Photovoltaic Module Hail Impact Testing: IEC 61215-2 Standards, Methods and Requirements

Photovoltaic (PV) module hail impact testing is a critical mechanical robustness evaluation that verifies whether a solar module can withstand repeated hailstone strikes without glass breakage, cell cracking, or power loss. Performed by accredited third-party laboratories in accordance with IEC 61215-2 (Test MQT 10) and related standards such as ASTM E1038, the test launches calibrated ice balls at controlled velocities against a fully assembled module. This article explains the applicable standards, sample requirements, test items, procedures, pass criteria, and typical applications of PV module hail impact testing.

Technical Overview

Hail impact testing is one of the mechanical stress evaluations included in the design qualification and type approval of terrestrial photovoltaic (PV) modules. The test simulates the kinetic energy transfer of falling hailstones by propelling solid ice balls of calibrated diameter and mass against the front surface of a fully assembled module at precisely controlled velocities. Its purpose is to demonstrate that the module can tolerate realistic hail events without glass fracture, encapsulant damage, cell micro-cracking, or hazardous electrical degradation.

Hail is one of the leading causes of catastrophic field failures in solar power plants. A single severe storm can shatter front covers, fracture cells, and sharply reduce string output across an entire site. Cracked cells may also develop hot spots, snail trails, and insulation faults over time, creating long-term safety and financial risks for asset owners and insurers. Hail impact testing quantifies impact resistance before modules are deployed, and higher test severities are frequently specified for projects located in hail-prone climates.

The test is primarily governed by the following standards and their national adoptions:

• IEC 61215-2 (Terrestrial photovoltaic modules – Design qualification and type approval – Part 2: Test procedures), which defines the hail impact test as MQT 10, adopted in Europe as EN IEC 61215-2.
• GB/T 9535 and related Chinese national standards aligned with the IEC 61215 series.
• ASTM E1038, which determines hail resistance of PV panels by impact with replicated ice balls.
• UL 61730 and IEC 61730, the module safety standards, which complement design qualification with additional mechanical robustness requirements for market access.

Testing Samples

Testing is performed on production-representative modules that are complete and fully functional: frame installed (for framed designs), junction box fitted, and cables and connectors included as shipped. One module is normally sufficient for the standard hail test sequence, while additional samples may be required for enhanced or comparative protocols. Modules are tested in the as-delivered state unless a special conditioning regime, such as cold soaking, has been agreed to simulate severe winter storm conditions.

Crystalline Silicon Modules

Conventional glass-backsheet and dual-glass crystalline silicon modules, both framed and frameless, represent the majority of hail test samples. These products use tempered or heat-strengthened front glass, and the test verifies that the specified glass thickness and tempering quality can absorb the standardized impacts.

Thin-Film Modules

Cadmium telluride, CIGS, and amorphous silicon modules are covered by the IEC 61215 series. Depending on whether the product is substrate- or superstrate-based, the active surface may be more sensitive to localized impact, making verified hail resistance particularly important for fragile glass structures.

Bifacial and Dual-Glass Modules

Because bifacial modules collect light and may be exposed to debris on both faces, test plans can be extended to assess the rear glass as well as the front surface, especially for installations with elevated or tracking mounting systems.

Flexible, Lightweight and Building-Integrated Modules

Modules with polymer front sheets do not shatter like glass, but must still resist puncture, penetration, and encapsulation breach. Building-integrated PV products are additionally evaluated in their intended mounting configuration, since edge supports and installation hardware strongly influence impact behavior.

Testing Items

During and after the hail impact test, accredited third-party laboratories evaluate the following items:

• Front cover integrity: verification that the glass or polymer front sheet shows no fracture, penetration, or through-thickness damage at any impact point.
• Cell and interconnect damage: electroluminescence (EL) imaging before and after the test to reveal hidden cell micro-cracks and broken interconnects that are invisible to the naked eye.
• Electrical performance: measurement of maximum power (Pmax) at Standard Test Conditions (STC) to quantify degradation caused by the impacts.
• Visual defects: inspection against the major visual defect definitions of IEC 61215-1, including delamination, bubbles, and broken external surfaces.
• Electrical safety: verification of insulation resistance and, where required, wet leakage current compliance after impact exposure.
• Structural components: assessment of frame deformation, backsheet damage, junction box integrity, and cable or connector displacement.

Testing Methods

Test Apparatus

The core equipment is a compressed-air ice ball launcher, often called a hail cannon, equipped with interchangeable barrels matched to each ice ball diameter. The launcher is supported by precision ice ball molds, a low-temperature freezer, a calibrated balance and measuring tools for ball mass and diameter, optical velocity sensors (light barriers) positioned a fixed distance apart, and laser or optical sighting to place each shot accurately on the target point.

Ice Ball Preparation

Ice balls are molded from purified water and frozen until completely solid. Each ball is inspected to confirm it is free of air bubbles and cracks, and its diameter and mass are verified against the tolerances of the applicable standard. The balls are conditioned at a standardized low temperature, typically around −10 °C, and launched promptly so that the ice remains fully rigid at the moment of impact. Common ball diameters and corresponding nominal launch velocities include:

• 20 mm diameter at approximately 21.0 m/s
• 25 mm diameter at 23.0 m/s (the default severity, ball mass approximately 7.5 g)
• 35 mm diameter at 27.2 m/s
• 45 mm diameter at 30.7 m/s
• 55 mm diameter and larger, with correspondingly higher velocities, for enhanced-severity assessments agreed between manufacturer and customer

Test Procedure

Before impact testing, the sample undergoes baseline characterization: visual inspection, EL imaging, power measurement at STC, and insulation testing. The module is then rigidly mounted with its target surface perpendicular to the launch trajectory, and ice balls are fired at a defined pattern of impact points, with one impact per location. Under IEC 61215-2 (MQT 10), the standard pattern comprises 11 impacts distributed across the module, including a point near a corner approximately 50 mm from the edges, the midpoints of the long sides approximately 12 mm from the edge, the module centre, and further specified positions covering cell boundaries and structural features such as the junction box area. The measured velocity of each shot must fall within a tight tolerance, typically ±5 %, of the specified value. After the final impact, the full post-test characterization sequence is repeated.

Pass Criteria

To pass the hail impact test, the module must show no major visual defects as defined in IEC 61215-1, no breakage of the front cover at any impact location, a maximum power degradation at STC within the specified limit (typically no more than 5 % compared with the initial measurement), and continued compliance with insulation and wet leakage requirements. Typical failure modes include radial glass cracking from impact points, shattered front covers, hidden cell fractures detected by EL imaging, and power loss exceeding the allowed degradation threshold.

Applications

Hail impact test reports from accredited third-party laboratories are used throughout the PV value chain:

• Certification: the test is a mandatory component of IEC 61215 design qualification and type approval sequences required for market access and most tender submissions.
• Project engineering: developers in hail-prone regions specify elevated severities, such as 35 mm or 45 mm ice balls, to match local climatic risk profiles.
• Insurance and finance: underwriters and lenders use verified hail resistance data to assess bankability and to set coverage conditions for utility-scale assets.
• Research and development: manufacturers compare glass types, thicknesses, tempering processes, encapsulants, and cell layouts during product optimization.
• Quality assurance and field analysis: production quality control, incoming module inspection, and root-cause investigation of hail damage claims reported from operating plants.

FAQ

What is the default hail test severity in IEC 61215-2?

The default severity is a 25 mm ice ball with a nominal mass of approximately 7.5 g launched at 23.0 m/s, delivered as single impacts at 11 defined locations on the module. Higher or lower severities may be selected by agreement to reflect the expected hail climate at the installation site.

Why are ice balls used instead of steel balls?

Ice balls replicate the density, hardness, and fracture behavior of real hailstones, producing realistic contact pressure and energy transfer. Steel ball impact is a separate mechanical abuse test used in module safety standards and produces a different damage mechanism, so it cannot substitute for the hail simulation required in design qualification.

Can the test simulate extreme hailstorms?

Yes. Ice ball diameters up to 75 mm with correspondingly higher launch velocities are available for enhanced-severity protocols. Some programs also combine the impacts with cold-conditioned modules to represent worst-case winter storms, in which materials are more brittle and damage propagation is more likely.

How is invisible cell damage detected?

Electroluminescence imaging is performed before and after the impacts. Cracked cells appear as dark lines in the EL image, allowing laboratories to identify micro-cracks that cause no visible surface damage yet reduce power output and long-term reliability.

Is hail impact testing required for every PV module type?

The test applies to all terrestrial module technologies covered by the IEC 61215 series, including crystalline silicon, thin-film, bifacial, and flexible products, because any module installed outdoors faces hail exposure. Specific severities may vary with product design and target market requirements.

How long does hail impact testing take?

Ice ball production and freezing typically require 12 to 24 hours, while the impact phase itself is completed within minutes. Including baseline and post-test characterization such as EL imaging, STC power measurement, and insulation verification, the complete test is commonly completed within a few working days, depending on the laboratory workflow and any enhanced-severity requirements.