Fault: Insulation fault in PV systems

The message often appears in the morning, after rain, or in high humidity: the inverter starts late, does not connect to the grid, or reports too low an insulation resistance. As soon as the system dries out, the fault seems to have disappeared. This is precisely what makes insulation faults so insidious: the symptom may be temporary, but the cause persists.

What is an insulation fault in a PV system?

In a photovoltaic system, current-carrying parts must be reliably separated from earth, the substructure, housings, and other touchable conductive parts. If this separation is compromised by moisture, aging, mechanical damage, or a component defect, the insulation resistance decreases. This can lead to an undesirable leakage or fault current to earth.

The inverter monitors the insulation of the connected PV generator. If the determined value falls below the permissible limit for the specific device and system, the inverter usually prevents grid connection or interrupts operation. The error message thus initially identifies the electrical symptom, but not automatically the location of the fault.

How can an insulation fault be identified?

Typical indicators are:

• The inverter reports, for example, "insulation fault", "Riso too low", "earth fault", or a manufacturer-specific error code.
• The system starts later on damp days than comparable system parts or remains completely out of operation.
• The fault occurs after rain, fog, dew, or snowmelt and temporarily disappears after drying.
• A single inverter or MPP tracker shows recurring abnormalities, while other system parts operate normally.
• Protective devices trip or monitoring shows a falling or strongly fluctuating insulation value.

For operators, it is particularly important to document the time, weather conditions, error code, and affected inverter. This information helps the specialist company to specifically reproduce a sporadic fault and prepare for the service call. A dry, trouble-free period is not proof that the insulation is intact again.

Where do insulation faults occur in PV systems?

The causes can be usefully divided into three areas: solar modules, the rest of the PV generator on the DC side, and the inverter. The following overview serves as a guide; clear assignment requires measurements by a qualified electrician.

1. Faults in the solar modules

For modules, damaged backsheets and glass surfaces are among the typical causes. Cracks or delaminations can open the way for moisture to current-carrying parts. Leaky junction boxes, damaged connection cables, or aged seals are also possible. Some damage is visible to the naked eye, others lie in inner foil layers and only become apparent upon close inspection or measurement.

Glass damage is not just a mechanical problem. If water penetrates the module structure, the insulation resistance can gradually drop. The fault pattern may initially only occur when wet and later develop into a permanent fault. An affected module should not be replaced based on a mere assumption; it must be clearly assigned electrically and spatially.

2. Faults in the generator and DC cabling

Outside the modules, faults often occur in DC cables and connectors. Cables can chafe on sharp edges, be pinched under modules, become brittle due to UV radiation, or be laid in permanently wet areas. For buried cables, damage can be particularly difficult to find. Animal bites, unsuitable fastening, and mechanical stress from wind can also be triggers.

For connectors, incompletely tightened screw connections, assembly errors, contaminated sealing areas, or unapproved combinations of different brands lead to leaks and contact problems. Penetrating moisture can promote corrosion, a drop in insulation resistance, and in unfavorable cases, thermal damage or arcing. Generator junction boxes, fuse holders, and DC surge protection devices should also be checked.

3. Faults in the inverter

If modules and DC cabling show no abnormalities during a professional inspection, the cause may lie in the inverter. Possible causes include damaged components at the DC input, internal surge protection components, moisture, contamination, or an electronic defect. A tripped or damaged varistor can influence the insulation value.

The inverter may only be opened and inspected according to manufacturer specifications and by appropriately qualified personnel. Before replacing a device, the DC side should be clearly delimited. Otherwise, there is a risk of replacing a functioning device while the actual fault in the generator persists.

What danger does an insulation fault pose?

An insulation fault is more than an yield problem. If the resistance to earth decreases, unwanted fault currents can flow through the substructure, housing, or other conductive parts. Depending on the fault location, system concept, and protective measures, this can lead to touch hazards, malfunctions of protective devices, arcing, or thermal damage. At the same time, start delays and shutdowns lead to yield losses.

How is an insulation fault systematically found?

Efficient troubleshooting proceeds from the overall system to the affected section. Prerequisites are reliable system documentation, an up-to-date string plan, and measuring devices suitable for the existing DC voltage and the intended test procedure. Manufacturer specifications, system configuration, and relevant test rules, including those from the IEC or DIN EN 62446 series of standards, are decisive.

1. Evaluate monitoring and events:

   Compare error code, timestamp, weather, insulation value, affected MPP tracker, and start-up behavior. Recurring patterns narrow down the fault.

2. Check system and documentation:

   Compare string plan, cable routes, connector types, previous repairs, and manufacturer instructions. Missing or incorrect plans significantly prolong the search.

3. Perform visual inspection:

   Examine modules, backsheets, glass, junction boxes, cables, connectors, fastenings, generator junction boxes, and inverters for cracks, chafing, moisture, corrosion, and thermal traces.

4. Separate fault area:

   Separate the DC strings from the inverter using a safe procedure and check whether the abnormality can be attributed to the generator, a cable section, or the inverter.

5. Isolate affected string:

   Measure the insulation values of the strings under comparable conditions and gradually subdivide conspicuous strings. Perform the measurement as far as possible under the conditions in which the fault occurs.

6. Locate fault:

   Depending on the system structure, division measurements, suitable fault location procedures, or a voltage balance may be considered. For optimizers, module inverters, and special topologies, manufacturer specifications are particularly important.

7. Repair and recheck:

   Professionally replace defective components, assemble plug systems in accordance with manufacturer specifications, and then update insulation measurement, functional test, and documentation.

What can operators do specifically?

Operators can significantly speed up diagnosis without interfering with the electrical system themselves:

• Photograph the error message and error code or export them from the portal.
• Note date, time, and weather conditions; especially record dew, rain, fog, or snowmelt.
• Check whether only one inverter, one tracker, or the entire system is affected.
• Compare yield and start times with comparable days or other inverters.
• Keep system documentation, string plan, module and inverter data, and previous service reports ready.
• In case of recurring messages, promptly commission a qualified specialist company, even if the system starts again later.

How can insulation faults be avoided and remedied?

Not every insulation fault is avoidable: materials age, components can fail, and extreme weather leaves its marks. However, planning, installation, and maintenance significantly influence the risk.

During planning and installation

• Only use mutually approved connectors and the intended mounting tools.
• Lay DC cables strain-relieved, UV-resistant, without sharp edges, and with distance from permanently wet surfaces.
• Protect cables from chafing, pinching, animal bites, and movements due to wind.
• Install and check seals, cable glands, and housings according to manufacturer specifications.
• Completely document the string plan, cable routes, plug systems, and commissioning measurement values.

During ongoing operation

• Regularly evaluate monitoring and event logs, instead of just looking at the annual yield.
• Take recurring start delays in humidity seriously as an early warning sign.
• Have visual and repeat inspections carried out risk-oriented and according to the maintenance concept.
• Arrange an unscheduled inspection after storms, roof work, overvoltage events, or animal infestation.

During repair

The permanent solution is to eliminate the clearly identified cause. Provisional sealing of an already moistened module or merely resetting the error message does not reliably restore the insulation. Defective modules, cables, connectors, surge protection devices, or inverter components must be replaced or repaired according to findings and manufacturer specifications. Subsequently, the measured values must be documented and safe system operation must be demonstrated.

Conclusion: First narrow down, then repair specifically

Insulation faults often manifest as weather-dependent, but their causes range from individual modules to cables and connectors to the inverter. For operators: document error messages, do not work on the DC side yourself, and have recurring faults checked early. For installers: monitoring, documentation, visual inspection, and systematic metrological isolation save time and avoid unnecessary component replacements.

The earlier falling insulation values and moisture-dependent start-up problems are detected, the sooner a sporadic fault can be turned into a planned repair. This protects people, the system, and the yield.