The photovoltaic module leakage current test, most commonly performed as the wet leakage current test specified in IEC 61730-2 (MST 16) and IEC 61215-2 (MQT 15), verifies the insulation integrity between a module's live electrical circuit and its frame or other accessible conductive surfaces. This article explains the test principle, applicable sample types, test procedures, acceptance criteria, and typical applications in PV manufacturing, certification, and field diagnostics.
Technical Overview
Leakage current in a photovoltaic (PV) module is the unintended current that flows through, or across the surface of, the insulation system between the current-carrying circuit (cells, interconnect ribbons, busbars, and cables) and accessible conductive parts such as the frame, mounting holes, junction box enclosure, and module edges. Under dry conditions, a healthy module exhibits insulation resistance in the gigaohm range. However, when the module surface is wetted by rain, morning dew, snowmelt, condensation, or cleaning operations, both surface and bulk resistivity drop dramatically, and latent insulation defects become electrically visible.
The wet leakage current test therefore evaluates module safety under a worst-case humidity condition. It is a mandatory safety and design qualification test defined in IEC 61730-2 (safety qualification, MST 16, wet insulation resistance) and IEC 61215-2 (design qualification and type approval, MQT 15, wet leakage current test). In North America, the equivalent requirements are covered by UL 61730, which harmonizes the former UL 1703 standard with the IEC framework. In addition, leakage current measurement plays a central role in potential induced degradation (PID) susceptibility testing under IEC TS 62804.
The safety significance is substantial. Modules in a utility-scale string operate at DC system voltages of up to 1500 V relative to grounded frames and mounting structures. Degraded insulation can result in electric shock hazards for installation and maintenance personnel, nuisance ground-fault trips in inverters, accelerated corrosion at cell edges, and PID-related power loss. The leakage current test is consequently one of the most frequently repeated checkpoints within the entire qualification sequence.
Testing Samples
Crystalline Silicon Modules
Framed glass-backsheet and glass-glass modules based on monocrystalline or multicrystalline silicon cells are the most common test articles. The frame, mounting holes, junction box, cables, and connectors are all treated as accessible conductive parts and must be included in the measurement circuit.
Thin-Film Modules
CdTe, CIGS, and amorphous-silicon thin-film products, including flexible and lightweight designs, are tested with particular attention to edge sealing and the integrity of the front insulating layer, since the circuit is often located close to the substrate surface.
Frameless, Bifacial, and Building-Integrated Modules
For modules without a metallic frame, accessible conductive parts are simulated using a wetted conductive mesh or metal foil applied to the mounting interface and exposed surfaces, as permitted by the standards. Bifacial glass-glass modules and building-integrated PV (BIPV) laminates are evaluated in their intended mounting configuration.
Pre- and Post-Aging Specimens
Wet leakage testing is performed both on as-manufactured modules and on modules that have completed accelerated stress tests, typically damp heat (1000 h), humidity-freeze, and thermal cycling, to confirm that insulation performance is retained after long-term environmental aging.
Testing Items
The scope of leakage current related testing typically includes the following items:
- Wet leakage current / wet insulation resistance test (IEC 61730-2 MST 16; IEC 61215-2 MQT 15)
- Dry insulation resistance and dielectric withstand verification of the module insulation system
- Leakage current monitoring during PID stress testing in accordance with IEC TS 62804
- Insulation assessment of accessory parts evaluated together with the module, including the junction box, cables, and connectors
- Comparative insulation resistance measurement before and after environmental stress sequences
Testing Methods
Sample Preparation
The module is inspected visually, and its + and - output terminals, including attached cables, are shorted together. The frame, any exposed conductive hardware, and, for frameless designs, a wetted conductive foil or mesh are connected as the counter electrode. The specimen is then wetted over its entire surface, including the junction box area, while cable ends remain dry and exit the wetting zone so that the measurement is not shunted through the water path.
Wet Leakage Current Test Procedure
The module surface is sprayed with, or immersed in, an aqueous solution containing a wetting agent (surfactant) with a resistivity not greater than 3500 ohm-centimeters, ensuring uniform wetting of hydrophobic glass surfaces. A DC test voltage equal to 500 V or the maximum rated system voltage of the module, whichever is greater, is applied between the shorted terminals and the frame or accessible parts. The voltage is maintained for approximately two minutes, or until the resistance reading stabilizes, and the insulation resistance is then recorded.
Dry Insulation Resistance Test
As a complementary check, a dielectric withstand or insulation test is performed in the dry state at a higher voltage, typically 1000 V plus twice the maximum system voltage. The test verifies that no dielectric breakdown, puncture, or surface tracking occurs across the encapsulant, backsheet, or edge seal.
Acceptance Criteria
The pass criteria for the wet leakage current test are defined by module area:
- For modules with an area greater than 0.1 square meters, the measured insulation resistance shall be not less than 40 megohm multiplied by the module area in square meters divided into the resistance requirement (equivalently, R multiplied by A shall be at least 40 megohm per square meter).
- For modules with an area of 0.1 square meters or less, the insulation resistance shall be not less than 400 megohms.
- No dielectric breakdown, flashover, or visible tracking is permitted during voltage application.
Failure of the criteria indicates compromised insulation, which may originate from backsheet cracking or abrasion, poor edge sealing, cell edge corrosion, encapsulant hydrolysis, moisture ingress into the junction box, or damaged cable insulation.
Instrumentation
Testing is performed with a calibrated insulation resistance tester (megohmmeter) capable of stable output at the required DC voltage, together with a temperature- and humidity-controlled laboratory environment, a water tank or spray system for the wetting solution, and conductive foils or meshes for frameless configurations. Accredited third-party laboratories maintain traceable calibration of all instruments and document ambient conditions for each measurement.
Applications
Leakage current testing serves a wide range of industry needs:
- Certification and market access: IEC 61215 and IEC 61730 based safety and performance certification required by most global markets, supported by UL 61730 listing in North America.
- Manufacturing quality control: periodic sampling of production lots, incoming inspection of encapsulants, backsheets, and junction boxes, and root-cause screening when field returns show insulation anomalies.
- Research and development: comparative evaluation of edge seal designs, encapsulant and backsheet materials, and junction box sealing technologies against moisture-driven leakage.
- PID studies: leakage current trending under voltage, temperature, and humidity stress in accordance with IEC TS 62804 to assess susceptibility and validate mitigation strategies.
- Field diagnostics and asset management: safety audits of operating power plants, investigation of inverter ground-fault alarms, and condition assessment of modules after hail, flooding, fire exposure, or extended service life.
- Extended reliability programs: retesting after prolonged damp heat (for example 2000 hours) or combined multiple-stress sequences to demonstrate long-term insulation durability beyond standard qualification.
FAQ
What is the difference between the insulation test and the wet leakage current test?
The dry insulation test applies a high withstand voltage under normal ambient conditions to verify that the insulation system does not break down. The wet leakage current test applies a lower measurement voltage while the module surfaces are deliberately wetted, and it quantifies the insulation resistance under worst-case moisture exposure, which is far more sensitive to surface and edge-seal defects.
What test voltage and duration are used for the wet leakage test?
A DC voltage of 500 V or the maximum system voltage, whichever is greater, is applied between the shorted module terminals and the accessible conductive parts for about two minutes, and the insulation resistance is recorded once the reading stabilizes.
Why must the water contain a wetting agent?
PV glass and backsheet surfaces are often hydrophobic, and pure water forms droplets that leave dry patches. Adding a surfactant reduces surface tension so the entire module, including the junction box perimeter and edges, is uniformly wetted, ensuring the test reproduces the harshest realistic wetting condition.
How are frameless modules tested?
Since there is no metallic frame to contact, accessible surfaces are simulated with a wetted conductive mesh or metal foil placed against the module in its intended mounting configuration, and the measurement is made between the shorted terminals and this electrode.
What are the typical failure causes?
Common causes include backsheet cracks, pinholes, or inner-layer adhesion loss, edge seal delamination in glass-glass modules, moisture ingress at the junction box or cable gland, cell edge corrosion from PID, and mechanical damage during transport or installation.
Is the wet leakage test repeated during qualification?
Yes. Within the IEC 61215-2 and IEC 61730-2 test sequences, the wet leakage test is repeated after major environmental stresses such as damp heat, humidity-freeze, and thermal cycling to confirm that insulation integrity survives long-term aging.
How does leakage current relate to PID?
In PID mechanisms, sodium and other ions migrate through the encapsulant driven by the voltage difference between the cells and the grounded frame. The magnitude of the leakage current path resistance is a direct indicator of PID susceptibility, which is why IEC TS 62804 procedures monitor leakage current and require power loss to remain within defined limits, typically five percent, after stress.