The 35mm hail impact test is an advanced mechanical stress test that propels a 35mm diameter ice ball at high velocity against a specimen to simulate severe hailstorm conditions. It is widely applied to photovoltaic modules, building materials, and outdoor equipment to verify impact resistance beyond the baseline requirements of IEC 61215. This article explains the technical principles, test equipment, procedures, acceptance criteria, and typical applications of 35mm hail impact testing.
Technical Overview
The 35mm hail impact test is an enhanced mechanical load evaluation in which a spherical ice ball with a nominal diameter of 35 millimeters is launched against the surface of a test specimen at a controlled velocity. The purpose of the test is to reproduce the kinetic energy delivered by large natural hailstones during severe convective storms, allowing manufacturers and certification bodies to assess whether a product can survive such events without functional or safety-critical damage.
While the standard hail impact requirement in IEC 61215-2:2016 (Test MQT 17) specifies a 25mm ice ball, many markets with documented severe hail exposure—such as the Alpine regions of Switzerland, Australia, the Great Plains of North America, and parts of South Africa—require verification with larger ice balls. The 35mm test is one of the most common enhanced hail levels requested by project developers, insurers, and certification agencies.
- Ice ball diameter: 35 mm nominal
- Ice ball mass: approximately 20.7 g (pure frozen deionized water)
- Ice temperature: typically -4 °C ± 2 °C at launch
- Impact velocity: commonly 27.2 m/s ± 2% (exact value defined by the applicable specification)
- Kinetic energy: approximately 7.6 J per impact at 27.2 m/s
- Impact angle: perpendicular (90°) to the specimen surface
Testing Samples
Photovoltaic Modules
Crystalline silicon and thin-film PV modules are the primary specimens for 35mm hail impact testing. Glass-front laminates, glass-glass bifacial modules, and flexible lightweight modules are all evaluated, since hail is one of the dominant causes of field failures such as front glass fracture and cell micro-cracking. Test severity may be adjusted based on glass thickness, module architecture, and target market requirements.
Building and Construction Materials
Roof tiles, skylights, polycarbonate and acrylic glazing sheets, facade panels, waterproofing membranes, and composite cladding are frequently tested for hail resistance. Certification schemes for roofing products in hail-prone regions often mandate larger ice ball sizes than the standard 25mm level.
Automotive and Transportation Components
Windshields, panoramic roofs, headlamp lenses, radar and LiDAR covers, and camera housings are subjected to hail impact evaluation to confirm resistance under real-world storm conditions and highway-speed relative impact scenarios.
Outdoor Equipment and Enclosures
Telecommunication enclosures, outdoor display panels, EV charging stations, greenhouse glazing, and aerospace surface components may also be qualified using 35mm hail impact protocols defined by manufacturer specifications or industry standards.
Testing Items
A complete 35mm hail impact evaluation typically includes the following test items and assessments:
- Single-impact resistance at each designated impact point on the specimen
- Repeated-impact capability at critical locations where specified (commonly up to three impacts per point)
- Verification of defined impact locations, including module corners, edges, cell centers, inter-cell gaps, busbar regions, and areas near the junction box or frame
- Post-test visual inspection for cracks, fractures, delamination, and surface damage
- Electroluminescence (EL) imaging to detect hidden cell cracks
- Maximum power (Pmax) measurement to quantify performance degradation, commonly limited to 5% relative degradation
- Insulation resistance and wet leakage current testing to confirm electrical safety after impact
Testing Methods
Test Equipment
The test is performed using a pneumatic hail impact tester, which consists of a compressed-air launcher (hail cannon), precision ice ball molds, a velocity measurement system based on light gates or photodiodes, and a specimen mounting fixture positioned at a defined standoff distance. The launching system must deliver the ice ball with a velocity tolerance within ±2% and with repeatable trajectory accuracy at the target point.
Ice Ball Preparation
Ice balls are produced from deionized or purified water in calibrated molds to guarantee dimensional accuracy and mass consistency. The molded ice balls are stored in a freezer, conditioned to the required test temperature (typically -4 °C ± 2 °C), and inspected visually for bubbles, cracks, or surface defects before use. Only intact, transparent ice balls within the specified mass tolerance are accepted for testing.
Velocity Calibration
Prior to testing, the launch pressure is calibrated so that the ice ball velocity measured by the chronograph system matches the specified value, for example 27.2 m/s for a typical 35mm test level. Velocity verification is repeated periodically during the test sequence to confirm system stability, and results are recorded for the test report.
Test Procedure
- Condition the specimen at standard ambient conditions, typically 23 °C ± 5 °C, unless otherwise specified.
- Mark the required impact points according to the applicable standard or client-defined map.
- Load a conditioned 35mm ice ball into the launcher and fire it perpendicular to the specimen surface at each target point.
- Record velocity, impact point accuracy, and ice ball condition at each shot.
- After all impacts are completed, allow the specimen to stabilize and perform post-test inspections: visual examination, EL imaging, electrical performance measurement, and safety tests as required.
Acceptance Criteria
- No broken front glass, through-cracks, or exposed live parts
- No major visual defects as defined by the applicable standard
- Power degradation within the specified limit, commonly no more than 5%
- Insulation resistance and wet leakage results within standard requirements
- No significant delamination, cell fracture beyond allowable limits, or functional failure
Applications
35mm hail impact testing supports product qualification, certification, and engineering development across multiple industries:
- PV project bankability: Developers and lenders in hail-prone regions increasingly require enhanced hail certification above the IEC baseline to reduce insurance risk and long-term performance losses.
- Certification programs: Testing laboratories apply 35mm protocols within IEC 61215-based certification sequences, extended hail test programs, and jurisdiction-specific requirements such as Swiss and Australian market specifications.
- Insurance and risk assessment: Insurers use verified hail resistance levels to classify roofing and PV products for premium calculations in severe hail zones.
- Research and development: Module and material manufacturers use 35mm impact data to optimize glass thickness, encapsulant selection, cell layout, and frame design.
- Quality control: Serial production lots may be spot-checked with hail impact testing to confirm consistency of lamination and glass quality.
FAQ
Why is a 35mm ice ball used instead of the standard 25mm ball?
The 25mm ice ball defined in IEC 61215 represents a moderate hailstone. A 35mm ice ball carries roughly 2.7 times more kinetic energy and better represents severe hail events recorded in many parts of the world, providing stronger assurance of field durability for hail-exposed installations.
What velocity is used for the 35mm hail impact test?
A velocity of approximately 27.2 m/s is widely applied for the 35mm level, corresponding to the estimated terminal velocity of an ice ball of this size and delivering about 7.6 J of kinetic energy. The exact velocity is always defined by the governing specification and verified with a calibrated measurement system.
How many impacts are applied during the test?
The number of impacts depends on the applied protocol. Certification-style tests typically impact a defined set of points once each, while enhanced or research programs may apply up to three impacts per location and increase the total number of impact points to cover corners, edges, and cell regions.
What happens if a specimen fails the 35mm test?
Failure indications include front glass fracture, excessive power loss, safety test non-conformity, or unacceptable cell cracking visible in EL imaging. Failed specimens require design changes—such as thicker or heat-strengthened glass or modified encapsulation—followed by retesting.
Can 35mm hail impact testing be combined with other tests?
Yes. Hail impact testing is frequently performed as part of a full IEC 61215 and IEC 61730 sequence and may be combined with damp heat, thermal cycling, and mechanical load testing to evaluate cumulative stress effects on long-term reliability.