The vertical burning test is a standardized laboratory method for evaluating the flammability of plastic and polymeric materials by applying a controlled small flame to a vertically mounted specimen and measuring its afterflame time, afterglow behavior, and dripping characteristics. Governed by widely recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, and ASTM D3801, this test assigns critical flammability classifications (V-0, V-1, and V-2) that are essential for material selection in electronics, appliances, and automotive applications. This guide explains the test principle, specimen requirements, procedures, classification criteria, and typical applications of vertical burning testing.
Technical Overview
The vertical burning test is one of the most widely adopted flammability evaluation methods for polymeric materials. In this test, a standardized 20 mm blue flame generated from a methane or natural-gas burner is applied to the lower edge of a specimen that is clamped in a vertical orientation. The test quantifies how long the specimen continues to burn after the ignition source is removed (afterflame time), how long it glows without a flame (afterglow time), and whether flaming drips or particles fall and ignite a cotton indicator placed beneath the sample.
The vertical orientation makes this test significantly more severe than the horizontal burning test, because the flame propagates upward against the specimen and preheats the material above the burning zone, accelerating combustion. As a result, the vertical burning classification (V-0, V-1, or V-2) is generally regarded as the key benchmark for flame-retardant plastics used in consumer electronics, electrical equipment, and other safety-critical products.
The principal standards covering vertical burning testing include UL 94, IEC 60695-11-10, ISO 1210, GB/T 2408, GB/T 5169.16, and ASTM D3801 for the 50 W vertical test; IEC 60695-11-20, GB/T 5169.17, and ASTM D5048 for the more severe 500 W test (5VA/5VB classifications); and ASTM D4804 and related methods for thin-film materials (VTM classifications). These standards are technically harmonized in most respects, allowing results to be recognized across major global markets.
Testing Samples
Rigid Plastic Specimens (V-Rated Testing)
For the standard 50 W vertical burning test, specimens are cut to 125 mm in length and 13 mm in width, with the thickness corresponding to the minimum and maximum wall thicknesses of the intended application. A complete test set consists of at least five specimens per thickness, and additional sets are prepared if retesting is required. Specimens must be free of burrs, flash, and sharp edges that could influence flame behavior, and reference marks are typically applied at defined intervals to observe the extent of burning.
Thin-Film Specimens (VTM Testing)
Flexible films and membranes thinner than approximately 0.25 mm cannot be tested in the standard configuration because they distort or shrink in the vertical position. For these materials, the specimen is wrapped around a mandrel and supported in a vertical frame, and it is evaluated against the VTM-0, VTM-1, or VTM-2 classifications in accordance with UL 94 and ASTM D4804.
Plaque Specimens (5VA and 5VB Testing)
The 5VA and 5VB classifications require additional square plaques (typically 125 mm × 125 mm) in addition to bar specimens, because the 500 W flame is applied to both the edge of the bar and the flat surface of the plaque to assess whether the flame burns through the material. These classifications represent the most demanding flammability requirements for enclosures of electronic devices.
Cellular and Foam Materials
Foam materials intended for decorative or acoustic applications are generally assessed using separate horizontal and vertical foam burning procedures covered by ISO 9772, ISO 9773, and corresponding UL 94 foam classifications (such as HF-1 and HF-2), since the burning behavior of cellular polymers differs substantially from that of solid plastics.
Testing Items
During a vertical burning test, the following parameters are recorded and evaluated for each specimen:
- Afterflame time after the first flame application (t1): the duration for which the specimen continues to burn with a visible flame after the ignition source is first removed.
- Afterflame time after the second flame application (t2): the duration of continued flaming after the second and final flame exposure.
- Afterglow time (t3): the duration of glowing combustion without a flame following the cessation of afterflame after the second application.
- Total afterflame time: the sum of all individual afterflame times across the complete set of five specimens.
- Flaming drips and particles: whether incandescent or flaming material falls from the specimen and whether it ignites the dry absorbent cotton pad placed 300 mm below the specimen.
- Flame spread and burn-through: whether the specimen burns up to the clamp, or, in the case of 5VA/5VB plaque testing, whether the 500 W flame perforates the plaque.
- Final classification: assignment of V-0, V-1, V-2, or no classification (NC) based on the combined results of all specimens in the set.
Testing Methods
Apparatus and Test Conditions
The test is conducted inside a draught-free enclosure or laboratory fume hood, with the ambient air movement controlled so that it does not disturb the flame. A Bunsen or Terrill burner with an inner tube diameter of approximately 9.5 mm is supplied with technical-grade methane or natural gas adjusted to produce a blue flame of 20 mm in height. A layer of 100 percent absorbent surgical cotton, approximately 50 mm × 50 mm and no thicker than 6 mm, is positioned 300 mm below the bottom edge of the specimen to capture and indicate ignition by flaming drips. Laboratory ambient conditions are maintained at 23 ± 2 °C with a relative humidity of 50 ± 5 percent for specimen conditioning, which lasts at least 48 hours.
Specimen Preparation
Specimens are marked with reference lines to permit observation of the burned length, clamped so that approximately 10 mm of the upper end is held in the clamp and the longitudinal axis is vertical, with the lower end positioned 10 mm above the top of the burner. Any flash or surface contamination is removed, and the complete set of five conditioned specimens is tested in sequence.
Test Procedure
The standard test sequence is performed as follows:
- Apply the 20 mm blue flame to the center of the lower edge of the specimen for 10 seconds, then withdraw the burner.
- Record the afterflame time t1 and note whether any dripping material ignites the cotton.
- Once the afterflame from the first application has ceased, immediately reapply the flame for a further 10 seconds and withdraw the burner again.
- Record the afterflame time t2, the afterglow time t3, and any dripping or cotton ignition for the second application.
- Repeat the procedure for all five specimens in the set and compile the results for classification.
Classification Criteria
Based on the recorded data, specimens are classified in accordance with the following criteria:
- V-0: afterflame time for each specimen does not exceed 10 seconds; the total afterflame time for the set does not exceed 50 seconds; no flaming drips ignite the cotton; and afterglow does not persist beyond 30 seconds.
- V-1: afterflame time for each specimen does not exceed 30 seconds; the total afterflame time for the set does not exceed 250 seconds; no flaming drips ignite the cotton; and afterglow does not persist beyond 60 seconds.
- V-2: identical timing criteria to V-1, except that flaming drips are permitted provided they do not fully consume the cotton indicator in a manner that fails the standard, as the V-2 rating specifically allows brief ignition of the cotton by flaming particles.
- No classification (NC): any set of results that fails to satisfy the criteria for V-2 is considered unclassified, indicating that the material does not meet the minimum vertical burning requirements.
Applications
Vertical burning test results are prerequisites for material acceptance in numerous regulated and safety-critical industries. Typical applications include:
- Consumer electronics and information technology equipment: enclosures, internal brackets, connectors, and printed circuit board substrates for laptops, chargers, displays, and power adapters.
- Household appliances: housings, switches, terminal blocks, and insulating parts of refrigerators, washing machines, air conditioners, and kitchen appliances.
- Electrical and power distribution products: breaker housings, relay bodies, lamp holders, socket components, and wire and cable accessories.
- Automotive and new energy sectors: interior trim components, battery module housings, busbar insulation, and charging connector materials.
- Medical devices and instrumentation: equipment enclosures and structural polymers requiring documented flame-retardant performance.
- Research, development, and quality control: formulation optimization of flame-retardant compounds, incoming material inspection, and ongoing supplier verification within ISO/IEC 17025-accredited third-party laboratories.
FAQ
What is the difference between V-0, V-1, and V-2 classifications?
The three ratings differ mainly in permitted burning duration and dripping behavior. V-0 is the most stringent, allowing individual afterflame times of no more than 10 seconds and no flaming drips that ignite cotton. V-1 allows longer afterflame times of up to 30 seconds per specimen with no igniting drips, while V-2 accepts the same burning times as V-1 but permits flaming drips that may briefly ignite the cotton indicator.
How does the vertical burning test differ from the horizontal burning test?
In the horizontal burning test, the specimen lies flat and a burn rate is measured, which is suitable for materials used in less demanding positions. In the vertical test, the specimen stands upright, so heat and flame travel upward and continuously preheat the material, producing a far more severe condition. Consequently, a vertical rating is required for most enclosures and live parts of electrical equipment, whereas horizontal classifications (such as HB) are accepted only for less critical components.
Which specimen thickness should be tested?
Materials should be tested at the minimum thickness at which they will be used, because thinner sections burn more readily. For a complete evaluation, manufacturers typically test both the thinnest and thickest intended thicknesses, since the flammability classification is thickness-dependent and can only be claimed for the tested geometries.
Why is the flame applied twice during the test?
The double application simulates repeated ignition from an external source and evaluates whether the material, already heated by the first exposure, can still self-extinguish after a second ignition. This reveals whether flame retardancy is sustained under thermal stress, which is a more realistic representation of potential fault conditions inside electrical equipment.
Which standards are equivalent to one another?
UL 94, IEC 60695-11-10, ISO 1210, GB/T 5169.16, GB/T 2408, and ASTM D3801 define substantially equivalent 50 W vertical test procedures and classifications. Similarly, the 500 W procedures of IEC 60695-11-20, GB/T 5169.17, ASTM D5048, and the corresponding UL 94 sections are aligned, allowing test reports from accredited third-party laboratories to be accepted across different regional markets.
Is vertical burning testing mandatory for product compliance?
For most electrical and electronic products, product safety standards require that polymeric parts carrying live connections or forming enclosures be made of materials with a specified minimum flammability rating, commonly V-0 or V-1 depending on the application. Manufacturers therefore rely on vertical burning test reports from accredited third-party laboratories as objective evidence during product certification, customs inspection, and customer audits.
