Transformer Insulation Film Combustion Testing: Standards, Methods, and Flammability Ratings

Transformer insulation film combustion testing is a specialized flammability evaluation that determines how thin polymer dielectric films, such as PET, PEN, and polyimide films, behave when exposed to an ignition source inside transformers and other electrical equipment. Performed by accredited third-party laboratories in accordance with UL 94, IEC 60695, and GB/T standards, these tests establish flame-retardant classifications, quantify dripping and afterflame behavior, and verify compliance with global fire-safety requirements for insulation systems.

Technical Overview

Insulation films are among the most widely used dielectric materials in transformers, serving as interlayer insulation, wrap insulation, margin tape, and barrier layers between windings and cores. Because these films are located in close proximity to current-carrying conductors that generate heat during operation, and because they may be exposed to arcing, overload faults, or external fire scenarios, their combustion behavior is a critical safety parameter.

Transformer insulation film combustion testing subjects film specimens to standardized ignition sources under controlled laboratory conditions. The tests measure ignitability, flame propagation speed, afterflame time, afterglow time, and the tendency to release flaming droplets that could ignite adjacent materials. Results are expressed as flammability ratings, which transformer manufacturers, insulation system designers, and certification bodies rely upon to demonstrate that a material meets the fire-hazard requirements of product safety standards.

The governing test methods are drawn from the UL 94, IEC 60695, and GB/T 5169 families of standards, supplemented by limiting oxygen index (LOI) measurements under ISO 4589-2, ASTM D2863, and GB/T 2406.2. For thin films specifically, UL 94 also defines the VTM (vertical thin material) procedure, which addresses the tendency of very thin specimens to distort during vertical burning tests.

Testing Samples

PET Insulation Films

Polyethylene terephthalate (PET) films are the most common insulation films for Class B (130 °C) and Class F (155 °C) transformer insulation systems. Typical thicknesses include 25 μm, 50 μm, 75 μm, 100 μm, 125 μm, 188 μm, and 250 μm. Because flammability performance can vary with thickness and gauge, specimens are usually tested at each nominal thickness used in production. Standard PET film is combustible, so many transformer grades incorporate flame-retardant additives or coatings to achieve higher ratings.

Polyimide (PI) Films

Polyimide films are high-temperature dielectrics used in Class H (180 °C) and Class C (220 °C) insulation systems for dry-type transformers, traction transformers, and demanding industrial applications. They exhibit inherently high limiting oxygen index values, typically in the range of 36% to 38%, and generally achieve the highest vertical burning classifications without halogenated additives.

PEN, PPS, PTFE, and Specialty Films

Polyethylene naphthalate (PEN), polyphenylene sulfide (PPS), and fluoropolymer films are used where elevated temperature resistance, chemical resistance, or low flammability is required. Combustion testing confirms whether these materials meet the ignition and dripping requirements of the relevant end-product standards.

Composite, Coated, and Bonding Films

Epoxy-coated PET bonding films, B-stage adhesive films, aramid-fiber composites, and film-paper laminates combine polymers with different combustion characteristics. These products are tested in their final laminate construction, since coatings, adhesives, and bonding layers can significantly alter ignition behavior, melt dripping, and char formation.

Sample Preparation and Conditioning

• Vertical burning (V series): rectangular strips, typically 125 mm × 13 mm, at the minimum and maximum thicknesses to be qualified.
• Thin-material vertical burning (VTM series): strips of approximately 200 mm × 50 mm, rolled longitudinally into a cylindrical form for testing.
• Horizontal burning (HB): rectangular strips, typically 125 mm × 13 mm.
• Edges must be smooth and free of flash, cracks, or wrinkles that could influence flame behavior.
• Specimens are conditioned before testing, commonly at 23 ± 2 °C and 50 ± 5% relative humidity for at least 48 hours, or as specified by the governing standard.
• A sufficient quantity of specimens, generally 10 to 20 strips per thickness per test, should be submitted to allow for two complete test sets and any repeat testing.

Testing Items

Accredited third-party laboratories typically offer the following combustion and flammability test items for transformer insulation films:

• UL 94 vertical burning test (20 mm, 50 W flame) for V-0, V-1, and V-2 classifications, per UL 94, IEC 60695-11-10, and GB/T 5169.16.
• UL 94 VTM test for thin materials (VTM-0, VTM-1, VTM-2) for films that distort during standard vertical testing.
• UL 94 horizontal burning test for the HB classification, per UL 94, IEC 60695-11-10, GB/T 5169.16, and ASTM D635.
• Glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT) tests at 550 °C to 960 °C, per IEC 60695-2-12 and IEC 60695-2-13.
• Needle-flame test, per IEC 60695-11-5 and GB/T 5169.5, simulating small fault-induced ignition sources.
• Limiting oxygen index (LOI), per ISO 4589-2, ASTM D2863, and GB/T 2406.2.
• Determination of flaming drips, ignition of the cotton indicator, afterflame time, afterglow time, and linear burning rate.

Testing Methods

UL 94 Vertical Burning Test (V-0, V-1, V-2)

A specimen is clamped vertically with its lower edge 10 mm above the top of a 20 mm (50 W) premixed methane burner flame. The flame is applied to the lower edge for 10 seconds, then removed, and the afterflame time (t1) is recorded. The flame is applied a second time for 10 seconds, and the second afterflame time (t2) and afterglow time are recorded. A layer of surgical cotton is placed beneath the specimen to detect flaming drips. Key classification criteria include:

• V-0: individual afterflame time ≤ 10 seconds, total afterflame time ≤ 50 seconds for all specimens, no ignition of the cotton indicator, and afterflame or afterglow not persisting beyond 30 seconds after the second application.
• V-1: individual afterflame time ≤ 30 seconds, total afterflame time ≤ 250 seconds, no ignition of the cotton indicator.
• V-2: same timing limits as V-1, but brief flaming drips that ignite the cotton indicator are permitted.
• In all classifications, the specimen must not burn up to the clamp and must not exhibit glowing combustion that persists beyond the specified limit.

UL 94 VTM Test for Thin Films

Very thin films may shrink, curl, or melt away from a vertical flame, producing unreliable results. In the VTM procedure, the specimen is rolled around an approximately 13 mm mandrel into a self-supporting cylindrical form, mounted vertically, and exposed to two successive applications of the burner flame. The timing and dripping criteria for VTM-0, VTM-1, and VTM-2 mirror those of the V-0, V-1, and V-2 classifications, making VTM-0 the preferred flammability designation for high-performance insulation films.

Horizontal Burning Test (HB)

The specimen is clamped horizontally, marked at 25 mm and 100 mm from the free end, and ignited at one end with a 25 mm flame. The material passes HB if burning extinguishes before the 100 mm mark, or if the measured burning rate does not exceed 75 mm/min for thicknesses below 3 mm (40 mm/min for thicknesses of 3 mm to 13 mm). The HB rating is generally suitable only for lower-risk applications within equipment enclosures.

Glow-Wire Tests

A glowing nickel-chromium wire heated to a specified temperature (550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, or 960 °C) is pressed against the specimen for 30 seconds. For GWFI, the specimen passes if it does not ignite or self-extinguishes within 30 seconds without igniting the tissue paper placed beneath it. GWIT determines the temperature 25 K above which the material does not ignite during contact. These tests are frequently required by end-product standards for components located near live parts.

Needle-Flame Test

A small 12 mm high butane flame is applied to the specimen surface or edge for a defined duration, typically 5 to 120 seconds depending on the application. The specimen passes if flames do not spread, no flaming drips ignite the underlying tissue paper, and the specimen does not sustain combustion after the flame is removed. This test simulates small ignition sources produced by component failures.

Limiting Oxygen Index (LOI)

A vertical specimen is ignited at its top in a glass chimney through which a precisely controlled mixture of oxygen and nitrogen flows. The LOI is the minimum oxygen concentration, expressed as a percentage, that supports candle-like burning for a specified time or length. Materials with an LOI of approximately 28% or higher in air (21% oxygen) are generally considered self-extinguishing. Polyimide films typically reach 36% to 38%, while untreated PET films usually fall in the 20% to 22% range.

Applications

Combustion test results for transformer insulation films directly support the following engineering and compliance activities:

• Qualification of interlayer, interwinding, and wrap insulation films for power transformers, distribution transformers, and dry-type cast-resin transformers, including fire-behavior requirements under IEC 60076-11 and GB/T 1094.11.
• Compliance demonstrations for the safety of transformers, power supply units, and switch-mode power supplies under the IEC 61558 series and related national standards.
• Electrical insulation system (EIS) evaluation and recognition under UL 1446 and IEC 61857 frameworks, where flammability data form part of the thermal and fire-performance dossier.
• Component-level material selection for audio/video, information, and communication technology equipment under IEC 62368-1, which assigns fire-hazard requirements based on the material thickness and location within the equipment.
• Incoming material inspection and supplier qualification programs for transformer and coil manufacturers.
• Research and development of flame-retardant film formulations, including halogen-free systems, where LOI and UL 94 data guide additive optimization.
• Support for product certifications and market-access documentation requiring test reports from accredited third-party laboratories.

FAQ

What flammability rating do transformer insulation films typically require?

Requirements depend on the end-product standard and the location of the film within the transformer. For insulation close to live parts or in unenclosed constructions, VTM-0 or V-0 is commonly specified. V-2, VTM-2, or HB ratings may be acceptable for films used inside compliant enclosures or at greater distances from ignition sources.

Can films thinner than 0.1 mm be tested to UL 94 V-0?

Very thin films often distort when clamped as flat strips, so the dedicated VTM procedure is used instead. VTM-0 carries equivalent severity for thin materials and is the standard route for qualifying films at thicknesses where flat vertical testing is impractical.

What is the difference between GWFI and GWIT?

GWFI (glow-wire flammability index) is the highest temperature at which the material either does not ignite or self-extinguishes within 30 seconds without igniting underlying tissue paper. GWIT (glow-wire ignition temperature) is the temperature 25 K above the highest temperature at which the material does not ignite during glow-wire contact. Both values are used by end-product standards to set fire-hazard boundaries.

How many samples are required for a combustion test campaign?

Vertical and horizontal burning tests normally require two complete sets of five specimens per thickness, plus contingency specimens. Glow-wire, needle-flame, and LOI testing each require additional specimens. Laboratories typically request 10 to 20 strips per thickness per test item, conditioned according to the governing standard.

What information appears in the final test report?

A professional report from an accredited third-party laboratory includes the applied standard and edition, material identification and thickness, conditioning details, specimen dimensions, individual and total afterflame and afterglow times, observations of dripping and cotton ignition, burner and flame parameters, the resulting flammability rating, and photographic or observational records where applicable.

Does LOI testing replace UL 94 testing?

No. LOI is a screening and material characterization method that indicates relative self-extinguishing tendency, but it does not generate a flammability rating recognized by end-product standards. Most certification programs require UL 94, IEC 60695, or equivalent GB/T tests, with LOI serving as complementary evidence of flame-retardant performance.