Eaton® 9390 (100–160 kVA) UPS Installation and Operation Manual 164201604—Rev F 46
modules, depending on the system configuration through the internal continuous-duty static switches. If a
module is taken offline, the other modules remain online to support the load. If more modules than can support
the load must be taken offline, the load must be transferred to maintenance bypass or shut down.
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.
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.
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The UPMs transfer to Battery mode automatically if a utility power outage occurs, or if the utility power does
not conform to specified parameters. In Battery mode, the battery provides emergency DC power that the
inverter converts to AC power.
Figure 16 shows the path of electrical power through the parallel system when operating in Battery mode.
Understanding UPS Operation
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