Solar Lattice

FailureMode

Automatic backup unit fault

ID
fm:backup-unit-fault
IRI
https://solarlattice.org/kb/fm/backup-unit-fault
Status
active
Also known as
Backup module error, Automatic Backup Unit error, backup box hardware error, backup module relay error, backup module installation error, backup module faulty configuration, Automatic transfer switching device error, Backup module bimetal switch

This fault covers problems in the automatic transfer switch or backup module that moves loads to the battery-backup supply when the grid fails. Typical signs are event messages for a faulty relay, the bimetal switch, N-PE monitoring, overtemperature, or lost communication with the backup unit controller. In the field, the switch may not transfer when the grid fails, or it may click over while the backup loads still get no power. To confirm, simulate a grid failure by switching off the pre-meter fuse, wait for the set delay and listen for the transfer click. Check the transfer switch fuses and the B6 pre-fuse, make sure the backup type is set to automatic backup operation, and make sure the SPS IN and SPS OUT cables are not swapped. Check that the communication cable meets its requirements, then run a communication test; for backup overvoltage, check the backup port wiring and restart. If the relay or other hardware in the transfer switch is faulty, contact the manufacturer of the switching device.

Claims

affects

c1 verified · fact Transfer relay

Event 9208: relay in the automatic transfer switching device is faulty.

verified by: review-workflowverified on: 2026-10-24

c2 drafted · lead Transfer switch controller

SMA Backup Unit Controller installed in the automatic transfer switch.

caused_by

c3 drafted · lead Relay failure

c4 drafted · lead Installation error

SPS IN and SPS OUT cables miswired or swapped; installation of the transfer device to be checked on overtemperature.

c5 drafted · lead Inverter parameter settings

Backup type parameter not set to automatic backup operation, or switch-on delay not yet elapsed.

presents_as

c6 drafted · lead Backup mode unavailable

c7 drafted · lead Transfer switch transfer failure

Device does not switch on grid failure, or switches but backup loads stay unpowered.

c8 drafted · lead Backup unit overtemperature

c9 drafted · lead Communication loss

Communication between inverter and Backup Unit Controller disrupted.

c10 drafted · lead Firmware update failure

Backup Unit Controller update failed.

c11 drafted · lead Backup output AC overvoltage

review note: support check: partial - OV-Vbackup backup overvoltage messages exist, but they are inverter alarms and the passages do not tie them to the automatic backup unit fault.; review: partial - The Solis rows describe a backup overvoltage (OV-VBackup) alarm on the inverter's backup port, but none of them connect it to an automatic backup unit or transfer switch fault.

detected_by

c12 drafted · lead Backup transition test

Simulate a grid failure by switching off the pre-meter fuse and listen for the transfer click.

c13 drafted · lead Communication test proposed

mitigated_by

c14 drafted · lead External fuse check

Check transfer device fuses; B6 pre-fuse and monitoring relay pre-fuse must be switched on.

review note: support check: partial - Checking transfer device fuses, the B6 pre-fuse and the monitoring relay pre-fuse appear, but the B6 passages describe backup continuing after grid return, not a cause of this fault.

c15 drafted · lead Communication cable check

c16 drafted · lead Inverter settings review

c17 verified · fact Firmware update

Inverter firmware 3.12.15.R or later needed to set backup parameters.

verified by: review-workflowverified on: 2026-10-24

c18 drafted · lead Wiring and terminal check

c19 drafted · lead Backup load reduction

review note: support check: partial - Reducing backup loads appears for backup short circuit and LLC overcurrent, not directly for the automatic transfer switch fault.

c20 drafted · lead Inverter restart

review note: support check: partial - Restarting the inverter appears only for the OV-Vbackup alarm, not as a general mitigation for the transfer unit fault.

c21 verified · fact Contact manufacturer service

Contact the manufacturer of the automatic transfer switching device for relay, bimetal switch or N-PE monitoring errors.

verified by: review-workflowverified on: 2026-10-24

Data