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Understanding UPS Operation
The parallel system can be transferred from Normal mode to Bypass mode manually.
However, the parallel system automatically switches to Bypass mode whenever the UPMs
can no longer supply the critical load. If the parallel system transfers to Bypass mode from
Normal mode due to an output voltage deviation, the parallel system automatically
attempts to return to Normal mode (up to three times within a 10-minute period). After
three transfer attempts or an overload, the system locks the critical load to the bypass
source and requires operator intervention to transfer.
Bypass mode is a normal operating mode, not an alarm condition. However, if the parallel
system is unable to return to Normal mode following an automatic transfer to Bypass
mode, an alarm condition is recorded.
The Bypass may also be used when the modules in the system must be shut down to
perform routine maintenance or repairs.
In the Parallel Redundant (N + 1) arrangement, the continuous-duty static switch in each
module operates to support the applied loads on bypass. If both units are in NORMAL and
one unit trips offline, the remaining unit does not go to bypass as long as it has the
capacity to support the load.
In the Parallel Capacity (N + 0) arrangement, if one unit trips offline and goes to BYPASS,
the remaining units also go to BYPASS.
7-13
Powerware
9390 UPS (40–80 kVA) Installation and Operation Manual S 164201535 Rev C
®
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