Vertical Burning Standard Testing: UL 94 V-0/V-1/V-2 Flammability Test Methods and Ratings

Vertical burning standard testing is a laboratory flammability test used to evaluate the ignition resistance and self-extinguishing properties of plastic materials, electronic components, and textiles when a small flame is applied in a vertical orientation. Based on internationally recognized standards such as UL 94, IEC 60695-11-10, GB/T 2408, and ASTM D3801, the method assigns the widely accepted V-0, V-1, and V-2 flammability ratings. This article explains the test principle, sample requirements, classification criteria, operating procedures, and typical industrial applications of vertical burning standard testing.

Technical Overview

Vertical burning standard testing is a small-scale flame test in which a rectangular specimen is clamped with its long axis vertical, and a standardized gas burner flame is applied to the lower edge of the specimen. The specimen is ignited twice for 10 seconds each time, and the laboratory records the afterflame time, afterglow time, and whether flaming drips fall onto a cotton pad positioned below the specimen. Based on these observations, the material is classified as V-0, V-1, or V-2, where V-0 represents the highest flame-retardant performance among the three grades.

The vertical orientation is deliberately severe: flames and hot gases rise along the specimen surface, preheating the material ahead of the flame front and promoting sustained combustion. As a result, vertical burning ratings are demanded by most product safety standards for plastics used in fire enclosures, live parts insulation, and structural components of electrical and electronic equipment.

The principal standards governing vertical burning standard testing include:

• UL 94 (ANSI/UL 94): Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, the origin of the V-0/V-1/V-2 classification.
• IEC 60695-11-10: Fire hazard testing, 50 W horizontal and vertical flame test methods, technically equivalent to the UL 94 V method.
• IEC 60695-11-20: 500 W flame test methods for the more severe 5VA and 5VB classifications.
• GB/T 2408 and GB/T 5169.16: Chinese national standards adopting the IEC 50 W vertical flame method.
• ASTM D3801: Comparative burning characteristics of solid plastics in a vertical position.
• ISO 9773: Vertical burning test for thin flexible plastic specimens.
• GB/T 5455, ASTM D6413, and ISO 6941: Vertical flame tests for textiles and fabrics.

Testing Samples

Rigid Plastic Materials

The standard specimen for UL 94 and IEC 60695-11-10 vertical testing measures 125 mm in length and 13 mm in width, cut at the intended commercial thickness, commonly 1.5 mm or 3.0 mm. Two sets of five specimens (10 pieces in total) are prepared for each thickness so that the classification is validated on a statistically meaningful sample. Edges should be smooth and free of burrs, flash, and stress marks, since surface defects can influence dripping behavior. Before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 48 hours.

Thin and Flexible Plastics

Films and very thin sections that distort when clamped vertically are tested according to ISO 9773, in which the specimen is rolled around a mandrel into a prescribed cylindrical shape to maintain rigidity during flame application. This allows thin insulation films, flexible printed circuit substrates, and adhesive tapes to be evaluated under a vertical configuration consistent with their end use.

Textiles and Fabrics

For vertical flame testing of fabrics, GB/T 5455 specifies specimens of 300 mm × 80 mm, while ASTM D6413 uses specimens of approximately 305 mm × 76 mm. Specimens are cut in both the length and width directions, and for regulatory programs such as children's sleepwear flammability requirements, fabrics may need to be tested after defined laundering cycles to represent the aged condition.

Foams, Compounds, and Finished Parts

Cellular plastics, thermosetting compounds, and potting materials can also be evaluated, provided representative plaques are molded at the relevant thickness. Accredited third-party laboratories frequently test finished component samples such as connectors, coil formers, and enclosures by preparing specimens from the actual production material.

Testing Items

Vertical burning standard testing measures and reports the following parameters:

• Afterflame time after the first flame application (t1), in seconds.
• Afterflame time after the second flame application (t2), in seconds.
• Total afterflame time for the set of five specimens.
• Afterglow time after the second application (t3), indicating continued glow without flame.
• Whether flaming droplets or particles fall and ignite the dry cotton indicator placed 300 mm below the specimen.
• Whether afterflame or afterglow propagates up to the holding clamp.
• Final classification: V-0, V-1, or V-2, and optionally 5VA or 5VB under the 500 W method.
• For textiles: afterflame time, smoldering time, and damaged length (char length) after a defined ignition period.

Testing Methods

The test is performed in a draft-free enclosure or fume hood with the ambient temperature controlled within the standard laboratory range. A Bunsen-type burner with a tube of approximately 100 mm length and 9.4 mm inner diameter is fueled with methane of at least 98 % purity, adjusted to produce a blue flame of 20 ± 1 mm height, corresponding to approximately 50 W output.

The general procedure follows these steps:

• Clamp the specimen vertically with its long axis vertical and the lower longitudinal edge positioned 10 mm above the top of the burner tube.
• Place a pad of dry, clean, undyed cotton approximately 50 mm × 50 mm exactly 300 mm below the lower edge of the specimen.
• Apply the flame to the center of the lower edge of the specimen for 10 seconds, then withdraw the burner and record the afterflame time t1.
• As soon as flaming stops, immediately reapply the flame for a second 10-second period, then withdraw and record afterflame time t2 and afterglow time t3.
• Observe and record cotton ignition by flaming drips and any burning up to the clamp.
• Repeat the sequence on the remaining specimens and evaluate the complete data set against the classification criteria.

The classification criteria under UL 94 and IEC 60695-11-10 are:

• V-0: each afterflame time not exceeding 10 s; total afterflame time for the five specimens not exceeding 50 s; no ignition of the cotton by flaming drips; no afterflame or afterglow reaching the clamp.
• V-1: each afterflame time not exceeding 30 s; total afterflame time not exceeding 250 s; no ignition of the cotton by flaming drips; no afterflame or afterglow reaching the clamp.
• V-2: each afterflame time not exceeding 30 s; total afterflame time not exceeding 250 s; brief flaming drips that may ignite the cotton are permitted; no afterflame or afterglow reaching the clamp.

For applications requiring the highest flame performance, the 500 W method of IEC 60695-11-20 and UL 94 subjects bar specimens and plaques to a 125 mm flame applied five times for 5 seconds each. Materials that extinguish within 60 seconds after the fifth application without damaging drips qualify as 5VA, or 5VB when a hole through the plaque is tolerated without flaming ignition.

For textile materials, the vertical method of GB/T 5455 or ASTM D6413 applies a defined igniter flame, typically for 12 seconds, to the lower edge of a vertically hung strip. Afterflame time, smoldering time, and char length are recorded, and pass or fail is judged against the applicable product specification.

Applications

Vertical burning ratings are fundamental compliance evidence across many industries:

• Consumer electronics and IT equipment: fire enclosure plastics for monitors, adapters, power supplies, and batteries are commonly required to meet V-1 or better under IEC 62368-1 and related safety standards.
• Household appliances: insulating parts, switches, terminal blocks, and motor components under IEC 60335-1 must satisfy minimum flammability grades such as V-0, V-1, V-2, or HB depending on the part function and proximity to live parts.
• Electrical components: connectors, relay housings, circuit breaker parts, PCB laminates, and cable accessories are specified with V-0 material as standard practice.
• Automotive engineering: interior non-metallic components, charging infrastructure parts, and high-voltage insulation components use vertical burning data as part of material qualification, complementing horizontal burning requirements such as FMVSS 302-equivalent regulations.
• Medical devices: equipment enclosures and insulating parts under IEC 60601 rely on vertical burning grades to support fire risk management.
• Textiles and apparel: protective clothing, children's sleepwear, curtains, and furnishings are assessed with vertical flame standards such as GB/T 5455, ASTM D6413, and ISO 6941.
• Research and quality control: compounders and molders use vertical burning tests during flame-retardant formulation development, incoming material inspection, and periodic production verification in accredited third-party laboratories.

FAQ

Which vertical burning rating is the best, V-0, V-1, or V-2?

V-0 is the strictest of the three grades, allowing the shortest afterflame times and tolerating no flaming drips. V-1 permits longer burn times but still prohibits dripping that ignites cotton, while V-2 accepts brief flaming drips. Product safety standards typically dictate the minimum acceptable grade for each application.

Are UL 94, IEC 60695-11-10, GB/T 2408, and ASTM D3801 mutually equivalent?

These standards describe the same 50 W vertical flame method with minor editorial differences, and test reports from accredited third-party laboratories are widely accepted across multiple standard designations. However, certificates should always cite the exact standard edition and the tested thickness.

Does specimen thickness affect the rating?

Yes. A vertical burning rating applies only to the thickness actually tested. Flame retardancy is generally more difficult to achieve at thinner wall sections, so manufacturers often qualify materials at both the thinnest and nominal production thicknesses.

What is the difference between vertical and horizontal burning tests?

In the horizontal configuration, the flame is applied to one end of a specimen lying flat, and the material is typically graded HB if burning proceeds slowly or self-extinguishes. The vertical test is far more severe because the flame attacks the lower edge and buoyant hot gases preheat the specimen above, which is why vertical grades are required for parts near ignition sources.

How many specimens are required and how long does testing take?

The vertical classification normally uses two sets of five bar specimens per thickness. Including 48 hours of conditioning, burner calibration, and report preparation, accredited third-party laboratories typically complete a standard vertical burning evaluation within three to seven working days.

Can vertical burning results predict performance in a real fire?

No. Vertical burning standard testing is a small-scale screening method for comparing the relative ignitability of materials. It does not measure heat release, smoke production, or toxic gas evolution, and it cannot by itself predict the behavior of a complete product in a full-scale fire. Additional fire hazard assessments may be required depending on the application and regulatory framework.