the modules on-line,
or if a UPS fault occurred.).
3. Overload has since dropped below 100% of the rated SCC current.
4. Enough UPS modules are on-line to supply the critical load.
5. OK to Transfer signal received from the control logic for at least 10 seconds, within 5 minutes of the system
overload transfer. (A manually initiated retransfer from bypass is required for overloads lasting 5 minutes or
more.)
6. Cyclic-type overloads, which occur up to five (select range is 0 to 5) times in 60 minutes, are automatically
returned to the UPS system for each event including the Nth overload. A manually initiated retransfer from
bypass is required for the N + 1 overload.
NOTE
A load transfer to the bypass line will be completed whenever an automatic transfer
to bypass is initiated 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.