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Understanding UPS Operation
5.2.4
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
an optional backfeed protection contactor. 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
contactor (if fitted), 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 contactor opens to isolate the inverter. The backfeed protection
contactor (if fitted) 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.
Bypass Input
Rectifier
Input
5-4
Eaton 93E UPS (15-80 kVA, 380/400/415V) Installation and Operation Manual
Bypass Mode
Bypass Switch (Optional Dual Input)
Input Switch
Rectifier
Input
Relay
Relay
Switch
Closed
Open
Figure 5-3. Path of Current Through the UPS in standard Bypass Mode
CAUTION
Static
Switch
Inverter
Battery
Converter
Battery
Relay
Battery
Output
Output
Relay
Main Power Flow
Trickle Current
Energised
De-Energised
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