Photovoltaic Cable Flame Retardant Performance Testing: Standards, Methods, and Compliance Requireme

Photovoltaic cable flame retardant performance testing verifies that solar DC cables resist ignition, limit flame propagation, and minimize smoke and corrosive gas release under fire conditions. Conducted in accordance with IEC 62930, EN 50618, UL 4703, and the IEC 60332 series, these tests are essential for product certification, quality control, and safe deployment of PV systems in residential, commercial, and utility-scale installations.

Technical Overview

Photovoltaic (PV) cables are flexible, single-core, double-insulated DC cables—typically with cross-linked polyolefin (XLPO) insulation and sheath—used to interconnect solar modules, strings, combiner boxes, and inverters. Rated for 1.5 kV DC operation with maximum conductor temperatures of 90 °C to 120 °C and a design life of 25 years or more, these cables are permanently exposed to sunlight, heat, and mechanical stress. Because large cable quantities are routed through roofs, cable trays, trenches, and sometimes occupied buildings, their fire behavior is a critical safety property.

Flame retardant performance testing evaluates whether a PV cable will resist ignition, self-extinguish after removal of an ignition source, and limit the release of smoke, halogenated acids, and toxic combustion products. The governing product standards include IEC 62930 (cables for photovoltaic power generation systems), EN 50618 (electric cables for photovoltaic systems), and UL 4703 (photovoltaic wire). The principal test methods are drawn from the IEC 60332 series (flame propagation), IEC 61034 (smoke density), IEC 60754 (halogen content and corrosivity), IEC 60695-2 (glow-wire tests), UL 2556 (which includes the VW-1 flame test), and UL 1685 (vertical tray flame test). In China, the corresponding national standards are the GB/T 18380 series, which are harmonized with IEC 60332, along with GB 31247 for fire hazard classification of cables.

Testing Samples

Finished Cable Samples

Tests are performed on production-length samples of finished single-core PV cable, covering the full range of cross-sections declared in the product specification (commonly 1.5 mm² to 240 mm², or the equivalent AWG sizes). For single vertical flame testing per IEC 60332-1-2, sample lengths of 600 mm ± 25 mm are cut from three consecutive positions along the cable. For bunched cable flame spread tests per IEC 60332-3, multiple test lengths of approximately 3.5 m are prepared. All samples are conditioned at 23 °C ± 5 °C for at least 16 hours before testing, with the conductor, insulation, and sheath exactly as supplied.

Material and Compound Samples

Insulation and sheath compounds are tested in material form—either as raw pellets or as compression-molded plaques—for limiting oxygen index (LOI), halogen acid gas evolution, pH and conductivity of combustion gases, and glow-wire flammability. These material-level results support the formulation development and incoming inspection of halogen-free flame-retardant compounds used in PV cable construction.

Assemblies and Installed Configurations

Where cables are qualified for tray installation or building-integrated photovoltaic systems, samples may be mounted on vertical ladders or trays in grouped configurations, including representative bundles with connectors and fixing hardware, to evaluate realistic flame spread, smoke production, and dripping behavior.

Testing Items

Typical flame retardant performance test items for photovoltaic cables include:

• Single vertical flame propagation test (IEC 60332-1-2 / GB/T 18380.12)
• Flame spread test on vertically mounted bunched cables, categories C or D (IEC 60332-3-23 / IEC 60332-3-24)
• VW-1 vertical wire flame test (UL 2556)
• Vertical tray cable flame test with optional smoke measurement (UL 1685)
• Smoke density test on cables burning under defined conditions (IEC 61034-2)
• Halogen acid gas evolution test (IEC 60754-1)
• Determination of acidity by pH and conductivity of combustion gases (IEC 60754-2)
• Glow-wire flammability index, GWFI (IEC 60695-2-12) and glow-wire ignition temperature, GWIT (IEC 60695-2-13)
• Limiting oxygen index of insulation and sheath materials (ASTM D2863 / ISO 4589-2)
• Fire hazard classification for the Chinese market, combining flame spread, smoke density, and toxicity results (GB 31247)

Testing Methods

Single Vertical Flame Propagation Test (IEC 60332-1-2)

A 600 mm sample is clamped vertically with its axis aligned to a standard 1 kW premixed propane burner inclined at 45° to the sample. The flame is applied to the sample for a duration determined by sample diameter—60 seconds for diameters up to and including 25 mm, 120 seconds for diameters above 25 mm up to 50 mm, and longer durations for larger diameters. After the burner is removed, the extent of charring and any afterflame or afterglow are recorded. The sample passes if the charred or affected portion does not extend to a point closer than 50 mm below the top support clamp. This test is the baseline flame retardancy requirement in IEC 62930 and EN 50618.

Bunched Cable Flame Spread Test (IEC 60332-3)

Multiple test lengths are secured to the front face of a vertical steel ladder at defined spacings, with the number of lengths calculated from the volume of non-metallic material per meter (0.5 L/m for category D, 1.5 L/m for category C). A horizontal propane burner with an output of approximately 20.5 kW is applied for 20 minutes for categories C and D. The sample passes if the charred portion does not extend more than 2.5 m above the point of application of the burner. This method evaluates behavior in grouped installation, which is representative of PV plant cable routes.

VW-1 and Vertical Tray Flame Tests (UL 2556 / UL 1685)

In the VW-1 test, a vertical sample fitted with a kraft paper flag is subjected to a gas flame applied for 15 seconds and removed for 15 seconds over five cycles, with surgical cotton positioned beneath the sample. The pass criteria require that the cotton is not ignited, no more than 25% of the flag is charred, and flaming extinguishes within 60 seconds after each flame application. PV wire listed to UL 4703 is normally evaluated with VW-1, while larger tray-rated constructions may additionally be subjected to the UL 1685 vertical tray flame test, which exposes a vertical cable bundle on a tray to a 20 kW flame source for 20 minutes.

Smoke Density Test (IEC 61034-2)

One-meter cable lengths, in a number depending on cable diameter and non-metallic volume, are mounted on a horizontal support inside a 3 m³ enclosure and ignited by a standard alcohol bath. A horizontal light path with a photometric system records the minimum light transmittance during and after combustion. Low-smoke halogen-free PV cable designs are typically required to achieve a minimum light transmittance of 60%, ensuring visibility for evacuation and firefighting in enclosed spaces.

Halogen Content and Combustion Gas Corrosivity (IEC 60754-1 / IEC 60754-2)

For halogen-free constructions, a weighed sample of insulation or sheath material is pyrolyzed in a tube furnace, and the evolved gases are absorbed in water. IEC 60754-1 determines the halogen acid content by titration, with halogen-free compounds required to release no more than 0.5% hydrogen halides. IEC 60754-2 measures the pH and conductivity of the gas absorption solution, with typical acceptance criteria of pH ≥ 4.3 and conductivity ≤ 10 µS/mm, demonstrating low corrosivity toward electronic equipment and metallic structures.

Glow-Wire Tests (IEC 60695-2-12 / IEC 60695-2-13)

Insulation and sheath materials are contacted with a glow-wire at elevated temperatures to determine the glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT). PV cable compounds commonly achieve GWFI of 960 °C and GWIT of 750 °C or higher, confirming resistance to ignition from hot components and fault-induced overheating.

Limiting Oxygen Index (ASTM D2863 / ISO 4589-2)

Material specimens are burned in a vertical glass chimney under a controlled oxygen-nitrogen atmosphere. The minimum oxygen concentration that supports candle-like burning is reported as the LOI. Because ambient air contains approximately 21% oxygen, flame-retardant XLPO compounds used for PV cables typically achieve LOI values of 30% or greater, providing a rapid screening indicator of flame retardant loading during compound development.

Applications

Flame retardant performance testing of photovoltaic cables supports the entire product and project lifecycle across the solar industry:

• Type testing and certification of PV cables against IEC 62930, EN 50618, and UL 4703 by accredited third-party laboratories
• Factory production control and periodic batch release testing by cable manufacturers
• Research and development of halogen-free flame-retardant XLPO insulation and sheath compounds
• Specification, incoming inspection, and acceptance testing by engineering, procurement, and construction contractors
• Installation qualification for rooftop, building-integrated, cable-tray, and trench-routed DC cabling where grouped fire behavior, smoke density, and corrosivity are safety-critical
• Market access compliance, including fire hazard classification under GB 31247 for projects in China and construction product fire performance requirements in other jurisdictions
• Root cause analysis and forensic assessment following field fires or thermal events in operating PV plants

FAQ

Which standards govern the flame retardancy of PV cables?

The product standards are IEC 62930, EN 50618, and UL 4703. They reference test methods from the IEC 60332 series for flame propagation, with supporting tests for smoke, halogen content, and glow-wire performance from IEC 61034, IEC 60754, and IEC 60695-2. UL-listed PV wire is evaluated using flame tests described in UL 2556 and, for tray-rated constructions, UL 1685.

Why do low-smoke and halogen-free requirements accompany flame testing?

Flame retardancy prevents fire spread, while low-smoke and halogen-free characteristics protect people and equipment during a fire. Halogenated materials can release hydrogen halides that form corrosive acids, damaging inverters, electronics, and steel structures. IEC 61034 and IEC 60754 therefore quantify smoke transmittance, halogen release, and gas acidity alongside flame spread results.

What is the difference between IEC and UL flame test approaches?

The IEC system, reflected in IEC 62930 and EN 50618, applies a 1 kW burner to a single vertical cable per IEC 60332-1-2 as the baseline requirement, with bunched tests in IEC 60332-3 for grouped installations. The UL system for PV wire relies on the VW-1 test and, for larger or tray-rated cables, the UL 1685 vertical tray flame test. Products intended for global markets are commonly verified against both approaches by accredited third-party laboratories.

How many samples are required and what is a typical test cycle?

Single vertical flame tests typically use three samples per cable size, while bunched, smoke, and material tests require additional lengths or compound specimens depending on cable diameter and non-metallic volume. A complete flame retardant performance package for one cable family is generally completed within two to four weeks, including conditioning and reporting.

Does flame retardancy affect other cable properties?

Flame retardant additives must be balanced with thermal aging resistance, mechanical strength, and electrical performance required for a 25-year service life. For this reason, flame tests are performed together with the full type test program—insulation resistance, tensile and elongation after aging, hot set, weathering, and cold bend—rather than in isolation.

Are flame tests required for every production batch?

Flame propagation and smoke tests are normally type tests performed during qualification and at defined periodic intervals, supplemented by routine checks such as material LOI verification as part of factory production control. Retesting is mandatory after any change to compound formulation, structure, or manufacturing process that could influence fire performance.