Vertical burning flame resistance evaluation is a fundamental small-scale flammability test that classifies how quickly plastic and polymeric materials self-extinguish after exposure to a standardized ignition flame, producing the globally recognized V-0, V-1, and V-2 ratings of ANSI/UL 94 and IEC 60695-11-10. This article explains the test principle, specimen requirements, classification criteria, and typical applications, and shows how accredited third-party laboratories help manufacturers demonstrate compliance with international fire safety requirements.
Technical Overview
Vertical burning flame resistance evaluation, commonly known as the vertical burning test or UL 94 V test, is a laboratory procedure that measures the comparative flammability of polymeric materials when a calibrated 50 W premixed gas flame is applied twice, for 10 seconds each time, to the lower edge of a vertically oriented bar specimen. The specimen's tendency to sustain flaming combustion, to continue glowing after the flame is removed, and to release flaming droplets or particles is recorded and converted into a standardized classification.
The vertical orientation is deliberately severe: molten material and flame propagation move upward along the specimen under the buoyancy of the flame, so the method discriminates effectively between materials that self-extinguish quickly and those that continue to burn. Principal standards include ANSI/UL 94, IEC 60695-11-10, ISO 1210, ASTM D3801, and the corresponding Chinese national standards GB/T 2408 and GB/T 5169.16. Related methods include the 500 W vertical test for 5VA and 5VA/5VB classifications covered by IEC 60695-11-20, as well as complementary fire hazard techniques such as the glow-wire tests of the IEC 60695-2 series, the needle-flame test of IEC 60695-11-5, and limiting oxygen index determination under ISO 4589 and ASTM D2863.
The resulting classifications are widely referenced by end-product safety standards. For example, hazard-based standards for audio/video, information, and communication technology equipment, such as IEC 62368-1, assign minimum vertical burning classes to enclosures, fire enclosures, printed wiring boards, and insulating components. Consequently, vertical burning data are essential inputs for material selection, design verification, and market access documentation worldwide.
Testing Samples
Standard Bar Specimens for V-0, V-1, and V-2
The basic specimen is a bar measuring 125 mm in length and 13 mm in width, prepared at the relevant end-use thickness. Because flammability performance is thickness dependent, testing is typically performed at two thicknesses, commonly 3.0 mm and 1.5 mm, or at the minimum and maximum thickness intended for production. Two sets of five bar specimens are required for each thickness and each color. Specimens must be representative of the molded or extruded material, free from flash, burrs, and edge defects, and marked near the end that will be gripped by the clamp.
Thin and Flexible Materials
Films, membranes, and thin flexible sheets that distort, shrink, or curl when exposed to flame cannot be tested as flat bars. Under ANSI/UL 94, such materials are rolled longitudinally, secured with pressure-sensitive tape, and supported on a mandrel so that a stable vertical geometry is maintained. These specimens are classified as VTM-0, VTM-1, or VTM-2 using timing criteria analogous to the V classifications.
Bar and Plaque Specimens for 5VA and 5VB
The 500 W vertical test, which supports the more stringent 5VA and 5VA/5VB classifications, requires both bar specimens of 125 mm by 13 mm and square plaques of 150 mm by 150 mm at the declared thickness. The plaque test determines whether the flame burns through the material, which distinguishes 5VA from 5VB. These classifications are frequently specified for fire enclosures and components located near potential ignition sources.
Finished Components and Parts
Vertical burning evaluation is fundamentally a material test performed on standard specimens rather than on finished goods. Complete devices are assessed under their applicable end-product safety standards; however, component-level vertical flame data, together with glow-wire and needle-flame results, form the technical basis on which those standards accept polymeric parts.
Testing Items
During each vertical burning determination, the laboratory records a defined set of observations from which the final classification is derived. The principal measured and observed items are listed below.
- Afterflame time after the first flame application, designated t1
- Afterflame time after the second flame application, designated t2
- Afterglow time following cessation of flaming after the second application, designated t3
- Summed afterflame time for each five-specimen set
- Ignition of the cotton indicator by flaming droplets or falling incandescent particles
- Whether flaming or glowing reaches the clamped upper end of the specimen
- Final classification assignment, such as V-0, V-1, V-2, VTM-0, VTM-1, VTM-2, 5VA, or 5VB
Accredited third-party laboratories report these results together with material designation, color, thickness, conditioning history, and the specific standard edition applied, in test reports consistent with the quality requirements of ISO/IEC 17025.
Testing Methods
Specimen Conditioning and Apparatus
Prior to testing, specimens are conditioned for at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity. The test is conducted in a draft-free enclosure or fume hood so that air movement does not influence the flame. A laboratory burner fueled by methane or natural gas produces a calibrated blue flame approximately 20 mm in height with a nominal power of 50 W. The specimen is clamped with its long axis vertical, and the top of the burner tube is positioned 10 mm below the lower edge of the specimen so that the flame cone impinges directly on the bottom edge. A pad of dry absorbent cotton, approximately 50 mm by 50 mm, is placed 300 mm below the specimen to capture and indicate flaming drips.
Flame Application Procedure
The flame is applied to the lower edge of the specimen for 10 ± 0.5 seconds and then withdrawn. The afterflame time t1 is recorded, and any dripping onto the cotton is noted. After flaming ceases, the flame is applied a second time for 10 ± 0.5 seconds, after which the afterflame time t2 and the subsequent afterglow time t3 are measured. The procedure is repeated for all five specimens in the first set, then for the second set, and for each declared thickness and color. For the 500 W 5VA/5VB procedure, a 127 mm flame is applied five times for 5 seconds with 5-second intervals to both bars and plaques.
Classification Criteria
The classification is assigned based on the worst-performing specimen within each set. The core criteria for the common classifications are summarized below.
- V-0: each individual afterflame time does not exceed 10 seconds; the total afterflame time for each five-specimen set does not exceed 50 seconds; the combined t2 plus t3 does not exceed 30 seconds; the cotton indicator is not ignited; and flaming or glowing does not reach the clamp.
- V-1: each individual afterflame time does not exceed 30 seconds; the total afterflame time per set does not exceed 250 seconds; the combined t2 plus t3 does not exceed 60 seconds; and the cotton is not ignited by drips or particles.
- V-2: the timing criteria are identical to V-1, but brief flaming drips that ignite the cotton indicator are permitted.
- 5VA and 5VB: afterflame and afterglow do not exceed 60 seconds after the fifth flame application and no flaming drips occur; 5VA additionally requires no burn-through hole in the plaques, while 5VB permits hole formation.
If any specimen in a set fails the criteria for the intended class, the set is classified at the lower level, or the material is retested at a greater thickness. Because a material may hold different classifications at different thicknesses, test reports must always state the classification together with the tested thickness.
Applications
Vertical burning classifications are among the most frequently cited polymer performance requirements in electrical and electronic safety specifications. Typical application areas include the following.
- Plastic enclosures, fire enclosures, and decorative parts of IT, communication, and audio/video equipment under IEC 62368-1
- Printed wiring board laminates, insulating barriers, and creepage protection sheets
- Connectors, terminal blocks, coil formers, relay bodies, and switch housings
- Battery enclosures, charger housings, adapters, and power supply cases
- Household appliances, luminaires, and electric power tools
- Automotive interior electronic modules and cable accessories
- Flame-retardant resin development, incoming material control, and certification submissions
Manufacturers rely on vertical burning data to select halogenated or halogen-free flame-retardant compounds, to verify that production materials remain within specification, and to prepare compliance dossiers for markets that accept UL-, IEC-, EN-, GB-, or JIS-based flammability evidence.
FAQ
What is the difference between V-0, V-1, and V-2?
The three ratings differ in permitted afterflame duration and dripping behavior. V-0 demands the fastest self-extinguishing performance, with individual afterflame times of no more than 10 seconds and no flaming drips. V-1 allows afterflame times up to 30 seconds but still prohibits drips that ignite cotton. V-2 matches the V-1 timing limits while accepting brief flaming drips that may ignite the cotton indicator.
How does the vertical burning test differ from the horizontal HB test?
In the horizontal configuration of ANSI/UL 94, IEC 60695-11-10, and GB/T 2408, the flame is applied to the free end of a horizontally mounted specimen and burning rate is measured, producing an HB classification for materials that burn slowly or self-extinguish. The vertical configuration is far more severe because the flame and any molten material travel upward along the specimen, and it is therefore reserved for materials intended to provide meaningful resistance to ignition, such as those used in fire enclosures.
Why are two thicknesses and two sets of specimens required?
Flammability depends strongly on section thickness, since thin sections heat up and pyrolyze faster than thick ones. Testing at two thicknesses defines the thickness range over which the classification is valid. Two sets of five specimens address material and processing variability, and the classification reflects the worst-performing set, which ensures a conservative safety margin.
Are UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408 results mutually recognized?
The technical procedures and classification criteria of these standards are closely aligned, and data generated under one method are generally transferable in engineering terms. However, acceptance depends on the certification scheme and the destination market, because some regulatory systems specify a particular standard edition. Manufacturers should confirm acceptance requirements with accredited third-party laboratories before finalizing a test plan.
Does a V-0 rating mean a material cannot burn?
No. A V-0 classification is a comparative, small-scale indicator showing that a material extinguishes rapidly after removal of a small ignition flame under controlled laboratory conditions. It does not guarantee fire performance in a full-scale fire, which is why end-product standards combine vertical burning data with glow-wire, needle-flame, and other fire hazard assessments to evaluate the complete device.