The short answer
Disconnect, measure +/PE, -/PE and +/-, locate the fault, otherwise do an insulation resistance measurement.
Plain explanation
From the SMA manual for the Sunny Boy: Installer disconnects. Measure +/PE, -/PE, +/-. The rest of the checklist is below; work through it in order. Installer only: DC and AC work on a live PV system is dangerous, so owners should note the code and call their installer.
Steps from the source
- Installer disconnects
- Measure +/PE, -/PE, +/-
- Locate ground fault
- Else insulation resistance measurement
Source
SMA Sunny Boy 3.0-5.0 Operating Manual (SB30-50-1AV-40-BE-en-10), §12.2 p67, §12.3 p80files.sma.de ↗
Where installers ask this
- Insulation Resistance - Check Generator (r/solar)
- reddit.com/r/solar/comments/1ke6i3y/
- reddit.com/r/solar/comments/1ellz9q/
- reddit.com/r/solar/comments/1ii81o5/
General guidance based on the manufacturer's manual or the named source. Always follow the current manual for your exact model and your national code. DC and AC electrical work is for qualified installers only. Not affiliated with or endorsed by any brand named; trademarks belong to their owners.