Photovoltaic Module Wet Leakage Current Test: Standards, Procedures, and Acceptance Criteria

The wet leakage current test is a mandatory safety and reliability evaluation for photovoltaic (PV) modules, verifying that the module insulation system remains sound when the module is wet from rain, condensation, melted snow, or cleaning water. Specified in IEC 61215-2 (test MQT 13) and IEC 61730-2 (test MST 13), the procedure measures the insulation resistance of a fully immersed or sprayed module and compares the result against strict minimum requirements. This article explains the test principle, applicable standards, sample requirements, equipment, step-by-step methodology, acceptance criteria, and typical industrial applications of the wet leakage test for PV modules.

Technical Overview

The wet leakage current test, also known as the wet insulation resistance test, evaluates whether a photovoltaic module maintains adequate electrical insulation between its internal current-carrying circuitry and externally accessible conductive parts—such as the frame, mounting surface, edges, and cable entries—while the module is thoroughly wetted. In field service, PV modules are routinely exposed to rain, morning dew, melted snow, and wash water. Moisture entering through defective backsheets, edge seals, or junction boxes can create conductive leakage paths that lead to ground faults, electric shock hazards, inverter protective shutdown, corrosion of interconnects, and accelerated degradation mechanisms such as potential-induced degradation (PID).

The test principle is straightforward: the module is immersed in, or continuously sprayed with, water of controlled resistivity, and a DC voltage equal to the module's rated maximum system voltage (with a minimum of 500 V DC) is applied between the short-circuited output terminals and the water itself. The resulting insulation resistance is measured; a low value indicates that moisture has established an unacceptable leakage path into or through the module insulation system.

Governing Standards

• IEC 61215-2:2021, Terrestrial photovoltaic modules — Design qualification and type approval — Part 2: Test procedures, test MQT 13 (wet leakage)
• IEC 61730-2:2021, Photovoltaic module safety qualification — Part 2: Requirements for testing, test MST 13 (wet leakage current)
• EN IEC 61215-2 and EN IEC 61730-2, European adoptions of the IEC test sequences
• UL 61730, the North American adoption harmonized with IEC 61730 for module safety certification
• GB/T 9535 and GB/T 18911, Chinese national standards adopting IEC 61215-type qualification sequences for crystalline silicon and thin-film modules respectively

Because modern utility-scale plants operate at system voltages up to 1500 V DC and modules remain accessible to installers, maintenance personnel, and emergency responders, the wet leakage test is treated as a critical pass/fail safety screen in nearly every certification and qualification program worldwide.

Testing Samples

Applicable Module Types

• Crystalline silicon modules, including monocrystalline, multicrystalline, PERC, TOPCon, and heterojunction architectures
• Thin-film modules based on CdTe, CIGS, or amorphous/microcrystalline silicon
• Glass–backsheet and glass–glass (bifacial) laminate constructions
• Framed, frameless, and flexible modules, as well as building-integrated photovoltaic (BIPV) laminates tested with mounting arrangements representative of installation

Sample Condition and Preparation

Samples must be production-representative and complete with all parts delivered in service, including the frame, junction box, cables, and connectors. The test may be performed on new modules or, more commonly, on modules that have undergone preceding climatic or mechanical stress tests such as damp heat (85 °C / 85 % RH for 1000 h), humidity-freeze, thermal cycling, UV preconditioning, or mechanical load testing. Repeating the wet leakage measurement after these stresses allows the insulation system to be tracked throughout the qualification sequence. After any climatic conditioning, the module should be stabilized at room ambient conditions for at least 4 hours before measurement.

Sample Quantity and Special Cases

For a standalone determination, one to two modules are typically sufficient; full design qualification per the IEC 61215 series uses a defined sample set in which individual modules are reused across sequential tests. Frameless modules are fully immersed with accessible conductive parts bridged to the water as defined by the standard, while very large-format modules may be evaluated by the spray method when tank immersion is impractical. Modules with factory-attached power electronics are tested according to the manufacturer's installation documentation or with the electronics evaluated separately.

Testing Items

A complete wet leakage evaluation comprises the following measured and verified items:

• Insulation resistance of the fully wetted module at the rated maximum system voltage (the pass/fail decision item)
• Derived wet leakage current, calculated from the applied voltage and the measured resistance
• Verification of the water solution resistivity: not greater than 3500 Ω·cm (approximately 286 µS/cm conductivity)
• Verification of water and module temperature: 22 °C ± 3 °C
• Post-test visual inspection for water ingress into the junction box, connectors, and laminate edges

The decisive acceptance requirements are an insulation resistance of not less than 40 MΩ for modules with an area greater than 0.1 m², and not less than 400 MΩ for modules with an area of 0.1 m² or smaller, with no dielectric breakdown or flashover during voltage application.

Testing Methods

Apparatus and Materials

• An insulation resistance tester (megohmmeter) providing stable DC output up to the required test voltage, with a 1500 V class instrument used for modern high-system-voltage modules
• A non-conductive immersion tank sized for complete module immersion, or a spray rack for the alternative spray method
• A conductive immersion electrode, such as a stainless steel plate, connected to the return terminal of the tester
• A water resistivity/conductivity meter and a calibrated thermometer
• A water supply with resistivity not exceeding 3500 Ω·cm; a non-ionic wetting agent may be added in accordance with standard practice to ensure complete wetting without trapped air bubbles
• Shorting leads or a busbar for the module output terminals, plus appropriate personal protective equipment for high-voltage work near water

Test Conditions

• Test voltage: DC equal to the module's marked maximum system voltage, and never less than 500 V DC
• Voltage duration: maintained for 2 minutes, with the resistance value recorded at the end of the period
• Wetting: complete immersion of the module, including the junction box where it is exposed in normal installation, for at least 2 minutes before and continuously during measurement; mated connectors may remain above the waterline
• Temperature: water solution and module maintained at 22 °C ± 3 °C throughout the test

Step-by-Step Procedure

• Verify the module identity, marked maximum system voltage, and initial visual condition; record module area for the applicable pass criterion
• Prepare the water solution and confirm its resistivity does not exceed 3500 Ω·cm and its temperature is 22 °C ± 3 °C
• Short-circuit the module positive and negative output leads together
• Fully immerse the module in the tank, ensuring no air bubbles are trapped under the laminate or within the frame cavity, and allow at least 2 minutes of soaking
• Connect the megohmmeter output terminal to the shorted module leads and the return terminal to the immersion electrode in contact with the water
• Apply the DC test voltage, hold it for 2 minutes, and record the insulation resistance (or the corresponding leakage current)
• Discharge and de-energize the circuit, remove the module from the tank, and allow it to dry before a post-test visual inspection
• Compare the recorded resistance with the applicable criterion and document all test conditions, including voltage, temperature, solution resistivity, and the method of wetting

Safety Precautions

• Only personnel trained in high-voltage testing should operate the equipment, and the test area must be cordoned off with clear warning signage
• Use an isolated, current-limited test instrument and verify full discharge of the module before any handling
• Maintain dry, insulated access to the tank controls and use insulated tools when connecting leads
• Keep all personnel clear of the tank and module during voltage application, and follow the laboratory electrical safety management system at all times

Applications

Design Qualification and Type Approval

Within the IEC 61215 series, the wet leakage test is performed on new modules and repeated after major climatic stresses, most notably after 1000 hours of damp heat at 85 °C and 85 % relative humidity, after humidity-freeze cycling, and frequently after thermal cycling and mechanical load tests. This repetition demonstrates that the insulation system survives long-term environmental exposure rather than merely passing in a pristine state.

Safety Certification and Market Access

As test MST 13 of IEC 61730-2, the wet leakage determination is a core element of PV module safety certification, whether under the IEC, EN, or UL 61730 frameworks. Accredited third-party laboratories conduct the test as part of the conformity assessment evidence required for product certification and market entry in most regions of the world.

Manufacturing Quality Control

Manufacturers apply the wet leakage test as a periodic production check and as a validation tool whenever materials change—new backsheets, encapsulants, edge-seal chemistries, junction box potting compounds, or connector systems—or whenever lamination, framing, and potting process parameters are modified. It is a sensitive detector of process drift such as insufficient lamination or incomplete box filling.

Reliability and Degradation Research

In research and development laboratories, the test supports extended damp-heat studies beyond 2000 hours, PID susceptibility assessments conducted in conjunction with IEC TS 62804, comparative backsheet durability programs, and climate-specific product design for tropical, coastal, or high-humidity environments, where leakage paths develop fastest.

Field Diagnostics and Failure Analysis

Returned field modules, ground-fault investigations, warranty and insurance assessments, and safety evaluations of aging fleets frequently include a wet leakage measurement, because it reproduces the leakage conditions that trigger inverter residual-current trips during or immediately after rainfall.

FAQ

What is the wet leakage current test for PV modules?

It is a safety and reliability test that measures the insulation resistance of a photovoltaic module while the module is completely wetted by immersion or spraying. It simulates service exposure to rain, dew, and melted snow and confirms that no dangerous leakage path exists between the live internal circuit and accessible external surfaces.

Which standards specify the test?

The test is specified as MQT 13 in IEC 61215-2 and as MST 13 in IEC 61730-2, with equivalent requirements in the corresponding EN, UL 61730, and GB/T national adoptions. Accredited third-party laboratories perform it as part of design qualification and safety certification programs.

What are the pass criteria?

The measured insulation resistance must be at least 40 MΩ for modules with an area larger than 0.1 m², and at least 400 MΩ for modules with an area of 0.1 m² or smaller. No dielectric breakdown or flashover may occur during voltage application.

What test voltage is applied and for how long?

A DC voltage equal to the module's marked maximum system voltage is applied, with an absolute minimum of 500 V DC. The voltage is maintained for 2 minutes, and the resistance is recorded at the end of that period.

Why must the water resistivity not exceed 3500 Ω·cm?

The water resistivity defines the severity of the test. Water that is too pure is a poor conductor and would understate leakage, producing artificially high resistance values, while contaminated water would overstate leakage and fail compliant modules. Fixing the resistivity at a maximum of 3500 Ω·cm ensures consistent, reproducible severity and comparability between laboratories.

How does the wet leakage test differ from the dry insulation test?

The dry insulation test (MQT 03 / MST 16) is performed under ambient conditions at a higher applied voltage, typically 1000 V plus twice the maximum system voltage, and uses an area-normalized resistance requirement. The wet leakage test applies the system voltage to a submerged module and is specifically designed to reveal moisture-driven weaknesses, such as backsheet porosity or seal degradation, that are invisible in a dry measurement.

What are the most common causes of wet leakage failures?

Typical root causes include backsheet pinholes, scratches, and poor interlayer adhesion; incomplete junction box potting; leaking cable glands or connector seals; gaps between the frame and laminate edge, particularly at corners; degradation of edge seals after prolonged damp-heat exposure; and handling damage incurred during transport or installation.

Is the test destructive, and can it be performed in the field?

The test is not destructive: a module that passes can be dried and continue through the qualification sequence. Portable megohmmeters and spray rigs allow indicative on-site measurements, but certification results require the controlled solution resistivity, temperature, and wetting conditions that only a properly equipped laboratory can provide.