Rain, fog, dew, and melted snow are everyday realities for solar panels in the field. If moisture finds its way into a module's active circuitry, it can cause corrosion, earth faults, and serious safety hazards. The IEC 61215 MQT 15 wet leakage current test is designed to verify that a module's insulation is robust enough to prevent exactly that. Here is a complete breakdown of what the test involves.
What is the MQT 15 Wet Leakage Current Test?
MQT 15, defined in IEC 61215-2:2021, is one of the most safety-critical tests in the IEC 61215 qualification sequence. Its purpose is:
To evaluate the insulation of the module under wet operating conditions and verify that moisture from rain, fog, dew or melted snow does not enter the active parts of the module circuitry, where it might cause corrosion, an earth fault or a safety hazard.
MQT 15 is also used as a final measurement in several other tests in the IEC 61215 sequence — including MQT 11, MQT 12, MQT 13, and MQT 14 — making it a recurring checkpoint throughout the full qualification programme.
Required Apparatus
Four items of equipment are required:
• A shallow trough or tank of sufficient size to enable the module with frame to be placed in the solution in a flat, horizontal position. It shall contain a water/wetting agent solution sufficient to wet all surfaces of the module under test, meeting the following requirements:
o Resistivity: 3 500 Ω·cm or less
o Solution temperature: (22 ± 2) °C
o The depth of the solution shall be sufficient to cover all surfaces except junction box entries.
• Spray equipment containing the same solution.
• A DC voltage source with current limitation, capable of applying 500 V or the maximum rated system voltage of the module, whichever is more.
• An instrument to measure insulation resistance
Test Procedure
All connections shall be representative of the recommended field wiring installation. Precautions must be taken to ensure that leakage currents do not originate from the instrumentation wiring attached to the module.
Step 1 — Immerse the module
Immerse the module in the tank of the required solution to a depth sufficient to cover all surfaces except junction box entries. The cable entries shall be thoroughly sprayed with solution. If the module is provided with a mating connector, the connector should also be sprayed during the test.
Step 2 — Connect the electrical circuit
Connect the shorted output terminals of the module to the positive terminal of the test equipment. Connect the liquid test solution to the negative terminal of the test equipment using a suitable metallic conductor.
Note: Some module technologies may be sensitive to static polarization if the module is maintained at positive voltage to the frame. In this case, the connection of the tester shall be done in the opposite way. If applicable, information with respect to sensitivity to static polarization shall be provided by the manufacturer and documented in the test report.
Step 3 — Apply and hold the voltage
Increase the voltage applied by the test equipment at a rate not exceeding 500 V/s to 500 V or the maximum system voltage for the module, whichever is greater. Maintain the voltage at this level for 2 minutes. Then determine the insulation resistance.
Step 4 — Discharge
Reduce the applied voltage to zero and short-circuit the terminals of the test equipment to discharge the voltage build-up on the module.
Step 5 — Rinse
Ensure that the used solution is well rinsed off the module before continuing with any further testing.
Pass/Fail Criteria
The insulation resistance requirements depend on the module's active area:
• Modules with an area less than 0.1 m²: insulation resistance shall not be less than 400 MΩ.
• Modules with an area larger than 0.1 m²: the measured insulation resistance multiplied by the area of the module shall not be less than 40 MΩ·m².
Why MQT 15 Matters for PV Module Certification
Insulation failure is one of the most serious safety risks in a photovoltaic system. A module with compromised insulation under wet conditions can create a path for current to flow through the frame, the mounting structure, or a person who inadvertently touches the system. MQT 15 is the primary gate that confirms a module's electrical isolation holds up under realistic wet operating conditions.
Its repeated use as a final measurement after environmental stress tests, damp heat, thermal cycling, humidity freeze, and robustness of terminations, makes MQT 15 a cumulative indicator of how well a module maintains its insulation integrity after being subjected to a wide range of stresses. A module that consistently passes MQT 15 throughout the full test sequence provides strong evidence of long-term safety and reliability in the field.
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Source: IEC 61215-2:2021 — Terrestrial photovoltaic (PV) modules – Design qualification and type approval – Part 2: Test procedures, Section 4.15