Insulation resistance between PV/battery DC circuit and earth has fallen below protection threshold. Safety-critical fault. Most common on rainy days. Displayed fault ID: 039.
Common causes
ISO resistance threshold set too highShort circuit or damaged insulation on PV string or DC cablesMoisture ingress into connectors or cable runsDamaged battery cables or loose battery terminals
What to check
STEP 1
If raining or recently rained, wait for dry conditions and re-check — moisture commonly causes this. Check ISO resistance protection threshold in App/LCD — ensure it complies with local regulations.
STEP 2
With DC isolated: measure insulation resistance of PV and DC cables to ground. Inspect battery cables for damage. Check all battery terminals are tight and making good contact.
Escalate if
Dry conditions, correct threshold, cables and terminals confirmed good but fault persists — contact Sungrow support
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