Understanding UPS Operation
5-4 Eaton 9E UPS (40–60 kVA, 208/220V) Installation and Operation Manual P-164000058—Rev 4 www.eaton.com/powerquality
5.2.4 Bypass Mode
The critical load is not protected from voltage or frequency fluctuations or power outages while
the UPS is in Bypass mode.
The UPS automatically switches to Bypass mode if it detects an overload, load fault, or internal failure. The UPS
can also be transferred from Normal mode to Bypass mode manually. The bypass source supplies the
commercial AC power to the load directly. Figure 5-3 shows the path of electrical power through the UPS
system when operating in Bypass mode.
In Bypass mode, the output of the system is provided with three-phase AC power directly from the system
input. While in this mode, the output of the system is not protected from voltage or frequency fluctuations or
power outages from the source. Some power line filtering and spike protection is provided to the load but no
active power conditioning or battery support is available to the output of the system in the Bypass mode of
operation.
The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR) continuous static switch, and
a backfeed protection relay. The static switch is used instantaneously anytime the inverter is unable to support
the applied load. The continuous static switch is wired in series with the backfeed protection relay, and together
they are wired in parallel with the rectifier and inverter.
The static switch, being an electronically-controlled device, can be turned on immediately to pick up the load
from the inverter while inverter output relay opens to isolate the inverter. The backfeed protection relay is
normally closed, ready to support the static switch unless the bypass input source becomes unavailable.
If the UPS transfers to Bypass mode from Normal mode due to any reason other than operator intervention,
the UPS automatically attempts to transfer back to Normal mode (up to three times within a ten minute period).
The fourth transfer locks the critical load to the bypass source and requires operator intervention to transfer.
Figure 5-3. Path of Current Through the UPS in Bypass Mode
Bypass Input
Static
Switch
Backfeed
Protection Relay
Rectifier
Inverter
Output
Relay
Input
Relay
Input
BreakerRectifier
Input
Output
Battery
Converter
Battery
Relay
Main Power Flow
Relays
Breakers
Closed
Open
Battery
Trickle Current
De-Energized
Energized
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