AIO insulation resistance between DC wiring and earth is below minimum. This is the most common AIO fault on UK sites.
Common causes
Moisture in MC4 connectors after rain (most common)Damaged PV cable insulationPanel junction box seal failureBattery DC cable insulation fault
What to check
STEP 1
Disconnect all PV strings and battery. Test IR of each independently at 500V DC. Any reading below 1MΩ is suspect. The most common cause on UK sites is wet MC4 connectors — dry and retry
STEP 2
If PV strings test OK individually but fault returns when reconnected, inspect the AIO DC combiner/input for moisture. Check battery DC cable condition
Escalate if
No string identified as fault source and fault persists in dry conditions — AIO internal ISO detection circuit fault, escalate
Need full interactive diagnosis?
Faultpath walks you through decision trees, fault codes, and commissioning — built for solar & battery engineers.