GoodWe ISO Failure — PV insulation resistance too low

What it means: The insulation between the PV DC side and earth has fallen below the safe threshold, so the inverter has refused to connect. GoodWe lists this one without a number — it appears as ISO Failure in the app rather than as a numeric code, and on the ET-series display as Low Insulation Res. The four causes GoodWe names are a PV panel ground cable missing or poorly connected, damaged DC cable insulation, neutral and ground not well connected at the AC terminals, and high humidity at dawn, dusk or in rain. The last of those is the common, benign version. The others are not.

Why does GoodWe show ISO-Failure?

• The ground cable of the PV panels is not connected, or not connected well
• DC cable insulation has been damaged — abrasion, rodents, UV degradation, a crushed conduit run
• Neutral and ground cables not well connected at the AC terminals
• High humidity at dawn, at sunset or on rainy days, which is the version that clears on its own
• A PV string short-circuited to the earthed mounting structure

The Pakistan factor

Two Pakistani seasons produce most of these. Monsoon between July and September soaks rooftop DC runs and panel junction boxes that were never sealed properly, and the autumn-to-winter dew cycle wets a dusty array every night — dust plus dew is a conductive film, which is why the same system throws ISO Failure at dawn in November and runs perfectly by noon. The dangerous version is the one that is not seasonal. Rooftop DC cable run loose across a parapet or through unprotected conduit, rodent damage in a ceiling void, and panel frames on a chhat installation that were never bonded to earth at all are all common here, and all of them produce an ISO Failure that does not care what the weather is doing. If the fault is present on a dry day, the roof is not a place to investigate it yourself.

How to fix it

1. Note the time of day and the weather, then read the pattern. GoodWe is explicit that ISO Failure occurs on rainy days, in the early morning and at sunset when humidity is high, and that version normally clears as the array dries. If your fault appears at 7am in August and is gone by 10am, log it and watch it.
2. If it appears on a dry, sunny afternoon, stop treating it as weather. Leave the DC switch off and call your installer. A genuine insulation fault means a live DC conductor is finding a path to earthed metal, and the array cannot be switched off at the panel — it stays live all day.
3. Do not go on the roof and do not disconnect strings yourself. GoodWe's diagnostic is to connect the PV strings one at a time to find the culprit, then check the DC cable insulation resistance and the earth bond on every panel frame. That is installer work on a live DC array.
4. Ask the installer which threshold applies to your model, because GoodWe publishes different numbers for different series — the EM manual calls for the PV-to-earth resistance to be above 33.3 kilo-ohms, the ET manual for above 50 kilo-ohms to PE, and the troubleshooting guide flags DC cable insulation resistance below 10 kilo-ohms. Use your own manual's figure, not a number from a forum.
5. The installer should also check the AC side. GoodWe's third listed cause is neutral and ground not well connected at the AC terminals, with the same under-10 V neutral-to-earth check that error 25 uses.
6. Do not use the SolarGo ISO reset to make the fault go away. The ET manual only offers that reset for the low-resistance-on-rainy-days case. Using it to silence a real insulation fault leaves a live DC conductor leaking onto an earthed roof structure that people touch.