Understanding UPS Operation
Power Xpert
TM
9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716—Rev 8
www.eaton.com/powerquality
6-5
6.2.6
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 Online mode to Bypass mode manually. The bypass source supplies the
commercial AC power to the load directly. Figure 6-3 shows the path of electrical power through the UPS
system when operating in Bypass mode.
CAUTION
The critical load is not protected from voltage or frequency fluctuations or power outages while the UPS
is in Bypass mode.
Figure 6-3. Path of Current Through the UPS 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) static switch (SSW) and a
backfeed protection contactor K5. The static switch is rated as a continuous-duty device that is used anytime
the inverter is unable to support the applied load. The static switch is wired in series with the backfeed
protection contactor, 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 the inverter output contactor K3 opens to isolate the inverter. The backfeed protection
contactor is normally always closed, ready to support the static switch unless the bypass input source becomes
unavailable.
If the UPS transfers to Bypass mode from Online mode due to any reason other than operator intervention, the
UPS automatically attempts to transfer back to Online 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.
Static
Switch
K5
r
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t
r
e
v
n
I
r
e
i
f
i
t
c
e
R
K1
K3
Battery
Converter
Battery
Battery
Breaker
Main Power Flow
Trickle Current
Energized
De-Energized
Closed
Open
Breakers
Contactors
K2
Bypass Input
Rectifier
Input
Output
Input
Breaker
(CB1
Summary of Contents for Power Xpert 9395 550/275
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