UNDERSTANDING UPS OPERATION
Power Xpert
9395 UPS (450–550 kVA) Installation and Operation Manual
164201701 Rev 3
www.eaton.com/powerquality
6-6
6.2.5
Bypass Mode
The UPS automatically switches to Bypass mode if it detects an overload, load fault,
or internal failure. The bypass source supplies the commercial AC power to the load
directly. Figure 6‐4 shows the path of electrical power through the UPS system when
operating in Bypass mode.
C
A
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T
I
O
N
The critical load is not protected from voltage or frequency fluctuations or power outages while the UPS is in
Bypass mode.
Static
Switch
K5
Rectifier
Inverter
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
NOTE
On a UPS configured as an IOM, the bypass input, static switch, and K5 are not present.
Input
Breaker
(CB1
Figure 6‐4. 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.
