The vertical burning performance test is a small-scale laboratory flammability test that evaluates how quickly polymeric and plastic materials self-extinguish after exposure to a standardized flame, forming the basis of the widely recognized UL 94 V-0, V-1, and V-2 ratings. Governed by standards such as ANSI/UL 94, IEC 60695-11-10, and GB/T 2408, it is a core requirement for material qualification, quality control, and regulatory compliance of plastics used in electrical and electronic products. This article explains the test principle, specimen requirements, procedures, classification criteria, and typical applications of the vertical burning performance test.
Technical Overview
The vertical burning performance test is a laboratory-scale flammability procedure in which a vertically oriented test specimen is exposed, twice and in a defined manner, to a small, standardized gas flame. The test measures the afterflame time, afterglow time, and flaming-drip behavior of the material, allowing its relative flammability to be ranked and classified. It is one of the most frequently performed flammability tests worldwide for engineering plastics, thermoplastics, thermosets, films, foams, and composite materials.
Because the specimen is mounted vertically, flame propagation upward along the specimen is promoted by natural convection, making the vertical orientation significantly more severe than the horizontal burning test. As a result, the vertical test distinguishes self-extinguishing materials from materials that continue to burn, and it is the primary basis for material selection in applications where fire safety is critical.
The most important standards governing the vertical burning performance test include:
- ANSI/UL 94 – Tests for Flammability of Plastic Materials for Parts in Devices and Appliances (originating the V-0, V-1, V-2, 5VA, 5VB, VTM, and HF rating systems).
- IEC 60695-11-10 – Fire hazard testing, 50 W horizontal and vertical flame test methods.
- IEC 60695-11-3 – Fire hazard testing, 500 W flame test method (basis of the 5VA and 5VB ratings).
- ISO 9773 – Plastics, determination of burning behaviour of thin vertical specimens in contact with a small-flame ignition source.
- GB/T 2408 – Chinese national standard for the horizontal and vertical burning determination of plastics, technically aligned with IEC 60695-11-10, designating the vertical classes as FV-0, FV-1, and FV-2.
The principal classifications produced by the vertical burning test are summarized below, in order of increasing severity of test conditions:
- V-2 / VTM-2 / HF-2: self-extinguishing, but flaming drips or particles that ignite cotton are permitted.
- V-1 / VTM-1: self-extinguishing with longer time limits than V-0, and no flaming drips igniting cotton.
- V-0 / VTM-0: the highest 50 W rating, with the shortest afterflame times and no flaming drips igniting cotton.
- 5VB: self-extinguishing under a 500 W (125 mm) flame; burn-through of a plaque specimen is permitted.
- 5VA: self-extinguishing under a 500 W (125 mm) flame with no burn-through hole; the most severe UL 94 material rating.
Testing Samples
Rigid Plastic Bars (V Ratings)
Standard specimens for the 50 W vertical test are rectangular bars measuring 125 mm ± 5 mm in length and 13.0 mm ± 0.5 mm in width, with smooth edges and free of burrs or flash. The thickness is normally the minimum wall thickness used in the end application, and common reference thicknesses include 0.8 mm, 1.5 mm, 1.6 mm, and 3.2 mm. A full test normally uses one or two sets of five bars. Because flammability ratings are thickness-dependent, the reported classification is only valid for the thickness actually tested.
Thin and Flexible Films (VTM Ratings)
Films and sheeting thinner than approximately 0.25 mm often distort or retract under the standard bar test. Such materials are therefore evaluated using the thin-material vertical (VTM) method described in UL 94 and ISO 9773, in which the film is rolled around a mandrel into a tubular specimen. Flammability of thin specimens is strongly influenced by orientation and processing, so film grades are always qualified separately from their heavier-gauge counterparts.
Foamed Polymeric Materials (HF Ratings)
Cellular plastics used for cushioning, sealing, and thermal insulation are tested as rectangular foam blocks of defined length, width, and minimum thickness. The HF-1 and HF-2 classifications apply the vertical flame principle to foams, with HF-1 being the more severe rating in which no flaming drips are permitted.
Bars and Plaques for 5VA and 5VB Ratings
The 500 W test uses both vertical bar specimens and square plaques (typically 150 mm × 150 mm) tested in the horizontal orientation. The plaque test verifies whether the flame burns through the material, which is the decisive criterion separating 5VA from 5VB.
Finished Components and Parts
Although the vertical burning test is fundamentally a material test, injection-molded parts, laminates, and machined components are frequently submitted to accredited third-party laboratories to confirm that the material grade, colorant system, and processing conditions preserve the flammability classification claimed by the resin supplier. Different colors can affect ignition behavior, so dis-similar colors of the same material may require separate verification.
Testing Items
The vertical burning performance test measures and records the following parameters for every specimen:
- Afterflame time t1: duration of sustained flaming after the first 10 s flame application.
- Afterflame time t2: duration of sustained flaming after the second 10 s flame application.
- Total afterflame time: the sum of t1 and t2 over all specimens in a set.
- Afterglow time t3: duration of glowing combustion without flame after the second application.
- Flaming drips and particles: whether molten drops or particles fall and ignite the indicator cotton placed below the specimen.
- Flame or glow propagation: whether burning reaches the clamp or the upper portion of the specimen.
- Plaque burn-through (5VA/5VB): whether the 500 W flame creates a hole through a horizontal plaque.
- Damage length and marker thread ignition (VTM and HF methods): additional geometric criteria for thin and foamed specimens.
- Final classification: V-0, V-1, V-2, VTM-0, VTM-1, VTM-2, HF-1, HF-2, 5VA, or 5VB (or FV-0, FV-1, FV-2 under GB/T 2408).
Testing Methods
Specimen Conditioning
Before testing, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 48 hours. When specified, an additional set is aged at 70 °C ± 2 °C for 168 hours and cooled before testing, in order to evaluate materials intended for elevated-temperature service.
Apparatus and Flame Calibration
The test is performed in an enclosed, draft-free cabinet with controlled ventilation and exhaust. A Bunsen or Tirrill-type burner is fed with methane of at least 98% purity, and the gas flow is adjusted to produce a blue flame with a height of 20 mm ± 1 mm, corresponding to the standardized 50 W ignition source defined in the IEC 60695-11 series. A pad of dry, 100% surgical cotton (approximately 50 mm × 50 mm, maximum 6 mm thick) is placed 300 mm below the lower edge of the specimen to capture flaming drips. Flame height and burner geometry are verified regularly as part of laboratory quality assurance.
50 W Vertical Test Procedure (V-0, V-1, V-2)
The specimen is clamped with its long axis vertical so that its lower edge is 10 mm above the top of the burner tube. The test sequence is:
- The flame is applied to the center of the specimen's lower edge for 10 s ± 0.5 s and then withdrawn; the afterflame time t1 is recorded to the nearest 0.1 s.
- Immediately after flaming ceases, the flame is applied a second time for 10 s ± 0.5 s; the afterflame time t2 and afterglow time t3 are recorded.
- Any flaming drips igniting the cotton and any burning to the clamp are noted.
- The procedure is repeated for all five specimens in the set, and the results are compared with the classification criteria.
The classification criteria for the 50 W vertical test are:
- V-0: t1 or t2 not exceeding 10 s per specimen; total afterflame not exceeding 50 s per set; t2 plus afterglow not exceeding 30 s; no flaming drips igniting cotton; no specimen burning to the clamp.
- V-1: t1 or t2 not exceeding 30 s per specimen; total afterflame not exceeding 250 s per set; t2 plus afterglow not exceeding 60 s; no flaming drips igniting cotton; no specimen burning to the clamp.
- V-2: identical time limits to V-1, but flaming drips or particles that ignite the cotton are permitted.
If only a single specimen marginally exceeds one time criterion while all other requirements are satisfied, the standard permits a confirmation test on an additional set of five specimens before the final classification is assigned.
500 W Test Procedure (5VA and 5VB)
For the 5V ratings defined in IEC 60695-11-3 and UL 94, a more energetic 125 mm flame is applied five times for 5 s each, with 5 s intervals, to vertical bar specimens and to horizontal plaques at the center and near an edge. To pass, afterflame plus afterglow after the fifth application must not exceed 60 s and no flaming drips igniting cotton are allowed. A rating of 5VA additionally requires that the plaque show no burn-through hole, whereas 5VB permits a hole.
Thin-Material VTM Procedure
Film specimens are rolled into cylinders around a mandrel and mounted vertically with fine marker threads attached at defined heights. The small flame is applied briefly to the lower end, and the specimen is evaluated on afterflame and afterglow times, cotton ignition by drips, ignition of the marker threads, and the damaged length of the specimen. The resulting VTM-0, VTM-1, and VTM-2 ratings correspond in severity to V-0, V-1, and V-2.
Test Report
A complete test report from an accredited third-party laboratory identifies the applied standard, the material designation, color, and thickness, conditioning history, apparatus and gas details, individual and total afterflame and afterglow times for every specimen, observations of dripping and cotton ignition, and the final flammability classification valid for the tested thickness.
Applications
Vertical burning performance data are used throughout product development, material qualification, and market surveillance. Typical application areas include:
- Enclosures and structural parts of information technology and audio/video equipment evaluated under IEC 62368-1 and GB 4943.1.
- Insulating parts, connectors, switches, terminal blocks, and plug-and-socket components in household appliances covered by IEC 60335-1.
- Lamp holders, diffusers, and luminaire bodies assessed under IEC 60598.
- Printed circuit board laminates, battery housings, chargers, power adapters, and power supply enclosures.
- Flexible insulating films, adhesive tapes, and sleeving for cable management and winding insulation.
- Interior components, trim, and functional plastic parts for automotive, railway, and aerospace programs.
- Housings and accessories for medical electrical devices and industrial control equipment.
- Material datasheets and procurement specifications, where a minimum rating such as V-0 at a stated thickness is a common acceptance criterion.
Because end-product safety standards reference specific minimum flammability ratings for parts located near potential ignition sources or live parts, the vertical burning test directly supports design compliance, certification, and market access for electrical and electronic products.
FAQ
What is the difference between V-0, V-1, and V-2?
The three ratings differ in the permitted afterflame times and dripping behavior. V-0 is the strictest rating, allowing individual afterflame times of no more than 10 s and a set total of 50 s. V-1 allows longer burning times (up to 30 s per application and 250 s total). V-2 has the same time limits as V-1 but tolerates flaming drips that ignite cotton, whereas V-0 and V-1 do not.
How does the vertical burning test differ from the horizontal burning test?
In the horizontal test the specimen lies flat and flame spread along the length is measured, producing an HB classification; it is suitable for less critical parts. In the vertical test the specimen stands upright so that heat and convection assist upward flame propagation, making the test far more severe. Self-extinguishing V ratings, rather than HB, are typically mandated for parts near unattended live circuits, heat sources, or high-power components.
Which standards specify the vertical burning test?
The main specifications are ANSI/UL 94, IEC 60695-11-10 for the 50 W method, IEC 60695-11-3 for the 500 W method, ISO 9773 for thin films, and GB/T 2408 in China, which assigns the equivalent designations FV-0, FV-1, and FV-2.
How many specimens are required?
For the 50 W vertical test, one set of five bars is the basic requirement, with an additional set of five when a confirmation test is needed. The 500 W method uses five bars plus a defined number of plaques. Thin-film and foam methods specify their own specimen quantities in the relevant clauses of the standards.
Does a V-0 rating mean the material cannot burn?
No. A V-0 rating means the material extinguishes quickly after removal of a small ignition flame and does not produce flaming drips under the specified laboratory conditions. It is a relative ranking of burning behavior, not a guarantee of performance in a fully developed fire, and it does not replace large-scale fire hazard assessments.
Is the rating valid for all thicknesses of the same material?
No. Flammability classifications are thickness-dependent, and thinner sections generally burn more readily. The rating applies only to the thickness tested, so a V-0 result at 3.2 mm does not automatically qualify the same resin at 1.0 mm. Colorants and fillers can also influence results, which is why verification testing of production-colored parts is recommended by accredited third-party laboratories.
