The UN-BUS fault occurs when the inverter detects abnormally low DC voltage on the internal DC bus bar. This can also happen if the inverter experiences an internal failure. Either the internal circuit is damaged, or the measurement of that circuit is damaged. This can be caused by faulty components, shipping damage, or improper installation. Determine the PV string Isc(s) and then match them to the limitations outlined on the inverter datasheet. If the Isc exceeds the limit, the strings or MPPTs will need to be reconfigured by ensuring the string Isc no longer exceeds the limit.
UN-BUS01: Low DC voltage on the internal DC bus bar - check string Isc when the inverter is running at full power during the middle of the day with no clouds or obstructions
UN-BUS02: Busbar voltage detection failure - inverter cannot detect DC voltage - RMA the inverter
Note: if the inverter throwing the alarm is a Solis RHI-1P(5-10)K-HVES-5G series inverter, please be sure that the DSP firmware version is 08 and that the HMI firmware version is 09. If the firmware is older (lower number) then please update the firmware to prevent the UNB-BUS alarms from happening.
To check the firmware version go to