The plastic vertical burning test is a fundamental flammability evaluation used to classify polymeric materials in accordance with standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 5169.16. By measuring afterflame time, afterglow time, and the ignition of cotton by flaming drips, the test assigns V-0, V-1, or V-2 ratings that determine whether plastics can be safely used in electrical and electronic products. This article explains the sample requirements, test procedures, classification criteria, and typical applications of vertical burning testing for manufacturers seeking compliance verification in accredited third-party laboratories.
Technical Overview
The plastic vertical burning test is a small-scale laboratory procedure that evaluates the flammability characteristics of rigid plastic specimens mounted in a vertical orientation. It is one of the most widely requested flammability tests in the electrical, electronics, and appliance industries because it directly supports material selection and regulatory compliance for polymeric components.
The test exposes vertically clamped bar specimens to a standardized 20 mm (nominal 50 W) blue flame generated by a methane or natural gas burner. The flame is applied twice to the lower edge of the specimen, and the laboratory records the duration of flaming after each application, the duration of glowing combustion, and whether flaming particles drip onto and ignite a surgical cotton indicator placed below the specimen.
The most commonly applied standards include UL 94 (Tests for Flammability of Plastic Materials for Parts in Devices and Appliances), IEC 60695-11-10, ASTM D3801, and GB/T 5169.16. These standards define the vertical flame classifications V-0, V-1, and V-2. A related but more severe procedure, the 500 W (5V) test described in UL 94 and IEC 60695-11-20, yields the 5VA and 5VB classifications used for high-risk applications.
It is important to note that a vertical burning rating applies only to the specific material grade and thickness tested. A flame rating obtained at 3.0 mm thickness cannot automatically be extrapolated to thinner parts, which is why product designers frequently request testing at the minimum wall thickness used in production.
Testing Samples
Bar Specimens for V-0/V-1/V-2 Testing
Standard bar specimens measure 125 ± 5 mm in length and 13.0 ± 0.5 mm in width, with the thickness reflecting the actual end-use application (commonly 1.5 mm, 3.0 mm, or other specified values, generally up to 13 mm). Two sets of five bar specimens are required so that the test can be repeated on a conditioned set and, where applicable, on a thermally aged set.
Plaque and Bar Specimens for 5VA/5VB Testing
The 500 W vertical test requires both bar specimens (125 mm × 13 mm) and plaque specimens of 150 mm × 150 mm. The plaque test is essential for distinguishing between 5VA and 5VB, because a 5VA material must not exhibit burn-through (hole formation) in the plaque, whereas 5VB permits burn-through as long as flaming and ignition criteria are still met.
Specimen Conditioning
Before testing, specimens are typically conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for at least 48 hours. For certain requirements, an additional set of specimens is thermally aged at 70 °C for 168 hours and cooled in a desiccator for at least 4 hours at room temperature prior to testing.
Testing Items
The plastic vertical burning test evaluates the following parameters and observations:
- Afterflame time after the first flame application (t1), recorded in seconds for each specimen.
- Afterflame time after the second flame application (t2), recorded in seconds for each specimen.
- Afterflame plus afterglow time after the second application (t2 + t3), indicating continued glowing combustion.
- Ignition of the surgical cotton indicator by flaming drips or particles released from the specimen.
- Whether flaming or glowing combustion propagates up to the clamp holding the specimen.
- Total afterflame time across the complete set of five specimens, used to distinguish between classifications.
- Final classification of the material as V-0, V-1, or V-2 (or 5VA/5VB under the 500 W method).
Testing Methods
The vertical burning test is performed in a ventilated test chamber or fume hood with the specimen clamped vertically so that its lower end is positioned 10 mm above the top of an adjustable burner tube. A layer of dry, absorbent surgical cotton is placed 300 mm below the point where the specimen edge is exposed to the flame.
The general procedure for the 50 W vertical test (V classifications) is as follows:
- Adjust the gas supply to produce a 20 ± 1 mm blue flame with a clearly defined inner cone.
- Apply the flame to the midpoint of the lower edge of the specimen for 10 ± 0.5 seconds, then withdraw it and immediately start timing.
- Record the afterflame time t1. If flaming ceases before 30 seconds, reapply the flame for another 10 ± 0.5 seconds after the specimen has cooled.
- Record the afterflame time t2 and the afterglow time t3 following the second application.
- Observe whether flaming drips ignite the cotton indicator and whether the flame reaches the clamp.
- Repeat the procedure for all five specimens in the set and evaluate the complete data set against the classification criteria.
The classification criteria defined in UL 94 and equivalent standards are summarized below:
V-0 Rating
- Afterflame time for each individual specimen does not exceed 10 seconds.
- Total afterflame time for the set of five specimens does not exceed 50 seconds.
- No flaming drips that ignite the cotton indicator.
- No flaming or glowing combustion sustained long enough to reach the clamp.
V-1 Rating
- Afterflame time for each individual specimen does not exceed 30 seconds.
- Total afterflame time for the set does not exceed 250 seconds.
- No flaming drips that ignite the cotton indicator.
- Afterglow after the second application does not exceed 60 seconds.
V-2 Rating
- Afterflame time for each individual specimen does not exceed 30 seconds.
- Total afterflame time for the set does not exceed 250 seconds.
- Flaming drips are permitted provided the cotton indicator only briefly flames, and glowing particles do not ignite the cotton.
- Afterglow after the second application does not exceed 60 seconds.
For the more severe 500 W (5V) method, the burner produces a 125 mm flame with a 40 mm inner blue cone, applied five times for 5 seconds each with 5-second intervals. A 5VA classification requires no burning longer than 60 seconds after any application, no ignition of the cotton, and no burn-through of the plaque, while 5VB allows burn-through of the plaque but retains the other restrictions.
Product safety standards often stipulate minimum flame ratings depending on the distance of the plastic part from potential ignition sources and the part's function, so manufacturers should confirm the applicable requirement with the relevant end-product standard before commissioning testing.
Applications
Vertical burning test results are used across a broad range of industries wherever plastic parts may be exposed to electrical ignition sources:
- Enclosures and internal components of consumer electronics, chargers, power adapters, and IT equipment.
- Household appliances such as cookers, washing machines, refrigerators, and small kitchen devices.
- Electrical installation products including switches, sockets, distribution boxes, terminal blocks, and connectors.
- Luminaires and lighting components subject to IEC and EN safety requirements.
- Automotive interior and electrical components requiring flammability evidence alongside OEM material specifications.
- Medical device housings and laboratory equipment where fire risk must be minimized.
- Battery packs, power tools, and industrial control cabinets where higher-tier ratings such as 5VA are frequently specified.
- Material development and quality control by compounders and resin producers verifying flame-retardant formulations.
Manufacturers exporting products to North America, Europe, or Asia routinely submit materials to accredited third-party laboratories for vertical burning tests to demonstrate compliance with end-product standards referencing UL 94 or IEC 60695-11-10 classifications.
FAQ
What is the difference between V-0, V-1, and V-2?
V-0 is the most demanding 50 W classification, allowing a maximum of 10 seconds of afterflame per specimen and 50 seconds total for the set, with no igniting drips. V-1 allows up to 30 seconds per specimen and 250 seconds total, also without igniting drips. V-2 matches the V-1 timing requirements but tolerates flaming drips that briefly ignite the cotton indicator.
How does the vertical burning test differ from the horizontal burning test?
The vertical test applies a 50 W flame to the bottom edge of an upright specimen and classifies materials into V-0, V-1, or V-2, whereas the horizontal method (HB, per UL 94 and IEC 60695-11-10) applies the flame to the free end of a horizontally mounted specimen and measures burning rate. The vertical orientation is more severe and is typically required for parts closer to ignition sources.
Does thickness affect the flame rating?
Yes. Flame ratings are thickness-specific. A material rated V-0 at 3.0 mm may only achieve V-1 or fail at 1.5 mm. Testing must be performed at the thickness intended for the final part, and each thickness receives its own classification.
Which standards are considered equivalent?
UL 94 is harmonized in method with IEC 60695-11-10, ASTM D3801, and GB/T 5169.16 for the 50 W vertical procedure, and the 500 W procedure corresponds to IEC 60695-11-20. However, end-product standards may name a specific edition, so laboratories report the exact standard and edition used.
What happens if one specimen in the set fails the criteria?
If a second set of five specimens is tested following certain failure patterns of the first set, all requirements must be met by the second set for the classification to be granted. A complete failure of the criteria results in a lower classification or no rating for that thickness.
Can test results be used for product certification directly?
Vertical burning data supports material qualification, but product certification also depends on the applicable end-product safety standard, which may impose additional requirements such as hot-wire ignition, high-current arc ignition, or comparative tracking index tests. Accredited third-party laboratories can advise on the full test program required for a specific product family.