Solar Ground Faults, Low Isolation Faults and RCD Tripping During Heavy Rain
Author: Steve Fairless
Updated: 2nd September 2026
If a solar inverter repeatedly reports a ground fault or low-insulation fault during heavy rain, or the PV circuit trips an RCD/RCBO only in wet conditions, we treat that as an electrical fault to diagnose — not as normal rainy-weather behaviour. Moisture can expose weaknesses in module insulation, backsheets, connectors, junction boxes or DC cabling, and modern inverters may refuse to connect when measured insulation resistance is below their permitted threshold.
At Sustainable Energy Engineering, we have seen weather-dependent faults on older PV systems where the inverter behaves normally in dry conditions and then trips or delays start-up after prolonged rain or high humidity. That first-hand pattern is useful evidence, but it does not tell us which component has failed. We still isolate and test the system rather than assuming the panels are the cause.
Updated: 2nd September 2026.
Electrical Safety Comes First
PV arrays can remain at hazardous DC voltage whenever there is daylight, even if the inverter is switched off or the AC supply has tripped. A recurring isolation or ground-fault message should be investigated by a competent solar/electrical professional. We do not recommend opening connectors, testing live strings or repeatedly resetting protective devices as a DIY diagnostic method.
Why Can Heavy Rain Trigger a Solar Ground or Low-Iso Fault?
- Moisture can reduce insulation resistance. Existing cracks, degraded backsheets, damaged cable insulation or poor connectors may become electrically significant when wet.
- The inverter may detect the problem before generation starts. Many transformerless inverters perform an insulation check before connecting to the grid.
- The fault can be intermittent. A system may pass tests after it dries and fail again in the next period of rain.
- The panels are not the only possible cause. DC cables, connectors, junction boxes, isolators and inverter-side faults must also be considered.
- An RCD trip can have more than one contributor. Existing leakage from other circuits can combine with PV leakage depending on the installation and protective arrangement.
What We Have Seen in Real Systems
Over years of servicing PV systems, we have had customers report inverters that only show "Ground Fault", "Low Iso" or similar insulation messages after sustained rain. In some cases the system starts again when the roof and modules dry. In others the wet-weather symptom becomes more frequent over time until repair is required.
That behaviour is consistent with published inverter guidance. Fronius, for example, states that its low-insulation-resistance errors can be caused by problems with PV modules or cabling and notes that the error may occur only under certain conditions such as rain. SMA also publishes technical guidance explaining how modules, DC cabling and inverter components all contribute to a PV system’s insulation resistance relative to ground.
For customers using our solar servicing and repair service, the weather pattern is one clue in the fault history, not the diagnosis itself.
What Does Insulation Resistance Mean in a PV System?
The DC side of a solar array must be adequately insulated from earth and conductive parts. SMA's technical guidance describes the effective insulation resistance, often labelled Riso, as the combined result of modules, DC cabling and inverter components relative to ground.
As a PV system becomes larger, the parallel insulation paths mean total measured resistance can be lower than the resistance of an individual module. Inverter manufacturers therefore use system-appropriate thresholds and test methods rather than one universal resistance number for every array.
Where an Isolation Fault Can Originate
| Area | Examples we investigate |
|---|---|
| PV modules | Backsheet degradation, edge-seal failure, cracked glass, junction-box or lead damage. |
| DC cabling | Abrasion, UV ageing, trapped cable, rodent damage, water ingress or damaged insulation. |
| Connectors | Incorrect mating, poor assembly, damaged seals, loose terminations or water ingress. |
| DC isolators / enclosures | Moisture, failed seals, damaged glands or internal contamination. |
| Inverter | Internal measurement or insulation faults, depending on the error and manufacturer diagnostics. |
Why Ageing Backsheets Matter
A peer-reviewed study published in Progress in Photovoltaics investigated wet leakage resistance and backsheet types at a 5 MWp PV power station containing 20,530 modules and 314 inverters. The researchers found different moisture-related leakage-resistance behaviour between backsheet constructions and linked falling ground impedance with increased inverter shutdown risk.
The study is useful because it supports a mechanism we encounter in practice: water ingress can make an insulation weakness more visible to the inverter. It is not evidence that every older domestic module will fail after a fixed number of years. Product construction, exposure and condition vary significantly.
Rain Does Not Cause a Healthy Solar System to Become Unsafe
PV modules are outdoor electrical products and are designed to operate through normal rain. If a system only trips when wet, the rain is often revealing a weakness that is already present rather than creating a fault in an otherwise perfect system.
That distinction matters. Waiting for the weather to dry may allow the inverter to start again, but it does not repair cracked insulation, a degraded backsheet or a leaking connector. If the fault recurs, we want to locate it.
Why an RCD or RCBO Can Trip
RCDs and RCBOs respond to residual current imbalance. A PV system can contribute leakage current through its filters, capacitance and any actual insulation defect, while other circuits on the same protective device may contribute their own leakage.
We do not assume that moving the PV to a different protective device is a cure. Circuit design and RCD selection must follow the inverter manufacturer's requirements and the electrical installation standards. If there is a genuine DC insulation fault, changing protective arrangements without fixing the fault can mask the symptom rather than solve the problem.
Do Not Treat a Repeated RCD Trip as a Nuisance
Protective devices are there for a reason. Our first question is why the current imbalance exists and whether the PV system, another circuit or the protective arrangement is responsible.
How We Diagnose a Wet-Weather Isolation Fault
Intermittent faults are easiest to diagnose when we have the exact inverter message, date, time and weather conditions. Monitoring screenshots can be extremely useful. We then inspect and electrically test the system using a safe isolation procedure.
Typical Diagnostic Sequence
- Read and record inverter fault history and measured isolation values where available.
- Inspect modules, leads, connectors, cable management, isolators and visible enclosures.
- Confirm the AC protective arrangement and whether other circuits are contributing to RCD trips.
- Safely isolate the PV system and test insulation resistance using appropriate equipment and manufacturer procedures.
- Split the array into strings or sections where needed to narrow the fault location.
- Repair or replace the defective component and recommission the system.
- Review monitoring after the next wet period if the fault was strongly weather-dependent.
Why Testing After the Roof Has Dried Can Miss the Fault
A cable or module that measures acceptably in dry conditions can show much lower insulation resistance when wet. This is one reason a fault that disappears before the engineer arrives can be difficult to prove.
We use the fault history and repeatability to decide whether further sectional testing is justified. Manufacturer isolation-fault procedures can also help identify whether the issue sits on a particular string or component group.
When Module Replacement Becomes the Right Repair
If testing identifies one or more modules with compromised insulation, replacement is normally more appropriate than trying to seal or patch a safety-critical electrical laminate. On an older array, module matching and string voltage/current limits then need checking because an identical replacement may no longer be available.
Where failures are widespread across one module type or backsheet batch, a larger replacement strategy may be more sensible than repeated single-panel callouts. We decide that from evidence on the actual system, not from age alone.
Monitoring Helps Us Catch the Pattern
If an inverter shows low isolation only at dawn, after rain or during particular humidity conditions, monitoring can reveal the pattern. We encourage owners not to clear fault logs or repeatedly reboot equipment without recording the message first.
Our solar datasheet library also helps identify the correct manufacturer error definitions and operating limits for equipment we service.
What You Should Do if the Fault Appears Tonight
Do not access the roof or open DC equipment. Record the inverter message and take a screenshot if it is safe to do so. If an RCD or RCBO will not remain set, or there is any sign of burning, water inside electrical equipment or another immediate hazard, leave the affected system isolated and arrange professional assistance.
If the inverter restarts after drying, that information is useful but a recurring wet-weather fault should still be investigated.
Our Recommendation: Find the Fault, Do Not Normalise It
Rainy weather should reduce solar generation because irradiance is lower; it should not routinely produce ground faults. When it does, the best outcome is a documented diagnosis that identifies the defective module, cable, connector, enclosure or other component and restores the insulation integrity of the PV system.
Seeing Ground-Fault or Low-Isolation Errors After Rain?
Send us the inverter make, exact fault message, system age and any monitoring screenshots. We can plan a safe diagnostic visit and test the PV system rather than guessing from the weather.
