TL;DR

A static bypass fault means the SCR assembly or bypass synchronization has failed. The load is unprotected. Identify the root cause before transferring back to inverter.

What you might see

static bypass fault alarm active
load transferred to maintenance bypass unexpectedly
SCR temperature reading high
sync loss fault on bypass input

Likely causes

Tap to mark as ruled out

SCR (thyristor) in the static switch has failed open or shorted
Bypass input voltage out of sync with the inverter output causing transfer inhibit
Gate drive board fault preventing SCR firing
Bypass input frequency deviation beyond the sync window

Required tools

Multimeter (rated for the supply voltage and frequency class)
PPE: category-rated arc flash gear, insulated gloves
UPS event log printout

Safety first

The UPS cabinet contains 480 VAC (or higher) AC buses and a high-voltage DC bus. Internal work requires a qualified electrical engineer and full arc-flash PPE.
If the load is on static bypass, it is no longer isolated from supply disturbances. Prioritize restoring inverter protection or transferring to a known-good source.
Never open the maintenance bypass panel and the UPS cabinet simultaneously. This can create a back-feed path to energized equipment.

Procedure

7 steps · ~45 min

1
Check the current operating mode on the UPS display. Confirm whether the load is on inverter, static bypass, or maintenance bypass.
2
Review the fault log on the display for the specific bypass-related fault code and timestamp.[1]
3
Measure the bypass input voltage on all three phases and compare to the inverter output voltage. Both must be within the sync tolerance.[1]
4
Measure the bypass input frequency. It must be within the UPS sync window (typically plus or minus 0.5 Hz) for auto-transfer to be available.
5
Inspect the SCR heat sinks for discoloration or signs of thermal damage from the external vent opening without opening energized panels.
Warning: Internal UPS panels contain high-voltage DC and AC buses. Only qualified electrical personnel with appropriate PPE should open the cabinet.
6
If SCR thermal damage is suspected, contact the Eaton service team. SCR replacement requires a qualified field service engineer.
7
While the bypass fault is active, ensure the maintenance bypass switch is in a known state and document the UPS operating mode for continuity of protection.

Sources

Eaton Eaton 9395 / 93PM UPS System (Industrial) general technical documentation, Eaton
Eaton 9395 / 93PM UPS general static bypass and fault diagnosis procedures (general)

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