Vertical Burning Flammability Resistance Evaluation: Standards, Methods, and Rating Criteria

Vertical burning flammability resistance evaluation is a fundamental fire-safety test that measures how quickly a material ignites, burns, and self-extinguishes when a standardized flame is applied to a vertically mounted specimen. Governed by widely recognized standards such as UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408, this evaluation assigns classifications such as V-0, V-1, V-2, 5VA, and 5VB that are essential for product certification, regulatory compliance, and material selection. This article explains the test principle, sample requirements, evaluation items, test procedures, classification criteria, and typical applications of vertical burning flammability testing.

Technical Overview

Vertical burning flammability resistance evaluation is a laboratory fire-hazard screening test in which a specimen is clamped in a vertical orientation and exposed to a small, standardized ignition flame for a defined period. The test quantifies afterflame time, afterglow time, and the tendency of the material to produce flaming drips, and the results are converted into a material flammability rating. The vertical orientation is deliberately more severe than horizontal testing: combustion gases rise along the specimen surface and preheat the material above the flame front, so only genuinely self-extinguishing materials achieve the highest ratings.

The most widely applied vertical burning standards include UL 94 (United States), IEC 60695-11-10 and IEC 60695-11-20 (international), EN 60695-11-10 and EN 60695-11-20 (Europe), ISO 1210 (rigid plastics), ISO 9773 (thin flexible materials), ASTM D3801 and ASTM D5048 (United States), GB/T 2408, GB/T 5169.16, and GB/T 5169.17 (China), and the corresponding JIS adoptions of the IEC standards in Japan. These standards are technically harmonized, so a specimen tested by an accredited third-party laboratory to one scheme is broadly comparable to the others.

The 50 W vertical test (UL 94 V, IEC 60695-11-10, ISO 1210, ASTM D3801) yields the classifications V-0, V-1, and V-2, in decreasing order of flame resistance. The more severe 500 W vertical test (UL 94 5VA/5VB, IEC 60695-11-20, ASTM D5048) produces the classifications 5VA and 5VB, which represent the highest levels of small-scale ignition resistance for polymeric materials. Supplementary schemes cover thin films (VTM-0/VTM-1/VTM-2), cellular plastics (HF-1/HF-2), and textile fabrics.

Testing Samples

Rigid Plastic Bars for the 50 W Vertical Test

Specimens are bars of 125 mm × 13 mm, cut or injection molded at the thicknesses representative of the end-use part. Because flammability performance depends strongly on wall thickness, materials are normally evaluated at both their minimum and maximum commercial thicknesses. Two sets of five bars per thickness are prepared, with an additional replacement set reserved if any specimen produces an anomalous result.

Plaques and Bars for the 500 W Vertical Test

The 5VA/5VB evaluation combines 125 mm × 13 mm bars with flat plaques of approximately 125 mm × 125 mm at the relevant thickness. The plaque test determines whether the flame burns through the material, which is the distinguishing criterion between 5VA (no burn-through) and 5VB (burn-through permitted).

Thin and Flexible Materials

Films, tapes, and thin non-rigid sheets that distort or shrink when clamped as flat bars are tested using the VTM procedure of ISO 9773 and the corresponding UL 94 annex, in which the specimen is supported on a mandrel and rated VTM-0, VTM-1, or VTM-2.

Cellular and Foam Materials

Flexible and rigid foams are evaluated with specimens of approximately 150 mm × 50 mm using the HF-1, HF-2, or HBF classifications defined in ISO 9772, ASTM D4986, and the foam section of UL 94, since thin-bar geometry is unsuitable for low-density cellular materials.

Textile Fabrics

Vertical flame resistance of woven and knitted fabrics is assessed with rectangular specimens using dedicated textile methods such as ASTM D6413 and GB/T 5455, which measure afterflame time, smoldering time, and charred or damaged length after a timed vertical ignition.

Testing Items

During a standard 50 W vertical burning test, the laboratory records the following evaluation items for every specimen:

• Afterflame time after the first flame application (t1), recorded in seconds.
• Afterflame time after the second flame application (t2), recorded in seconds.
• Afterglow time after the second application (t3), defined as continued glowing without flame.
• Total afterflame time for the set of five specimens, calculated as the sum of t1 and t2 values.
• Occurrence of flaming or non-flaming drops and particles, and whether they ignite the dry cotton indicator placed beneath the specimen.
• Whether afterflame or afterglow propagates up to the specimen holding clamp.
• For 5VA/5VB testing, whether the flame burns a hole through the plaque specimen.
• Ancillary observations such as shrinkage, deformation, sooting, and, for textiles, damaged length after the test.

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut or molded with clean, square edges free of burrs and stress concentrations, and their dimensions are verified before testing. Unless otherwise specified, all specimens are conditioned for a minimum of 48 hours at 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291 or ASTM D618. The 100 % cotton indicator pads are dried in an oven before use so that they respond reproducibly to any flaming drip.

Apparatus and Test Setup

Testing is performed in a fume hood or draft-free enclosure with the exhaust turned off during the burn, because even slight air movement alters flame geometry. A Bunsen or Tirrill burner fueled by methane delivers a calibrated blue flame of 20 ± 1 mm, corresponding to a nominal 50 W premixed flame. The bar specimen is clamped with its long axis vertical and its lower edge 10 ± 1 mm above the top of the burner tube. A cotton pad of approximately 50 mm × 50 mm is positioned 300 mm below the bottom edge of the specimen to catch flaming drips.

Test Procedure for the 50 W Vertical Test

• Mount the conditioned specimen vertically on the clamp and confirm the 10 mm burner-to-specimen distance.
• Apply the flame to the center of the specimen's lower edge for 10 ± 0.5 seconds, then withdraw it and record the afterflame time t1.
• As soon as afterflame ceases, apply the flame a second time for 10 ± 0.5 seconds, withdraw it, and record t2 and the subsequent afterglow time t3.
• Observe and document any flaming drips, ignition of the cotton indicator, and burning to the clamp.
• Repeat the sequence for all five specimens in the set, then test the second set at the other thickness.

Test Procedure for the 500 W Vertical Test

For 5VA and 5VB classification, a burner flame of approximately 125 mm with a 40 mm blue inner cone is applied five times for 5 seconds each, at 5-second intervals, to the lower edge of the bar specimens and to the corner of the plaque specimens mounted at a defined angle. The laboratory records afterflame and afterglow after the fifth application, flaming drips, and whether the plaque is burned through.

Classification Criteria

For the 50 W vertical test, a set of specimens meets a given class when all of the following conditions are satisfied:

• Individual afterflame time after each application: V-0 requires 10 seconds or less; V-1 and V-2 require 30 seconds or less.
• Total afterflame time for the five-specimen set: V-0 requires 50 seconds or less; V-1 and V-2 require 250 seconds or less.
• Combined afterflame plus afterglow after the second application: V-0 requires 30 seconds or less; V-1 and V-2 require 60 seconds or less.
• No specimen may exhibit afterflame or afterglow extending to the holding clamp.
• Flaming drips that ignite the cotton indicator are not permitted for V-0 or V-1; V-2 permits brief flaming drips, including those that ignite the cotton.

If a set produces borderline or anomalous behavior, such as a single specimen exceeding the limit, a replacement set of five specimens may be tested and must fully satisfy the criteria. For the 500 W test, both 5VA and 5VB require no afterflame or afterglow exceeding 60 seconds after the fifth application and no flaming drips igniting the cotton; 5VA additionally requires that no hole is burned through the plaque, making it the most demanding small-flame classification for polymeric materials.

Applications

Vertical burning flammability ratings are among the most frequently cited material properties in electrical, electronic, and industrial product specifications. Typical application scenarios include:

• Enclosures, bezels, connectors, switches, and structural insulating parts for consumer electronics and information technology equipment, where product safety standards generally require V-1 or, more commonly, V-0 rated materials.
• Printed circuit board laminates, terminal blocks, coil bobbins, and battery module components that must self-extinguish to limit fire propagation inside energized assemblies.
• Household appliance components such as internal housings, fan blades, and wiring accessories subject to the material requirements of international appliance safety standards.
• Medical device enclosures and laboratory instrument parts where flammability performance supports overall device safety compliance.
• Automotive interior and electrical components, where vehicle manufacturers and their tier suppliers specify vertical burning classifications in addition to interior flammability regulations.
• 5VA and 5VB rated engineering plastics for high-risk locations such as parts positioned near unprotected arc sources or within power distribution equipment.
• Textile products, including protective workwear, curtains, and furnishings, evaluated by vertical flame methods to demonstrate flame resistance.
• Research, development, and quality control programs, where compounders and molders verify that flame-retardant formulations, colorants, and regrind content do not degrade the certified flammability rating.

Test reports issued by accredited third-party laboratories operating under ISO/IEC 17025 accreditation are accepted by regulators, certification bodies, and global supply chains as objective evidence of flammability performance.

FAQ

What is the difference between V-0, V-1, and V-2?

All three classifications describe self-extinguishing behavior after two 10-second flame applications, but the acceptance limits tighten progressively. V-0 allows the shortest afterflame times (10 seconds per specimen, 50 seconds total per set) and no flaming drips; V-1 allows longer afterflame times (30 seconds per specimen, 250 seconds total) and still no flaming drips; V-2 matches the V-1 time limits but tolerates brief flaming drips that may ignite the cotton indicator.

Which standards are technically equivalent to the UL 94 vertical burning test?

IEC 60695-11-10, EN 60695-11-10, ISO 1210, ASTM D3801, GB/T 5169.16, and GB/T 2408 define the 50 W vertical test with essentially the same apparatus, procedure, and V-0/V-1/V-2 criteria. IEC 60695-11-20, EN 60695-11-20, ASTM D5048, and GB/T 5169.17 correspond to the 5VA/5VB 500 W test.

Why must materials be tested at both minimum and maximum thickness?

Flammability performance improves as wall thickness increases, because thicker sections conduct heat away more effectively and resist burn-through. A vertical burning rating is therefore only valid for the thickness tested, and end products may only claim the classification demonstrated at a thickness equal to or greater than the certified minimum, which is why certification programs evaluate the full commercial thickness range.

What commonly causes a V-0 formulation to fail to V-2?

The most frequent cause is flaming drips that ignite the cotton indicator. Even if afterflame times meet the strict V-0 limits, molten burning particles falling from the specimen and igniting the cotton automatically restrict the result to V-2, which is why drip suppression is a key design element of flame-retardant formulations.

How long does vertical burning flammability testing take?

The burn sequence itself is short, but conditioning of at least 48 hours, multi-thickness testing, replacement sets, and report preparation mean that a routine single-material program is typically completed within one to two weeks. Programs covering many thicknesses, colors, or regrind levels require additional time.

Can test results from one country be used for products sold in another?

Yes. Because the major standards are harmonized, reports from accredited third-party laboratories testing to UL 94, IEC 60695-11-10, ISO 1210, or GB/T 2408 are widely accepted internationally, although some markets or certification schemes may require testing under a specifically designated standard, so the target market should be confirmed before commissioning the evaluation.