Impact resistance determination is one of the most critical quality tests for photovoltaic (PV) glass, verifying that solar module covers can withstand hail strikes, wind-borne debris, and mechanical abuse throughout a 25-year service life. This article explains the relevant international standards (IEC 61215, IEC 61730, ASTM, EN, GB), sample requirements, test methods such as steel ball drop and ice ball hail impact, and the acceptance criteria applied by accredited third-party laboratories and module manufacturers.
Technical Overview
Photovoltaic glass is the outermost protective layer of a crystalline silicon or thin-film solar module, shielding the encapsulated cells, polymeric interlayers, and electrical circuitry from environmental and mechanical stress. In service, the glass is routinely exposed to hail impacts, sand and gravel impact, wind-driven debris, snow loads, and handling shocks during transport and installation. Because glass is a brittle material, its impact resistance must be quantified through controlled, reproducible laboratory testing rather than inferred from thickness alone.
Most PV cover glass is low-iron soda-lime-silica glass that has been thermally tempered to induce a compressive surface stress, typically at least 90 MPa per EN 12150-1 or at least 69 MPa per ASTM C1048. Heat-strengthened glass, with a surface compression of roughly 24–52 MPa, is increasingly used for the rear panes of bifacial double-glass modules because its larger breakage fragments remain interlocked in the laminate. The tempering process raises bending strength and impact energy absorption by two to four times compared with annealed glass, but process deviations, edge defects, drilling damage, and coating stresses can all degrade performance. Impact testing therefore serves as both a qualification gate and a statistical quality-control tool.
The determination of impact resistance for PV glass and PV modules is governed by a framework of standards, the most important of which are listed below:
- IEC 61215-2 (Terrestrial photovoltaic (PV) modules – Design qualification and type approval – Test procedures), which specifies the hail resistance test using projected ice balls.
- IEC 61730-2 (Photovoltaic (PV) module safety qualification – Tests), which references impact and mechanical load requirements for module safety.
- ASTM E822, covering impact resistance of solar collector covers by hailstones.
- GB 15763.2 and the GB/T 30984 series, which define steel ball drop impact and fragmentation requirements for tempered glass used in Chinese solar and building applications.
- EN 12150-1 and EN 1863-1, which classify fully tempered and heat-strengthened glass, respectively, including fragmentation behavior.
- ANSI Z97.1 and EN 12600, which define soft-body pendulum impact methods occasionally applied to laminated glass-glass configurations.
Testing Samples
Tempered Ultra-Clear Patterned (Rolled) Glass
This is the classic front-cover material for conventional modules, most commonly 3.2 mm in thickness with a prismatic or inverted-pyramid textured surface. Coupons are typically cut to 300 mm × 300 mm or 610 mm × 610 mm panels, edge-polished, and tested in the finished tempered condition, since cutting after tempering is not possible.
Tempered or Heat-Strengthened Float Glass
Float glass with anti-reflective (AR) coatings, in thicknesses of 1.6 mm to 3.2 mm, is used for lightweight and bifacial double-glass modules. Both fully tempered and heat-strengthened variants are tested, and the test program should reflect the intended position (front or rear pane) and whether the glass is monolithic or laminated.
Laminated Glass-Glass Assemblies
For double-glass modules, impact testing may be performed on complete laminates with encapsulant (EVA, POE, or PVB) and cells or dummy inserts, because the interlayer absorbs energy and changes the breakage mode from through-thickness fracture to radial cracking confined to one pane.
Sample Preparation and Conditioning
Specimens must be free of edge chips and visible defects, cleaned, and conditioned at standard laboratory temperature (typically 23 °C ± 5 °C) before testing. The number of specimens per condition is normally at least 4 to 10, because glass strength is statistically distributed and a single result is not meaningful. Edges, holes, and coating condition are documented photographically before the test.
Testing Items
A comprehensive impact resistance determination for photovoltaic glass typically includes the following test items:
- Steel ball drop impact resistance at the center, corner, and near-edge zones.
- Hail impact resistance using calibrated ice balls projected at specified velocities (IEC 61215-2, ASTM E822).
- Fragmentation (breakage pattern) characterization after induced fracture.
- Post-impact integrity and residual strength of laminated configurations.
- Impact behavior at low temperature, simulating winter hail events and cold-climate installation.
- Statistical evaluation of breakage probability, often using Weibull analysis of ball-drop breakage heights.
Testing Methods
Steel Ball Drop Impact Test
The steel ball drop test is the most widely used component-level method for PV glass. A polished, hardened chrome-steel ball is released by an electromagnetic or mechanical release mechanism from a calibrated height so that it falls freely, without initial velocity, onto the horizontally supported specimen. Common protocols use a 227 g ball (approximately 38 mm in diameter) dropped from heights of 1.0 m to 2.0 m, corresponding to impact energies of roughly 2.2 J to 4.5 J, or a 1040 g ball (approximately 63.5 mm in diameter) dropped from 1.2 m, corresponding to approximately 12.2 J, consistent with practices in GB 15763.2 and related specifications.
The specimen is supported on a rigid frame or clamped perimeter fixture that matches the standard being applied, and impact points are marked at the geometric center and at defined distances from edges, corners, and drilled holes, where stress concentrations reduce strength. The pass criterion is survival without through-thickness fracture or spalling; if breakage occurs, the specimen is subjected to fragmentation evaluation. For statistical programs, the drop height is stepped up or down to determine the minimum breaking height and the 5% failure probability height.
Hail (Ice Ball) Impact Test per IEC 61215-2
The hail resistance test projects spherical ice balls at high velocity using a compressed-air launcher equipped with optical velocity measurement. Ice balls are molded from water, inspected for bubbles and dimensional accuracy, and stored in a freezer at approximately −10 °C until fired within seconds of removal. The standard test conditions specify:
- 25 mm ice ball at 23.0 m/s (approximately 2 J kinetic energy).
- 35 mm ice ball at 27.2 m/s (approximately 7.5 J kinetic energy).
- 45 mm ice ball at 30.7 m/s (approximately 20 J kinetic energy).
Eleven impact locations are targeted on the module or panel, including corners, edges near the frame, the geometric center, cell centers, and areas above the junction box and bypass diodes. Velocity must be verified before each shot and held within the tolerance of the standard. Acceptance at module level requires that no major visual defects are introduced, that maximum power at STC degrades by no more than 5%, and that insulation and wet-leakage requirements are still met. At glass coupon level, the criterion is the absence of through-cracks and unacceptable chipping.
Fragmentation (Breakage Pattern) Test
Because the safety value of tempered glass lies in how it fails, impact determination is complemented by a fragmentation test. The specimen is fractured with a pointed punch applied to a defined point, typically the center or the point of highest residual stress, and the fragment count is measured within a 50 mm × 50 mm counting zone. Fully tempered glass per GB 15763.2 and EN 12150-1 must produce a minimum particle count (commonly at least 40 particles within the counting zone) with no elongated dagger-like fragments exceeding the length limits of the standard. Heat-strengthened glass deliberately produces larger fragments, and its fragmentation is compared against the reference pattern classes of EN 1863-1.
Post-Impact Evaluation and Reporting
After each impact sequence, specimens are visually inspected under defined illumination, photographed, and, where relevant, re-tested for maximum power, insulation resistance, and dry and wet leakage. A complete test report includes specimen identification and thermal history, surface stress measurement results if available, ball mass and drop heights or ice ball diameter and velocity, impact locations, pass/fail results per location, fragmentation counts, and any statistical (Weibull) parameters derived from the data set. Accredited third-party laboratories performing these tests for certification purposes operate under ISO/IEC 17025 quality systems.
Applications
Impact resistance determination of PV glass supports the solar industry at every stage of the value chain, from raw glass production to field failure analysis. Typical applications include:
- Qualification of new glass suppliers, tempering lines, and AR coating processes before module design release.
- Type-approval and safety certification of PV modules under IEC 61215 and IEC 61730 by accredited third-party laboratories.
- Development and validation of lightweight 1.6 mm and 2.0 mm glass and bifacial double-glass module architectures.
- Routine statistical quality control of production lots, particularly after furnace settings, cutting, drilling, or edge-finishing changes.
- Insurance, bankability, and site-suitability assessments for projects located in severe hail regions.
- Root-cause analysis of field breakage, distinguishing thermal stress, point-contact damage, hail stones, and spontaneous nickel-sulfide fracture.
FAQ
What standards govern PV glass impact resistance testing?
The primary standards are IEC 61215-2 for the hail resistance test on complete modules, IEC 61730-2 for safety-related impact verification, ASTM E822 for hailstone impact on solar collector covers, and GB 15763.2 together with the GB/T 30984 series for steel ball drop and fragmentation requirements on the glass itself.
What is the pass criterion for the hail impact test?
At module level, the specimen must show no major visible defects, maximum power degradation must not exceed 5% at standard test conditions, and insulation and wet-leakage tests must still pass. At glass coupon level, the criterion is no through-thickness cracking or spalling at the specified impact energy.
What ball mass and drop height are used in the steel ball drop test?
Common protocols use a 227 g steel ball dropped from 1.0 m to 2.0 m, or a 1040 g ball from 1.2 m, applied to 610 mm × 610 mm or smaller coupons supported on a defined frame. The exact combination depends on the specification invoked by the purchaser or certifier.
How do tempered and heat-strengthened PV glass behave differently under impact?
Fully tempered glass carries higher surface compression and withstands higher impact energy, but shatters into small particles across the entire pane. Heat-strengthened glass fails at lower energy but breaks into larger radial fragments that remain engaged in the laminate, which is why it is preferred for rear panes where post-breakage integrity matters.
Why do identical glass specimens give different impact results?
Glass strength is governed by surface flaw statistics and follows a Weibull distribution, so breakage heights vary between nominally identical specimens. Valid determinations therefore use multiple specimens per condition and report breakage probability rather than a single pass or fail value.
Can hail tests be performed on glass panels instead of complete modules?
Yes, component-level hail shooting on framed or laminate-supported glass panels is routinely used for supplier QC and R&D. However, official IEC 61215-2 certification requires the hail resistance test to be performed on the complete module in its final configuration.