The UL94 vertical burning test is the most widely recognized small-scale flammability test for evaluating the self-extinguishing behavior of plastic materials used in electronic, electrical, and consumer products. It assigns the internationally accepted V-0, V-1, and V-2 ratings (as well as the more severe 5VA and 5VB ratings) that regulators, OEMs, and material specifiers rely on to confirm fire safety compliance. This article explains the test principle, specimen requirements, measurement items, detailed procedure, acceptance criteria, and typical applications of the UL94 vertical burning test.
Technical Overview
UL94, published by Underwriters Laboratories (UL), is the Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. Within this standard, the vertical burning test is the most frequently specified method because it subjects specimens to a far more severe condition than the horizontal burning (HB) test: the specimen is clamped vertically, and a standard 20 mm methane flame is applied directly to its lower edge. Heated combustion gases rise along the specimen surface, preheating the material above the ignition zone and promoting flame spread, which makes the vertical orientation a stringent challenge for self-extinguishing performance.
The test evaluates how quickly a material stops burning after the ignition source is removed, whether it produces flaming drips, and whether glowing combustion persists. Based on the measured results, plastics are classified into the following ratings, listed from the most severe to the least severe requirement:
- 5VA: highest flammability rating; no burn-through of a plaqued specimen and no flaming drips.
- 5VB: no flaming drips; burn-through of the plaque is permitted.
- V-0: burning stops within 10 seconds; no flaming drips that ignite cotton.
- V-1: burning stops within 30 seconds; no flaming drips that ignite cotton.
- V-2: burning stops within 30 seconds; flaming drips that ignite cotton are permitted.
- HB: horizontal burning classification, the lowest level in the UL94 system.
The vertical burning test is technically harmonized with several international standards, including IEC 60695-11-10 (fire hazard testing, 50 W vertical flame), IEC 60695-11-20 (500 W flame for 5VA/5VB), ASTM D3801, ASTM D5048, and GB/T 2408. Test reports issued under one of these standards are generally accepted across global supply chains, making UL94 V-ratings a common contractual and regulatory requirement for polymeric components.
Testing Samples
Specimen Geometry and Quantity
For the V-0, V-1, and V-2 classifications, test specimens are bars measuring 125 mm in length and 13 mm in width, with the thickness corresponding to the minimum and maximum wall thickness intended for use in the end product, up to a maximum of 13 mm. Common evaluation thicknesses are 1.5 mm and 3.0 mm because many product safety standards specify V-ratings at these nominal dimensions. A minimum of five specimens is tested for each thickness, and an additional set of five specimens may be required if the results are borderline or inconsistent.
For the 5VA and 5VB classifications, the sample program is expanded: vertical bars of 125 mm by 13 mm are tested for afterflame and dripping behavior, and flat plaques of approximately 150 mm by 150 mm at the relevant thickness are tested to determine whether the flame burns through the material.
Conditioning Requirements
Specimens must be conditioned before testing to ensure reproducible results. The standard conditioning environment is 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 48 hours. In full UL yellow-card and component-recognition programs, additional specimens are typically heat-aged (for example, 168 hours at 70 °C) to verify that the flammability performance is retained after long-term thermal exposure. Testing is performed inside a dedicated combustion chamber or fume hood with minimal air disturbance so that air currents do not influence flame behavior.
Testing Items
The UL94 vertical burning test measures a defined set of ignition and extinguishing parameters for every specimen. The key measurement items include:
- Afterflame time t1: the duration for which the specimen continues to flame after the first 10-second flame application is withdrawn.
- Afterflame time t2: the duration for which the specimen continues to flame after the second 10-second flame application is withdrawn.
- Afterglow time t3: the duration of glowing combustion without flame after the second application, measured from the moment flaming ceases.
- Total afterflame time: the sum of all afterflame times for the complete set of specimens, compared against the limit for each rating.
- Dripping behavior: whether molten or flaming particles fall, and whether falling flaming particles ignite the cotton indicator placed below the specimen.
- Flame travel: whether afterflame or afterglow extends upward to the specimen clamp.
- Burn-through: for 5VA/5VB plaque testing, whether the applied flame creates a hole through the plaque.
- Classification determination: the final assignment of V-0, V-1, V-2, 5VA, or 5VB, or no rating, based on all recorded observations.
Testing Methods
Apparatus and Setup
The test setup consists of a laboratory burner (Bunsen or Tirrill type) supplied with methane gas of at least 98% purity, adjusted to produce a blue flame of 20 ± 1 mm in height with a well-defined inner blue cone. The specimen is clamped at its upper end with the long axis vertical, and the burner is positioned so that the tip of the inner blue cone just contacts the midpoint of the specimen's lower edge, with the top of the burner tube approximately 10 mm below the bottom of the specimen. A pad of dry, absorbent surgical cotton approximately 50 mm by 50 mm and no more than 6 mm thick is placed horizontally 300 mm below the lower edge of the specimen to capture and record flaming drips. Timing devices with a resolution of at least 0.1 second are used to record afterflame and afterglow durations, and flame calibration is verified using the specified copper-block thermocouple method.
Procedure for V-0, V-1, and V-2
The test sequence for each specimen is performed as follows:
- Apply the test flame to the lower edge of the specimen for 10 ± 0.5 seconds, then withdraw the burner and immediately start timing. Record the afterflame time t1 and note any dripping particles.
- Once flaming from the first application has completely ceased, reapply the test flame for a second period of 10 ± 0.5 seconds, then withdraw and record the afterflame time t2.
- After flaming stops, continue timing the afterglow and record t3, the time until all glowing combustion ceases.
- Observe whether any flaming drips fall onto the cotton and whether the cotton is ignited.
- Verify whether afterflame or afterglow reaches the specimen clamp, and whether the specimen is completely consumed.
- Repeat the full sequence for all five specimens, ensuring that only one specimen is tested at a time so that heat from a previous test does not affect the next result.
Acceptance Criteria
All specimens must comply with every criterion of the target rating. The acceptance requirements are:
- V-0: individual afterflame time of each specimen does not exceed 10 seconds; total afterflame time for the set does not exceed 50 seconds; no ignition of the cotton indicator; no afterflame or afterglow extending to the clamp; afterglow time after the second application does not exceed 30 seconds.
- V-1: individual afterflame time does not exceed 30 seconds; total afterflame time does not exceed 250 seconds; no ignition of the cotton indicator by flaming particles; no afterflame or afterglow reaching the clamp; afterglow time does not exceed 60 seconds.
- V-2: identical timing and clamp requirements as V-1, except that flaming particles that ignite the cotton indicator are permitted.
Elevated Severity: 5VA and 5VB Testing
The 5V methods use a significantly more energetic flame of approximately 500 W applied five times for 5 seconds each, with a 5-second interval between applications, first to vertical bar specimens and then to horizontal plaques. To achieve 5VA, the material must show no flaming drips, extinguish within 60 seconds after the fifth application, and resist burn-through of the plaque. 5VB permits burn-through of the plaque while still prohibiting flaming drips. 5VA is the highest UL94 rating and is frequently demanded for critical enclosures, such as those of information technology equipment and certain battery systems.
Applications
Because product safety standards around the world reference UL94 flammability ratings as material-level requirements, vertical burning test results are embedded in the compliance chain of nearly every industry that uses polymeric parts near potential ignition sources. Typical application areas include:
- Enclosures and external parts of power adapters, chargers, and power supplies, where IEC 62368-1 requires V-rated materials for fire enclosures.
- Housings of information technology, audio-video, and communication equipment.
- Household appliance components such as switchgear, control panels, and internal supports under IEC 60335.
- Electrical components including connectors, terminal blocks, relay bodies, circuit breakers, and switch actuators.
- Printed circuit board laminates, where standard FR-4 material is typically qualified to V-0 at reduced thickness.
- Lithium-ion battery casings, module holders, and insulation components, where V-0 or 5VA grades are commonly specified.
- Medical device enclosures under IEC 60601 and luminaire components under IEC 60598.
- Automotive interior and electrical plastics, complementing horizontal burn requirements such as FMVSS 302.
Material producers publish V-ratings on UL yellow cards, allowing design engineers to select flame-retardant grades such as V-0-rated ABS, PC, PC/ABS, PA, PBT, and PP compounds that match the minimum wall thickness of their parts.
FAQ
What is the difference between UL94 V-0, V-1, and V-2?
The three ratings differ in permitted afterflame time and dripping behavior. V-0 requires each specimen to self-extinguish within 10 seconds and forbids flaming drips that ignite cotton. V-1 allows up to 30 seconds of afterflame per specimen while still prohibiting cotton ignition. V-2 has the same timing limits as V-1 but tolerates flaming drips that ignite the cotton indicator. V-0 is therefore the most demanding and most frequently specified of the three.
How does the vertical burning test differ from the horizontal HB test?
In the horizontal test, the specimen lies flat and flame spread is measured as a burning rate, which is a milder condition because the unburned material ahead of the flame is not strongly preheated. In the vertical test, hot gases rise along the specimen and continuously heat the region above the flame, so sustained burning is far more likely. A material rated V-0, V-1, or V-2 automatically satisfies the HB requirement, but the reverse is not true.
Why is specimen thickness so important?
A UL94 rating is only valid at the thickness at which it was tested. Thicker sections dissipate heat and extinguish more readily, so a compound may achieve V-0 at 3.0 mm but only V-1, V-2, or HB at 1.5 mm. Product designers must confirm that the rating of the selected material applies to the minimum wall thickness actually used in the part, and certification bodies always verify the rating at the thinnest relevant section.
Is UL94 equivalent to IEC 60695-11-10 and GB/T 2408?
Yes, these standards are technically aligned. IEC 60695-11-10 specifies the 50 W vertical flame method corresponding to V-0, V-1, and V-2, IEC 60695-11-20 covers the 500 W method for 5VA and 5VB, and GB/T 2408 is the Chinese national standard adopting the same vertical and horizontal procedures. Minor differences may exist in conditioning details, gas purity, and reporting formats, so laboratories typically state the exact standard edition on the test report.
Does a V-0 rating mean the material is non-flammable?
No. A V-0 rating means the material extinguishes quickly after removal of a small laboratory ignition flame, under strictly defined conditions. It does not guarantee behavior in a large-scale fire, during prolonged exposure to high heat flux, or in the presence of sustained electrical arcing. UL94 ratings are one element of fire-risk assessment and must be combined with end-product testing, thermal and electrical evaluations, and appropriate fire enclosure design.