How moisture affects PV isolation resistance testing and why it matters

Insulation faults in solar PV systems increase significantly during cold and humid periods. This makes fault finding more challenging precisely when assets should be prepared for the high-production season. Understanding how moisture influences test results is essential for anyone responsible for maintaining PV systems.

The basics: what isolation resistance testing measures

Insulation testing measures the electrical resistance between the active conductors of a PV string and ground. High resistance indicates healthy insulation. Low resistance indicates leakage paths, damaged insulation, or ground faults.

For PV systems up to 1000V DC, a common threshold is 1 MOhm. Values below this are generally considered indicative of an insulation fault. For modern 1500V systems, stricter thresholds apply.

Key principle: A single low reading does not always confirm a permanent fault. Environmental conditions, especially moisture, can temporarily reduce insulation resistance to levels that would normally trigger an alarm.

PV system grounding and insulation reference diagram
Grounding and insulation reference configuration in a PV string. Under normal operation, DC conductors are electrically isolated from ground and operate as a floating system. Moisture, contamination, or insulation damage can introduce leakage currents and reduce measured insulation resistance.

How moisture affects test results

Moisture from rain, fog, condensation, or high humidity can affect insulation measurements in several ways. The challenge is always distinguishing between a genuine defect and a temporary weather-related effect.

Surface moisture and water ingress. Moisture can condense on module surfaces, connectors, or cable insulation, creating temporary conductive paths. In damaged or aged cables, water can bridge conductors to ground, increasing leakage currents and lowering the measured value.

Morning dew and fog. Early morning dew or fog often causes lower insulation values, especially during startup tests. As the system dries during the day, resistance typically recovers. This is why an inverter may report a fault at startup that a manual test later in the day does not reproduce.

Creeping currents. High humidity combined with surface contamination can cause creeping currents. These surface leakage paths distort readings without indicating a true internal insulation failure.

Fluctuating readings. Measurements taken immediately after rainfall often show reduced resistance. Repeating the test under dry conditions may show acceptable values. This temporal variation is itself a useful diagnostic indicator.

Practical recommendations for technicians

  • Interpret results in context. Dry-weather testing is best for assessing overall system condition. Humid-condition testing is better for investigating suspected faults that appear intermittently.
  • Always repeat low readings. A single low insulation reading should be verified by repeating the test after a dry period. This determines whether the issue is weather-related or a permanent fault.
  • Understand inverter monitoring timing. Many transformerless inverters perform insulation checks at startup, when morning dew is present. Manual tests later in the day may miss the same fault. Neither is wrong — they are measuring under different conditions.
  • Perform visual inspections. Moisture-related problems often worsen alongside visible damage: cracked modules, damaged cable insulation, or poorly sealed connectors. Visual inspection remains essential.
  • Vary the test voltage. Testing at different voltages under humid conditions can help distinguish surface leakage from core insulation failure. Always use equipment designed specifically for PV systems.
  • Document environmental conditions. Recording humidity, temperature, and dew point at the time of testing is essential for meaningful comparison over time and across sites.

Traditional testing vs. field diagnostics

Understanding the difference between the two main diagnostic approaches helps explain why results can vary between instruments and methods.

Traditional insulation resistance testing
  • Applies fixed high DC voltage between conductors and earth
  • Widely used for commissioning and compliance
  • Surface leakage under humid conditions can skew results
  • Indicates whether insulation is acceptable or degraded
  • Limited fault localization capability
  • Requires isolation and discharge before testing
Advanced field diagnostics
  • Uses dynamic measurement methods tailored to PV systems
  • Less dominated by surface conductivity effects
  • More stable diagnostic results in moist environments
  • Can estimate fault position along the string
  • Reduces need for repeated disconnection
  • Complements traditional testing in real-world fault finding

The two approaches are complementary. Traditional insulation resistance testing remains essential for formal verification and standards compliance. Advanced field diagnostics add value during real-world fault finding, particularly under humid conditions where conventional measurements become ambiguous.

Industry perspective

Guidance from Megger’s PV insulation testing manuals supports these observations. High humidity and surface moisture can significantly reduce apparent insulation resistance. Guard terminals are recommended to reduce the influence of surface leakage. Documenting environmental conditions such as humidity and dew point is highlighted as essential for meaningful interpretation of results.

emazys Z300 PVT

Designed for real-world PV diagnostics

The Z300 PVT is designed specifically for PV fault finding in the field — including under challenging conditions such as high humidity. It captures Riso, Voc, Isc, polarity and Rs in a single 20-second test and syncs all data automatically to the emazys cloud.

See the Z300 PVT

Conclusion

Moisture is one of the most common sources of confusion in PV insulation diagnostics. A low reading is not always a fault — but a fault is not always visible in dry conditions either. The key is knowing when and how to test, and how to interpret results in context.

Repeated testing, awareness of environmental conditions, and the right combination of diagnostic tools are what separate reliable maintenance from reactive fire-fighting. Understanding the influence of moisture is a foundational skill for any technician responsible for the long-term performance of PV systems.

References

[1] Megger Instruments Ltd., “Application Note — Ground Fault Location in PV Systems,” Megger, Valley Forge, PA, USA, 2022.

[2] Megger Instruments Ltd., “Application Note — Megger Tools in PV Applications,” Megger, Valley Forge, PA, USA, 2021.