Wet Leakage Resistance and PV Backsheet Degradation: What the Research Means
Author: Steve Fairless
Originally Published: 30th November 2023 · Updated: 2nd September 2026
Moisture-sensitive backsheet degradation can reduce the electrical insulation of some ageing PV modules and contribute to inverter insulation faults or shutdowns. A peer-reviewed field study by Claudia Buerhop and co-authors investigated this effect in a 5 MWp PV plant and found markedly different leakage-resistance behaviour between backsheet groups.
This page is our technical explanation of that published research; it is not research carried out by Sustainable Energy Engineering. At Sustainable Energy Engineering, we are interested in the study because it helps explain a real fault pattern that can appear on older commercial and domestic PV systems: insulation warnings that become worse in damp or humid conditions.
What the Buerhop Study Found
- The researchers examined a 5 MWp PV power station with 20,530 modules and 314 inverters.
- Near-infrared absorption measurements were used to characterise backsheets on 518 modules associated with 20 inverters.
- The paper described two broad behaviours: “PA-like” and “FC-like” leakage-resistance behaviour.
- FC-like backsheets were reported as more susceptible to water, with a stronger leakage-resistance decline after around 6–8 years in the studied plant.
- The study linked falling ground impedance with increasing humidity and discussed inverter shutdown risk.
- The results do not prove that every module using a particular backsheet chemistry will fail. The study is evidence from a specific plant, population and operating history.
Why We Rebuilt This Page
The earlier version of this article followed the academic paper too closely and could give the impression that the field study was our own work. That is not appropriate. The original research belongs to Buerhop and co-authors and is published in Progress in Photovoltaics: Research and Applications.
Our role here is to explain what the research means for PV owners, asset managers and service engineers, while keeping the limitations visible.
What Is a PV Backsheet?
In a conventional glass-backsheet module, the backsheet is the outer rear layer protecting the encapsulated cells and interconnections from the environment. Depending on the module design, it may contain multiple polymer layers and coatings intended to provide electrical insulation, mechanical protection and environmental resistance.
If the backsheet cracks, embrittles, absorbs water or loses barrier properties, the electrical insulation of the complete module can be affected. That is why backsheet condition matters to both reliability and safety.
The Scale of the Field Study
The detailed on-site backsheet identification covered 518 modules from 20 inverters. The researchers combined those observations with laboratory wet-leakage measurements and historic inverter ground-impedance data.
What “PA-Like” and “FC-Like” Mean in the Paper
The terminology describes observed leakage-resistance behaviour associated with backsheet constructions in the study. The researchers characterised PA-like behaviour as a more moderate decline and FC-like behaviour as more water-susceptible, with higher leakage-resistance loss rates after years in the field.
Do Not Turn a Research Category Into a Universal Brand Verdict
A backsheet label is not enough to predict the future of every module. Manufacturing, formulation, layer thickness, climate, installation, system voltage, mechanical condition and batch-specific bill of materials can all matter. The study should guide diagnosis and risk awareness, not be used to condemn every product with a vaguely similar material description.
Why Humidity Can Make the Fault Look Intermittent
The study reports relationships between ground impedance, temperature and humidity. In practical terms, an ageing module may appear acceptable during dry weather and then contribute to an insulation warning during rain, fog, dew or periods of high absolute humidity.
That pattern can be diagnostically useful, but it is not proof that the backsheet is the cause. Damaged cables, connectors, junction boxes and other DC components can also produce weather-dependent insulation faults.
The 6–8 Year Observation Needs Context
The paper reports a stronger deterioration pattern for the FC-like group after around six to eight years of operation in the studied system. That is a valuable observation because it shows how a failure mechanism can emerge after years of apparently normal operation.
It is not a universal “backsheet lifetime”. We would not tell a customer that a panel will fail in year six or eight simply because a research paper observed that pattern in one population.
What the Ground-Impedance Data Showed
The researchers discussed a critical 400 kΩ ground-impedance level in the context of the inverter system studied. For two inverters associated with PA-based backsheets, the paper reports incidences below that level on 2.8% of days. For the FC-based group, it reports incidences up to 5% of days with an increasing trend and a sharper decline affecting part of that inverter population after six years.
Those numbers belong to the published study and its equipment configuration. They should not be copied into a different inverter or PV system as a universal trip threshold.
How This Connects With Wet Leakage Qualification
IEC 61215-2 provides the current design-qualification test-procedure framework for terrestrial PV modules, while IEC 61730-2 addresses safety qualification. Laboratory wet-condition insulation testing is therefore part of a much wider effort to evaluate how module constructions handle outdoor stresses.
Extended reliability programmes such as Kiwa PVEL’s Product Qualification Program add stress testing beyond minimum certification and track failure modes at bill-of-material level. That matters because a model name alone may cover changing materials over the product’s manufacturing life.
What This Means for an Operating PV Asset
When Weather-Related Insulation Faults Appear
- Record inverter fault codes, dates and weather conditions.
- Review whether the fault is associated with particular strings or MPPT inputs.
- Inspect accessible cables, connectors, junction boxes and modules safely.
- Carry out competent insulation/fault diagnosis rather than repeatedly resetting the inverter.
- Where module degradation is suspected, identify the exact module model, serial/batch information and rear-layer condition.
- Use the evidence to decide whether local module replacement, string remediation or a wider asset strategy is required.
For larger systems, our commercial solar and solar servicing work can include system review and fault investigation where ageing equipment is producing intermittent availability issues.
Why Simply Bypassing Failed Modules Is Not a Long-Term Strategy
The Wiley paper notes that removing low-insulation modules from a circuit can keep an inverter operating in the short term, but it also warns that identical modules may later experience similar faults and that shortening strings can create power and operating-range consequences.
That is a useful asset-management lesson: fixing the symptom is not the same as understanding the population risk.
Backsheet Degradation Is Only One Ageing Mechanism
PV fleets can also experience connector issues, cell cracks, solder/interconnection fatigue, potential-induced degradation, encapsulant changes, junction-box faults, diode failures, corrosion and mechanical damage. A good investigation keeps the fault tree broad until measurements narrow it.
We use the exact technical datasheets and service history where available because construction and electrical limits matter to diagnosis.
Our Interpretation of the Research
The strongest lesson is not that one backsheet type is “good” and another is “bad”. It is that long-term module reliability depends on materials and bills of materials in ways that are not visible from headline module efficiency. Humidity-linked insulation behaviour can also create intermittent faults that are easy to misdiagnose unless the weather pattern and string data are considered together.
For asset owners, that supports better record keeping, monitoring and targeted inspection. For new-system buyers, it supports looking beyond headline watts toward qualification, reliability evidence and manufacturer support.
How the Researchers Combined Laboratory and Field Evidence
The strength of the paper is that it did not rely on one observation alone. The authors used near-infrared absorption measurements to identify backsheet materials on a subset of field modules, laboratory wet-leakage measurements to examine material behaviour, and historic inverter ground-impedance data to see how insulation behaviour evolved in operation.
That combination allowed them to connect a material characteristic with a time- and humidity-dependent field signal. It is more informative than simply photographing cracked backsheets, although it still remains one study of a particular plant and module population.
The Study’s Limits Are Part of the Result
The plant was commissioned in 2012 in eastern Germany and used modules from one tier-one manufacturer across several power classes. The 518 modules characterised by NIRA represented a subset of the full installation. Those details matter because climate, bill of materials, system voltage and product generation can influence ageing.
Good Research Interpretation Includes “Where Does This Apply?”
We can use the study to recognise a plausible degradation mechanism and a diagnostic pattern. We cannot use it to state that every module with a fluorinated coating will fail, that every UK site will follow the same timeline, or that every wet-weather insulation fault is a backsheet fault.
What an Asset Manager Can Do With This Information
For a fleet with repeated insulation trips, the research suggests useful questions: Are faults concentrated on particular module batches or inverter groups? Do they correlate with humidity? Is visible rear-layer degradation present? Does the trend worsen year on year? Are identical modules beginning to show the same behaviour elsewhere on the site?
Those questions help decide whether the issue is an isolated component fault or a population-level ageing risk.
Replacement Strategy Needs to Consider String Design
If one or more modules are removed from an older string, replacement is not necessarily a simple one-for-one exercise. Original models may no longer be available, and new modules can have different current, voltage and dimensions. Any remediation needs to keep the inverter’s operating window and string compatibility valid.
| Remediation route | Potential benefit | Engineering question |
|---|---|---|
| Replace confirmed failed modules | Targets local faults | Are compatible replacements available and is the wider population stable? |
| Reconfigure affected strings | Can restore operation in some cases | Will voltage/current remain within inverter limits? |
| Wider module replacement | Addresses a population issue | Is there evidence that degradation is systemic enough to justify the cost? |
| Inverter/system redesign | Can modernise an ageing asset | Does it address the root insulation problem or only change protection behaviour? |
Monitoring Makes Ageing Mechanisms Easier to See
An isolated trip can be noise; a multi-year trend is evidence. Retaining inverter event logs and maintenance records makes it possible to see whether insulation conditions are gradually worsening and whether interventions actually improve availability.
Need Help Investigating Intermittent PV Insulation Faults?
We can review operating history and carry out system-level fault investigation before recommending replacement work.
