25mm Hail Impact Testing: Standards, Procedures, and Applications

The 25mm hail impact test is a standardized mechanical durability test that evaluates the ability of photovoltaic modules, glass products, and outdoor equipment to withstand hailstone strikes. Using precisely manufactured 25mm ice balls propelled at approximately 23 m/s, the test simulates real-world hail damage and is a mandatory requirement for IEC 61215 certification of solar panels. This article provides a comprehensive technical overview of the test principle, sample requirements, testing procedures, evaluation criteria, and typical applications.

Technical Overview

The 25mm hail impact test is a controlled mechanical impact test designed to reproduce the damage caused by severe hailstorms. A spherical ice ball with a nominal diameter of 25mm, weighing approximately 7.5 grams, is launched from a pneumatic air cannon at a calibrated velocity of about 23 m/s toward the surface of a test specimen. The resulting kinetic energy of roughly 2 joules is representative of the impact energy delivered by naturally falling hailstones of this size, which corresponds to quarter-sized hail and marks the threshold of severe hail events in meteorological classification.

This test is most widely associated with the certification of terrestrial photovoltaic (PV) modules under IEC 61215-2:2016, where it is designated as MQT 17 (hail impact test). Related requirements also appear in IEC 61730-2 for module safety and in national equivalents such as UL 61730 and GB/T 9535. The test verifies that the front cover glass, encapsulant, and internal cells of a module can survive hail impact without cracking, delamination, or significant electrical degradation.

The core parameters of the standard 25mm test configuration are summarized below:

• Ice ball diameter: 25mm (nominal)
• Ice ball mass: approximately 7.5 g
• Impact velocity: approximately 23 m/s, measured close to the specimen surface
• Impact energy: approximately 2 J
• Number of impact points: 11 locations on a standard PV module, including corners, edges, cell centers, and junction box areas
• Impact direction: perpendicular to the specimen surface at each impact point

Testing Samples

A wide range of products intended for long-term outdoor exposure can be evaluated using the 25mm hail impact test. The most common sample categories are described below.

Photovoltaic Modules

Crystalline silicon and thin-film PV modules are the primary test subjects. Full-size modules are tested because the response of glass, frame, and cell structure depends on the complete assembly. The module is inspected electrically and visually before and after impact to quantify any degradation caused by the hail strikes.

Glass and Glazing Products

Tempered glass, laminated safety glass, and coated architectural glazing for buildings, skylights, and greenhouses are frequently tested to confirm resistance to cracking, spalling, and coating damage under hail loading.

Building-Integrated Photovoltaics and Roofing Components

BIPV panels, solar tiles, transparent façade elements, and roofing assemblies are tested to demonstrate that hail exposure will not compromise weather-tightness or energy production. For roofing systems, larger hail sizes may also be applied under standards such as FM 4473, but 25mm remains the baseline reference size.

Automotive and Aerospace Components

Windshields, panoramic roofs, exterior trim, headlamp lenses, radar covers, and aircraft skin panels or radomes can be assessed using equivalent ice ball launch methods to validate resistance to hail during operation or ground parking.

Outdoor Enclosures and Electronic Equipment

Telecommunication enclosures, outdoor display panels, surveillance camera housings, and charging station cabinets are tested to ensure hail impact does not breach enclosure integrity or damage internal electronics.

Testing Items

The hail impact test itself is a mechanical exposure, but the evaluation of results combines physical inspection with electrical performance measurements. Typical testing items include:

• Pre-test visual inspection to document any existing defects in the specimen
• Maximum power output (Pmax) measurement at Standard Test Conditions (STC) before and after impact
• Power degradation assessment, with a typical pass threshold of no more than 5% loss relative to the pre-test value for PV modules
• Post-test visual inspection for glass breakage, cell cracks, delamination, bubbles, and surface damage
• Insulation resistance verification after impact
• Wet leakage current test to confirm the module remains safe when wet
• Electroluminescence (EL) imaging to reveal hidden micro-cracks in solar cells caused by the impact
• Verification of impact point accuracy, ice ball integrity, and impact velocity records for traceability

Testing Methods

The test procedure is carefully controlled to ensure repeatability and comparability between laboratories. The main stages are described below.

Specimen Conditioning

The specimen is conditioned at ambient laboratory temperature, typically 23°C ± 5°C, and is securely mounted in a rigid fixture that supports the rear surface in a manner representative of the intended installation. Secure mounting prevents artificial energy absorption from vibration or displacement during impact.

Ice Ball Preparation

Ice balls are molded from distilled or deionized water to eliminate inclusions and impurities that could alter their mechanical behavior. The ice balls are stored in a freezer at a specified low temperature, commonly at or below approximately -4°C, until immediately before firing. Each ice ball is visually inspected, and any ball containing cracks, air bubbles, or dimensional deviations is rejected, because internal flaws significantly change the impact force profile.

Launcher and Velocity Calibration

A compressed-air launcher fitted with a barrel of matching diameter accelerates the ice ball toward the specimen. The velocity is measured close to the target surface using optical light gates or photodiode sensors, and the air pressure is adjusted until the required speed, approximately 23 m/s, is achieved consistently. Velocity calibration is performed with dummy shots before formal impacts on the specimen.

Impact Procedure

The standard test sequence for a PV module involves firing ice balls at 11 designated impact points covering structurally critical areas, including the four corners, selected edges, cell centers, the geometric center of the module, and areas near the junction box. Each impact is delivered perpendicular to the surface. The impact locations, velocities, and results are recorded for each shot.

Post-Test Evaluation

After the impacts are completed, the specimen undergoes visual inspection, electrical characterization, insulation testing, wet leakage testing, and optionally EL imaging. The specimen passes if it exhibits no major visual defects, maintains power output within the allowed degradation limit, and satisfies all electrical safety requirements.

Applications

The 25mm hail impact test supports multiple engineering and commercial objectives across industries:

• Certification of PV modules under IEC 61215 and IEC 61730 for market access in most global regions
• Design verification of new front glass, encapsulant, and frame structures during product development
• Quality control and incoming inspection programs for module manufacturers and EPC contractors
• Comparative benchmarking of competing products or material formulations under identical impact conditions
• Failure analysis and insurance claim support following real-world hailstorm damage
• Qualification of building materials for hail-prone regions, including airport, stadium, and greenhouse glazing
• Validation of automotive and aerospace components intended for extreme weather environments

Because hail damage is one of the leading causes of field failures and insurance losses for solar installations, passing this test is widely regarded as a fundamental indicator of long-term product reliability in hail-exposed climates.

FAQ

What impact velocity is used in the 25mm hail impact test?

For PV module testing under IEC 61215-2, the 25mm ice ball is launched at approximately 23 m/s, which corresponds to the realistic terminal velocity of a hailstone of this size falling in a severe storm. The velocity is verified optically at the specimen surface for every test series.

How much impact energy does a 25mm ice ball deliver?

A 25mm ice ball weighs approximately 7.5 grams. At a velocity of 23 m/s, it carries a kinetic energy of approximately 2 joules, which is concentrated on a small contact area and is sufficient to fracture improperly tempered or laminated glass.

Which standards require the 25mm hail impact test?

The principal standard is IEC 61215-2:2016, clause MQT 17, for the design qualification of terrestrial PV modules. Related requirements appear in IEC 61730-2 for module safety, UL 61730, and national adaptations such as GB/T 9535. Roofing systems may additionally reference FM 4470 and FM 4473, which cover a range of hail sizes including 25mm.

Are hail sizes other than 25mm used in testing?

Yes. While 25mm is the standard baseline size for PV module certification, enhanced hail tests may use larger ice balls such as 35mm or 45mm for products intended for extreme hail regions. Automotive glazing and building product standards also specify their own size and velocity matrices depending on the expected service environment.

What results constitute a failure in the test?

A specimen fails if it shows broken front glass, visible cell cracks, delamination or bubbles beyond allowable limits, power degradation exceeding the specified threshold, or if it does not pass insulation resistance and wet leakage tests after impact.

How does the hail impact test differ from other mechanical load tests?

Unlike static mechanical load tests or cyclic pressure tests that apply distributed loading over seconds or hours, the hail impact test applies a concentrated, high-speed dynamic impulse lasting only milliseconds. It specifically evaluates local impact resistance and brittleness rather than overall structural deflection.

Can the test be performed on installed products or field samples?

Yes. Laboratories can test production samples, field-return modules, or new design prototypes. For forensic work, reproducing the impact conditions on undamaged reference samples often helps establish whether observed field damage is consistent with hail of a given size and velocity.