V-0 Flame Retardancy Rating Test: UL 94 Vertical Burn Testing, Criteria, and Applications

The V-0 flame retardancy rating test is a vertical burning test defined by the UL 94 standard that evaluates how quickly plastic materials self-extinguish after exposure to a small open flame. As the most stringent vertical classification among the V-0, V-1, and V-2 grades, it is widely required for electronic enclosures, PCB laminates, connectors, and charging equipment. This article explains the sample requirements, testing items, procedures, classification criteria, and practical applications of the V-0 flame retardancy rating test.

Technical Overview

The V-0 flame retardancy rating test is part of the UL 94 standard, "Tests for Flammability of Plastic Materials for Parts in Devices and Appliances," and is technically aligned with IEC 60695-11-10, EN 60695-11-10, ASTM D3801, and GB/T 5169.16. The test assesses the ability of a polymeric material to extinguish on its own after a standardized methane flame is applied twice to a vertically mounted specimen. A V-0 classification demonstrates that flaming stops within 10 seconds after each flame application, that the combined afterflame time of all specimens remains within strict limits, and that no flaming drips ignite a cotton indicator placed below the sample.

Within the UL 94 vertical burning hierarchy, materials are ranked according to their burning behavior:

• V-0: the highest vertical grade, with the shortest permitted afterflame time and no flaming drips that ignite cotton.
• V-1: slower self-extinguishing behavior, but still no flaming drips that ignite cotton.
• V-2: afterflame times identical to V-1, but brief flaming drips that ignite cotton are permitted.
• HB: a horizontal burning grade for materials that either extinguish slowly or burn slowly in a horizontal orientation.

Testing Samples

Specimen Dimensions and Preparation

Test specimens are rectangular bars measuring 125 mm in length and 13 mm in width. The thickness is determined by the intended end use or by agreement between the manufacturer and the testing laboratory, and common evaluation thicknesses include 1.5 mm and 3.0 mm. Specimens must be cut cleanly, with burrs removed and smooth edges, and each bar should represent the color, finish, and molding condition of the final part whenever possible.

Number of Specimens

A complete evaluation normally uses two sets of five bar specimens. If results are borderline or inconsistent, an additional set of five specimens is conditioned for 168 ± 2 hours at 70 ± 2 °C and then tested to confirm the final classification. When a rating is claimed at multiple thicknesses, each thickness is evaluated separately with its own specimen sets.

Conditioning Requirements

Prior to testing, specimens are conditioned for at least 48 hours at a temperature of 23 ± 2 °C and a relative humidity of 50 ± 5 %. Proper conditioning is essential because moisture content and surface contamination can significantly influence ignition and dripping behavior.

Testing Items

During the V-0 flame retardancy rating test, the laboratory records the following parameters for every specimen:

• Afterflame time t1, measured after the first 10-second flame application.
• Afterflame time t2, measured after the second 10-second flame application.
• Afterglow time t3, measured after flaming ceases following the second application.
• Occurrence of flaming drips or non-flaming drips released by the specimen.
• Ignition of the dry absorbent cotton indicator positioned 300 mm below the specimen.
• Total afterflame time, calculated as the sum of t1 and t2 for all five specimens in a set.
• Whether afterflame or afterglow propagates up to the holding clamp of the test stand.

Testing Methods

Test Apparatus

The test is performed in a draft-free enclosure or fume hood using the following equipment:

• A laboratory Tirrill-type burner fueled by methane or natural gas of at least 98 percent purity, adjusted to produce a 20 ± 1 mm blue flame corresponding to approximately 500 W.
• A ring stand with a clamp that holds the specimen in a vertical position with its lower edge 10 mm above the top of the burner tube.
• A pad of 100 percent absorbent cotton, approximately 50 × 50 mm in size and weighing no more than 0.08 g, placed 300 ± 10 mm below the bottom edge of the specimen.
• A timing device with a resolution sufficient to record afterflame and afterglow times to the nearest second.

Test Procedure

The test sequence for each specimen follows these steps:

• Mount the conditioned specimen vertically on the stand and confirm the 10 mm distance between the burner top and the lower edge of the bar.
• Apply the 20 mm blue flame to the center of the lower edge for 10 ± 0.5 seconds, then withdraw the burner and record the afterflame time t1.
• As soon as afterflaming from the first application ceases, immediately reapply the flame for another 10 ± 0.5 seconds, withdraw the burner, and record the afterflame time t2 and the subsequent afterglow time t3.
• Observe whether flaming or glowing particles drip and whether any drips ignite the cotton indicator.
• Repeat the sequence for all specimens in the set and calculate the total afterflame time.

Classification Criteria for V-0

To qualify for the V-0 rating, all specimens in a set must satisfy the following requirements:

• Afterflame time for each individual specimen does not exceed 10 seconds after either flame application.
• Total afterflame time for the five specimens does not exceed 50 seconds.
• Afterflame plus afterglow time for each specimen after the second application does not exceed 30 seconds.
• No specimen exhibits afterflame or afterglow that reaches the holding clamp.
• No flaming drips ignite the cotton indicator placed below the specimen.

For comparison, V-1 allows individual afterflame times up to 30 seconds with a total of 250 seconds while still prohibiting cotton ignition, whereas V-2 shares the timing limits of V-1 but permits brief flaming drips that may ignite the cotton.

Applications

The V-0 rating is a fundamental safety requirement for plastic materials used in electrical and electronic products, because it directly reduces the risk of fire propagation inside devices and enclosures. Typical applications include:

• Printed circuit board laminates and copper-clad materials, where standard FR-4 substrates are typically certified as V-0.
• Enclosures and housings for power adapters, chargers, power supplies, servers, and networking equipment.
• Connectors, terminal blocks, switches, relay bodies, and other current-carrying plastic components.
• Battery modules, battery packs, and related structural components in consumer and energy storage products.
• Household appliances, LED lighting housings, fan blades, and medical device enclosures.
• Automotive and electric vehicle components located near electrical systems, wiring, and power electronics.

FAQ

What does a V-0 rating actually mean?

A V-0 rating means that a vertically mounted test bar stops burning within 10 seconds after each of two 10-second flame applications, that the combined afterflame time of five specimens is 50 seconds or less, and that the material does not release flaming drips capable of igniting cotton. It is the highest classification in the UL 94 vertical bar test series.

Is a V-0 rated material non-flammable?

No. V-0 materials are self-extinguishing, not non-combustible. They can ignite when exposed to a sufficiently strong ignition source, but the rating demonstrates that combustion stops quickly once the source is removed and that burning droplets do not spread the fire to materials below.

Does the V-0 rating depend on thickness?

Yes. Flame retardancy is thickness dependent, and a material may achieve V-0 at one thickness while only reaching V-1 or V-2 at a thinner section. Certificates and Yellow Cards always state the minimum thickness at which the rating applies, so designers must verify that their part wall thickness meets or exceeds the certified value.

How is the V-0 test different from the 5VA and 5VB tests?

The 5VA and 5VB methods are more severe than the V-0 procedure. They combine bar specimens and larger plaques, use a higher-energy 125 mm flame with five applications, and in the case of 5VA require that the plaque is not burned through. A 5VA material automatically satisfies the V-0 requirements.

Which standards are considered equivalent to the V-0 test?

The vertical burning method in UL 94 is harmonized with IEC 60695-11-10, EN 60695-11-10, ASTM D3801, and GB/T 5169.16. Test parameters and classification criteria are substantially aligned, although minor differences in gas purity, apparatus, and reporting may exist between the versions.

How is the test result reported?

The laboratory report typically states the tested thickness, the individual afterflame and afterglow times for each specimen, the total afterflame time, dripping and cotton ignition observations, and the final classification. Manufacturers can also list the rating on a UL Yellow Card to support material selection and regulatory compliance.