The vertical burning length test is a standardized flammability test in which a small methane flame is applied to the lower edge of a vertically oriented specimen to measure afterflame time, afterglow time, burning length, and flaming drips. Performed in accordance with UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, it is the most widely used method for classifying plastic materials into V-0, V-1, and V-2 flammability ratings for electrical and electronic applications.
Technical Overview
The vertical burning length test (vertical flame test) is a small-scale laboratory procedure used to evaluate the relative flammability of polymeric materials. A bar-shaped specimen is clamped in a vertical position, and a standardized 20 mm blue flame is applied to its lower edge for a defined period. During and after flame application, the laboratory records the afterflame time, afterglow time, the occurrence of flaming drips that ignite a cotton indicator, and the burning length — the axial distance from the ignited lower edge of the specimen to the farthest point showing thermal damage such as charring, burning, or melting.
The test is primarily associated with the UL 94 vertical burning classification and its internationally harmonized equivalents, including IEC 60695-11-10, ISO 1210, and GB/T 2408 (the Chinese national standard). Historically, ASTM D3801 also covered vertical comparative burning of plastics before being superseded by the harmonized international methods. The results allow materials to be ranked as V-0, V-1, or V-2, with V-0 representing the most flame-retardant performance among the standard vertical ratings.
Because the specimen is oriented vertically, molten material and flaming drops fall away from the specimen under gravity, making this method significantly more severe than the horizontal burning method. As a result, the vertical test is specified for applications where a higher level of fire safety must be demonstrated, such as enclosures, printed circuit boards, and live parts of electrical equipment.
Testing Samples
Rigid Plastic Specimens
Standard test bars measure 125 ± 5 mm in length and 13.0 ± 0.5 mm in width, with the thickness selected according to the intended application. A thickness of 3.0 ± 0.2 mm is preferred for comparative purposes, but many product standards require testing at the minimum and maximum thickness used in production (commonly 0.8 mm, 1.0 mm, 1.5 mm, 1.6 mm, 2.0 mm, and 3.0 mm or greater). Edges should be smooth and free of burrs, flash, and obvious defects.
Specimen Quantity and Grouping
Two groups of five specimens (ten bars in total) are tested for each thickness and each color, since pigments and fillers can affect flammability behavior. If a specimen exhibits unusual behavior, such as ignition of the cotton or extended burning, additional sets may be required to confirm the classification.
Thin Materials and Other Product Forms
Films and thin flexible materials that cannot support themselves as rigid bars may be evaluated using specialized vertical procedures with mandrel support (VTM classifications). Cellular plastics and foams are typically assessed under separate vertical methods such as ISO 9772, while textiles are often evaluated for damaged length using vertical procedures such as GB/T 5455.
Conditioning Before Test
Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours in accordance with ISO 291 or the equivalent conditioning requirements of the applicable standard. The cotton indicator used to catch flaming drips is dried in an oven before use to ensure consistent ignition sensitivity.
Testing Items
The vertical burning length test generates the following measured and observed parameters:
- Afterflame time after the first flame application (t1), recorded in seconds for each specimen
- Afterflame time after the second flame application (t2), recorded in seconds for each specimen
- Total afterflame time for each set of five specimens
- Afterglow time, meaning the time the specimen continues to glow without a flame after the second ignition
- Burning length (damaged length), the distance from the lower edge of the specimen to the furthest point of carbonization, charring, or visible combustion damage
- Whether flaming or non-flaming drips are produced and whether drips ignite the cotton indicator placed 300 mm below the specimen
- Whether the specimen burns up to the holding clamp
- The final classification of the material: V-0, V-1, V-2, or no rating (failure to meet any vertical class)
Testing Methods
Apparatus and Test Setup
The test is conducted inside a laboratory fume hood or enclosed chamber with essentially no air draught during flame application. The key equipment includes a Tirrill-type laboratory burner with an inner tube diameter of approximately 9.4 mm, supplied with methane gas of approximately 37 MJ/m³ calorific value to produce a 20 ± 1 mm blue flame. The specimen is held by a clamp with its long axis vertical, and the lower edge of the specimen is positioned 10 ± 1 mm above the top of the burner tube. A pad of dry absorbent 100 % cotton, approximately 50 mm × 50 mm × 6 mm, is placed 300 ± 10 mm vertically below the point where the flame impinges on the specimen.
Test Procedure
The procedure for each specimen follows these steps:
- The burner is placed so that the tip of the blue flame contacts the lower edge of the specimen, and flame application is maintained for 10 ± 0.5 seconds.
- The burner is withdrawn at least 150 mm from the specimen, and the afterflame time t1 is recorded precisely.
- Immediately after the afterflame ceases, or once the specimen has stopped burning, the flame is re-applied for a second 10 ± 0.5 second interval.
- The burner is withdrawn again, and the second afterflame time t2 and any afterglow time are recorded.
- Throughout the test, the operator observes whether flaming particles or drops fall and whether they ignite the cotton indicator.
- After the specimen has cooled, the burning length is measured as the maximum extent of thermal damage from the original lower edge.
- The procedure is repeated for all five specimens in a set, then for the second set, and the results are evaluated against the classification criteria.
Classification Criteria
A material is classified based on the combined results of both sets of specimens:
- V-0: each individual afterflame time does not exceed 10 seconds, the total afterflame time per set does not exceed 50 seconds, no specimen burns to the clamp, and no flaming drips ignite the cotton.
- V-1: each individual afterflame time does not exceed 30 seconds, the total afterflame time per set does not exceed 250 seconds, no specimen burns to the clamp, and no flaming drips ignite the cotton.
- V-2: identical timing limits to V-1, but flaming drips are permitted provided the cotton is not ignited by flaming particles beyond the defined allowance, reflecting materials that melt and drip while extinguishing.
- No vertical rating: the specimen burns to the clamp, exceeds the time limits, or produces flaming drips that ignite the cotton beyond the criteria for V-2.
For more demanding applications, the related 500 W vertical burning procedures (5VA and 5VB classifications under UL 94 and IEC 60695-11-20) apply a more severe flame five times for five seconds each, with 5VA additionally requiring plaque specimens to show no burn-through.
Applications
Vertical burning length test results are used extensively across industries in which plastic parts are located near energized components or potential ignition sources:
- Enclosures and external parts of information technology and audiovisual equipment, where product safety standards such as IEC 62368-1 specify minimum V-1 or V-0 ratings
- Printed circuit board laminates, solderless connectors, terminal blocks, switches, coils, and insulating barriers inside electronic assemblies
- Household appliance components, including motor mounts, fan blades, and internal supports covered by IEC 60335
- Luminaires and lighting components governed by IEC 60598, including lamp holders and diffusers
- Battery enclosures, cell holders, and power tool housings
- Wire and cable insulation, for which related vertical flame procedures evaluate flame propagation along installed cables
- Material research and development, incoming quality control, flame-retardant formulation optimization, and regulatory compliance submissions to accredited third-party laboratories
Manufacturers frequently request vertical burning tests at the exact thickness used in the final product, because flammability ratings are only valid for the tested thickness and cannot be extrapolated to thinner sections without retesting.
FAQ
What does burning length mean in the vertical test?
Burning length, also called damaged length, is the distance measured from the lower edge of the specimen, where the flame was applied, to the furthest point of visible damage such as charring, carbonization, or melting. It provides a quantitative indication of how far combustion propagated along the specimen.
What is the difference between V-0, V-1, and V-2?
All three classifications demonstrate that the material self-extinguishes after removal of the flame. V-0 requires the fastest extinction (individual afterflame ≤ 10 s), V-1 allows ≤ 30 s per specimen, and V-2 matches the V-1 time limits but tolerates flaming drips within the standard allowance. The strictest common vertical ratings are 5VA and 5VB.
How many specimens are required for one test?
Ten specimens are generally needed: two sets of five bars for each thickness and color. Additional specimens may be required if retesting is necessary due to borderline results or anomalies such as cotton ignition.
Can a test result be transferred between thicknesses or colors?
No. Flammability ratings apply only to the specific material grade, thickness, and color tested. Thinner sections typically burn more easily, and pigments can alter flame-retardant behavior, so separate testing at each configuration is required.
How does the vertical method differ from the horizontal method?
In the horizontal method (HB rating under UL 94 and GB/T 2408), the flame spreads along the specimen against the effect of gravity and the test measures burning rate. The vertical method is far more severe because drops fall freely from the molten specimen, so it is reserved for materials and applications demanding higher fire safety performance.
What information is included in a typical test report?
A professional report from an accredited third-party laboratory includes the applied standard, material identification and thickness, conditioning details, individual and total afterflame and afterglow times, observations of dripping and cotton ignition, measured burning length values, and the final flammability classification with a clear pass or fail statement.
