Plastic Horizontal and Vertical Burning Test: Standards, Procedures, and Flammability Ratings

The plastic horizontal and vertical burning test is the core laboratory method for evaluating the ignitability, flame spread, and dripping behavior of polymeric materials used in electrical and electronic products. Based on internationally recognized standards such as UL 94, IEC 60695-11-10, IEC 60695-11-20, and GB/T 2408, the test assigns flammability classifications such as HB, V-0, V-1, V-2, 5VA, and 5VB that directly determine material acceptance in product safety certification. This article explains the sample requirements, test procedures, rating criteria, and typical applications of horizontal and vertical burning tests carried out by accredited third-party laboratories.

Technical Overview

The horizontal and vertical burning test measures the relative burning characteristics of plastic materials under controlled laboratory conditions. A standardized small-scale pilot flame, typically a 50 W or 500 W methane burner flame, is applied to a precisely dimensioned test specimen mounted in either a horizontal or a vertical orientation. The laboratory then records the afterflame time, afterglow time, linear burning rate, flame spread distance, and whether flaming droplets ignite a cotton indicator placed beneath the specimen.

Vertical exposure is significantly more severe than horizontal exposure because the combustion gases and hot flame front travel upward along the specimen surface, preheating the material ahead of the flame. Consequently, the vertical test is used to qualify materials for the most demanding applications such as fire enclosures, live parts, and unattended equipment. The horizontal test is generally applied to less critical components and provides a baseline HB (Horizontal Burning) classification.

The test does not by itself define the fire hazard of a finished product, but it provides a reproducible ranking of material flammability that is referenced by virtually all product safety standards for electrotechnical equipment. The principal standards governing the method include:

• UL 94: Tests for Flammability of Plastic Materials for Parts in Devices and Appliances
• IEC 60695-11-10: 50 W horizontal and vertical flame test methods
• IEC 60695-11-20: 500 W vertical flame test method
• GB/T 2408 and GB/T 5169.16: Chinese national adoptions aligned with the IEC methods
• ISO 1210 and ISO 9773: comparable horizontal and vertical specimen methods
• ASTM D635 and JIS C 60695-11-10: regional adoptions for horizontal and 50 W vertical testing

Testing Samples

Bar Specimens for 50 W Tests

Standard bar specimens for the horizontal (HB) and vertical (V-0/V-1/V-2) tests measure 125 mm in length by 13 mm in width, cut at the actual product wall thickness. Laboratories normally test at both the maximum and minimum intended thicknesses, with common reference thicknesses of 3.2 mm, 1.6 mm, 1.0 mm, and 0.8 mm. Edges must be smooth and free of burrs, flash, and visible defects that could influence flame behavior. Each test series typically requires two sets of five bars, one tested in the as-received conditioned state and one after additional preconditioning where the applicable product standard demands it.

Plaque Specimens for 500 W Tests

The 5VA and 5VB classifications require both bar specimens (125 mm × 13 mm) and flat plaques of approximately 150 mm × 150 mm. The plaque test is essential because it evaluates whether the flame burns through the material, which is the distinguishing criterion between 5VA and 5VB ratings.

Thin-Film and Flexible Specimens

Films and membranes below approximately 0.25 mm thickness, or materials that distort, shrink, or collapse when clamped as flat bars, may be tested rolled onto a mandrel using the VTM (vertical thin material) procedure described in ISO 9773 and the corresponding UL 94 thin-material annex.

Specimen Conditioning

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 48 hours, extended to 168 hours where the governing standard requires it. Certain end-product schemes additionally require thermal preconditioning, for example 7 days at 70 °C, to simulate aging before flammability evaluation.

Testing Items

Depending on the program selected, the laboratory reports the following results and classifications:

• Horizontal burning classification: HB rating based on linear burning rate and flame travel distance, or FH-1 to FH-4 classifications under the IEC and GB systems
• Vertical 50 W classification: V-0, V-1, or V-2 based on afterflame and afterglow times and dripping behavior
• Vertical 500 W classification: 5VA or 5VB based on five successive flame applications and burn-through resistance
• Thin-film classification: VTM-0, VTM-1, or VTM-2 for flexible materials
• Individual afterflame times (t1 and t2), total afterflame time for the specimen set, afterglow time (t3)
• Ignition of the surgical cotton indicator by flaming drips or particles
• Whether the specimen burns up to the clamp or the upper reference mark

Testing Methods

Horizontal Burning Test (HB)

The bar specimen is clamped horizontally with its long axis aligned, and reference marks are set at 25 mm and 100 mm from the free end. A burner tilted at 45 degrees applies a 20 mm blue flame to the free end for 30 seconds, or until the flame front reaches the 25 mm mark. The timer runs as the flame front travels between the 25 mm and 100 mm marks, and the linear burning rate is calculated in millimeters per minute. A material achieves the HB classification when the burning rate does not exceed 75 mm/min for thicknesses of 3.0 mm to 13 mm, or when burning ceases before the 100 mm mark; for thicknesses below 3.0 mm, the rate limit is 75 mm/min or burning must cease before the 25 mm mark. Under the IEC 60695-11-10 and GB/T 2408 framework, results are instead graded as FH-1, FH-2, FH-3, or FH-4 according to the extent of flame travel and the measured burning rate.

Vertical Burning Test, 50 W (V-0 / V-1 / V-2)

The specimen is clamped vertically with approximately 10 mm of the lower end exposed, and dry surgical cotton is placed 300 mm below the specimen. A 20 mm high blue flame is applied to the lower edge of the specimen for 10 ± 0.5 seconds, then removed, and the first afterflame time t1 is recorded. Immediately after flame extinction, the burner flame is reapplied for another 10 seconds, and the second afterflame time t2 and afterglow time t3 are recorded. Observers also note any flaming drips that ignite the cotton. The acceptance criteria are:

• V-0: each afterflame time ≤ 10 s, total afterflame time for all specimens ≤ 50 s, no cotton ignition from flaming drips, afterglow extinguished within 30 s
• V-1: each afterflame time ≤ 30 s, total afterflame time ≤ 250 s, no cotton ignition, afterglow extinguished within 60 s
• V-2: identical time criteria to V-1, but flaming drips that ignite the cotton are permitted

Vertical Burning Test, 500 W (5VA / 5VB)

The 5V test uses a higher-energy burner flame of nominally 500 W with a 125 mm overall flame height and a 40 mm inner blue cone. The flame is applied to the lower end of the vertical bar or to the center of the horizontal plaque for 5 seconds, removed for 5 seconds, and this cycle is repeated five times. To pass, the combined afterflame plus afterglow time after the fifth application must not exceed 60 seconds, and the cotton indicator must not be ignited. For the 5VA rating, the flame must not burn through the plaque, whereas 5VB permits a burn-through hole. 5VA is therefore the most demanding classification in this series and is frequently specified for fire enclosures and components near potential ignition sources.

Applications

Horizontal and vertical burning test reports are fundamental inputs to design, procurement, and regulatory compliance across many industries:

• Enclosures and internal parts of information technology and audio/video equipment qualified under IEC 62368-1 and its national adoptions
• Housings, switches, and non-metallic parts of household appliances under IEC 60335-1
• Medical electrical equipment components under the IEC 60601 series
• Connectors, terminal blocks, coil bobbins, sockets, and other wiring devices
• Printed circuit board laminates and copper-clad insulation systems
• Battery casings, charger housings, and power supply enclosures
• Automotive interior plastic components and low-voltage electrical parts
• Flame-retardant compound development, including halogen-free formulations, and incoming quality control of engineering resins such as ABS, PC, PC/ABS, PA, PBT, PP, and PVC

Manufacturers submitting products for national certification schemes routinely rely on accredited third-party laboratories to generate UL 94, IEC 60695-11-10, and GB/T 2408 test evidence demonstrating that all polymeric parts in the product meet the minimum flammability class required by the applicable end-product standard.

FAQ

What is the difference between the horizontal and vertical burning test?

The horizontal test (HB) assesses the linear burning rate of a specimen mounted flat, which is a milder exposure suitable for less critical parts. The vertical test applies the flame beneath a vertically clamped specimen, so heat and combustion gases travel up the sample, making it far more severe. Vertical ratings (V-0, V-1, V-2, 5VA, 5VB) indicate progressively higher flame-retardant performance and are required for most safety-critical applications.

What do V-0, V-1, and V-2 ratings actually mean?

The numbers define the maximum permitted afterflame times and total burning time across the specimen set. V-0 allows only 10 seconds of afterflame per ignition and 50 seconds total, V-1 allows 30 and 250 seconds respectively, and V-2 matches V-1 timing but tolerates flaming drips that ignite cotton. In practice, V-0 is the industry benchmark for self-extinguishing engineering plastics.

Are UL 94, IEC 60695-11-10, and GB/T 2408 equivalent?

The 50 W horizontal and vertical flame procedures in UL 94, IEC 60695-11-10, ISO 1210, and GB/T 2408/GB/T 5169.16 are technically aligned in burner, flame, timing, and classification criteria, so test data is broadly interchangeable for engineering purposes. However, certification bodies may require testing under the specific standard edition cited by the end-product specification, so laboratories typically issue reports referencing the exact standard requested by the client.

How many specimens are needed and how long does testing take?

A single classification normally requires ten bars (two sets of five) plus a plaque set for 5V testing. Because conditioning alone takes 48 hours or more, a complete test program from sample receipt to issued report typically requires five to ten working days, depending on the number of thicknesses and classifications evaluated.

Is a flammability rating valid for every thickness of the same material?

No. Burning behavior is strongly thickness-dependent. A V-0 rating at 3.2 mm does not guarantee the same performance at 0.8 mm, so ratings are always reported together with the minimum tested thickness. When a design uses thinner walls, the material must be requalified at the reduced thickness.

Why choose 5VA instead of V-0?

V-0 is a 50 W test lasting two flame applications, while 5VA is a 500 W test with five applications that also verifies resistance to burn-through on a plaque. Products located near high-energy ignition sources, such as fire enclosures for power supplies, are therefore specified with 5VA or 5VB materials rather than V-0.