EN IEC 62788-1-2 Measurement Procedures for Materials Used in Photovoltaic Modules - Part 1-2: Capsules - Measurement of Volume Resistivity of Photovoltaic Capsules and Other Polymeric Materials

The standard "EN IEC 62788-1-2 Measurement procedures for materials used in photovoltaic modules - Part 1-2: Encapsulations - Measurement of volume resistivity of photovoltaic encapsulations and other polymeric materials", which is based on a European standard and subsequently published by the International Electrotechnical Commission (IEC), describes a method and provides guidance for measuring the volume resistivity of materials used as encapsulations, edge seals, front sheets, back sheets or other insulating materials in a photovoltaic (PV) module. The test is carried out on dry, damp or wet preconditioned samples. In the case of front sheets and back sheets consisting of more than one layer, the measured resistivity is an effective value. This test is designed for measurement at room temperature, but can also be used at higher temperatures.

It is known that degradation of PV modules is caused in part by electrochemical corrosion and other potential-induced degradation processes. These processes can depend on the resistivity of a polymeric component. Therefore, the DC resistivity of polymeric components is relevant to module design and durability in the field. The resistivity can depend on the cure state, temperature, water content and voltage history. A number of options are included to allow the measurement to be performed consistently with representative field module conditions.

Most resistivity measurement methods and equipment become inaccurate and variable for materials with a bulk resistivity above 1016¹⁶ Ω∙cm. Therefore, this standard is used for measurements lower than 1 ⋅ 1017¹⁷ Ω∙cm.

Both monolithic and multilayer materials (e.g. front sheets and back sheets) are suitable for measurement. Methods are described for room temperature measurement with the addition of guidelines for testing at elevated temperatures.

Results vary with moisture content, so materials should be tested prior to use. Preconditioning procedures for dry, damp and wet environments have been included.

Depending on the material, the voltage history affects the measured result. The rate of change of current and the time it takes to reach equilibrium vary as it usually takes hours or days for the material to reach a static level. Therefore, long- and short-time methods have been added (Methods A and B).

The specified short-term alternating polarity method B is designed for qualitative comparison. Method A, long-term on/off polarity, is recommended for characterization in terms of PID resistance.

Measurements obtained using both methods can be used by material manufacturers for quality control of electrical insulation materials and for reporting purposes in product data sheets. PV module manufacturers can use these methods for material acceptance, material selection, process development, design analysis or failure analysis.

This measurement method can also be used to monitor the performance of electrical insulating materials after exposure to weathering and to evaluate their durability.

In preparing the sample, thin polymer films or laminates (up to 2 mm thick) are used as test specimens. A thickness of 0,5 mm is currently typical in PV encapsulation and edge seal applications and is the recommended thickness for these materials in this standard. The thickness should not vary more than 10 percent across the sample. For backsheet or frontsheet materials, samples should be in the form and thickness provided by the manufacturer, typically 0,15 mm to 0,30 mm. Samples should be cured to the manufacturer's specifications or processed to be typical for that material. Sample surfaces should be smooth, which may require curing or thermal treatment when held between flat, planar surfaces.

Test materials should be preconditioned under one of the following conditions and the method specified with the result reported should be applied:

• Dried: For dry resistance testing, 5 samples are preconditioned in a dried atmosphere of 48 degrees plus/minus 23 degrees at a relative humidity of not more than 2 percent for at least 5 hours prior to testing.
• Saturated: For wet resistance testing, 5 samples are preconditioned at 48 degrees plus/minus 23 degrees for at least 2 hours prior to testing by placing them in a closed container containing liquid water (the samples should not come into direct contact with the water). This can be accomplished using a typical desiccator jar. Simply place water in the bottom of the jar where the desiccant would normally be.
• Laboratory environment: Pre-condition in laboratory air at 48 degrees plus/minus 23 degrees and 2 percent plus/minus 50 percent relative humidity for at least 5 hours prior to testing. Alternatively, a humidity-controlled room may be used.

For some classes of material, such as desiccant filled edge seals, it may take significantly longer than 48 hours to reach equilibrium. In this case, verification of equilibrium is required. If the samples are laminated to metal foil on one side, they will need to reach equilibrium for 48 hours instead of 96 hours. When conditioning, samples cannot be stacked on top of each other and there should be at least a 2 mm air gap between the samples. Samples should be placed in the test fixture immediately after removal from the preconditioning chamber.

Since the samples are in close contact with the gas-tight electrodes, the moisture content is not expected to change significantly during the test. For dry conditions, this can be minimized by placing the test fixture in a dry environment or by adding a desiccant to the fixture. Similarly, conducting the test in a laboratory or room at 50 percent relative humidity eliminates this concern for moist samples. The thickness of each test specimen should be measured after conditioning, prior to testing. The thickness should be taken as the average of three measurements taken at different locations on the specimen.

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