question

Hi Peter, the problem could be ground loops and “disturbance”. I recommend try use the isolated USB-RS485 interface. There is a reference to it in the manual. Helmsjlund, is the brand if I remember correctly.

Matthijs

I have used an IMT Irradiance sensor in an offgrid installation, with a regular Victron RS485 to USB cable + long data extension cable and never experienced any drop out issues.

Can you describe your exact wiring arrangement? (Particularly if you use a data extension cable)

Have you read and followed the recommendations in the 'Installation notes' section of the manual? https://www.victronenergy.com/media/pg/Cerbo_GX/en/installation.html#UUID-d4b7dfe9-4ee0-53bc-8959-c567ed63cebb

FYI the actual IMT Irradiance sensor already has galvanic isolation between the power supply and RS485 modbus circuits.

Its a 10m long extension cable , CAT7 SSTP. Shield is bonded to main house earth only. I am running software on Venus that reads an SMA EM ( connected at connection point ) - Emulates the EM24 meter - so perhaps when grid is disconnected the Venus does not expect to "see" the EM24 however the emulation software is obviously still running on the Venus and provides EM24 like data , the data is obviously all zero . Further I occasionally - I have also had spurious Grid lost notifications even though the grid has been disconnected all the time . There is also a grid restore notification just prior .

Having given me a suggestion that it might be EMC related - i disconnected the screen from house earth and problem is solved. Also works if screen is connected to one of the DC power supply for the IMT sensor. This is not ideal as any induced voltages from near lightning strikes are not protected. Perhaps bond screen to earth at both ends or shorten the unscreened 3m fly leads from the RS485-USB convertor ( as suggested in the notes )