39 Eaton® 9390 (100–160 kVA) UPS Installation and Operation Manual 164201604—Rev F
If the UPS becomes overloaded or unavailable, the UPS switches to Bypass mode. The UPS automatically
returns to Normal mode when the overload condition is cleared and system operation is restored within
specified limits.
If the UPS suffers an internal failure, it switches automatically to Bypass mode and remains in that mode until
the failure is corrected and the UPS is back in service.
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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 11 shows the path of electrical
power through the UPS system when operating in Bypass mode.
The critical load is not protected while the UPS is 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 isolates the inverter. The backfeed protection contactor is
normally always closed, ready to support the static switch unless the bypass input source becomes
unavailable.
During an outage, the UPS prohibits transfers to bypass and provides upstream protection. The backfeed
protection contactor is opened, preventing system output voltage from bleeding across the static switch
snubber components to the bypass input source.
Understanding UPS Operation
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