UL 94 is the standard from the American test institute Underwriters Laboratories for the flammability of plastics in devices and parts. It rates a material by its behaviour in a defined flame test, from the weakest class HB to the strictest class 5VA. The best known level is UL 94 V-0: the material self-extinguishes within a few seconds after the flame is removed and does not drip while burning. For the design engineer the class is often a hard requirement for part approval, because device standards and customer specifications demand it directly.
What UL 94 is
UL 94 carries the full title Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. The standard checks how a plastic reacts to a flame: how fast it burns, whether it keeps burning by itself after the flame is removed and whether it gives off burning drips that can ignite material below. It explicitly does not check how hot a part may become in continuous service. The thermal class covers that separately.
A UL 94 class always applies to a specific material thickness. The same plastic can reach V-0 at 1.5 millimetres and only V-2 at 0.8 millimetres, because a thinner wall burns through faster. In an approval the class should therefore always be stated together with the tested thickness.
The classes HB, V-2, V-1, V-0, 5VB and 5VA
The classes form a ranking. HB is the lowest requirement, 5VA the highest. The table below names the decisive criterion for each class.
| Class | Test | Core criterion |
|---|---|---|
| HB | horizontal | slow burning, rate below 40 mm/min, or extinguishing before the mark |
| V-2 | vertical | afterflame up to 30 s per specimen, burning drips allowed |
| V-1 | vertical | afterflame up to 30 s per specimen, no igniting drips |
| V-0 | vertical | afterflame up to 10 s per specimen, no igniting drips |
| 5VB | vertical, large flame | afterflame up to 60 s, burn-through of the plaque allowed |
| 5VA | vertical, large flame | afterflame up to 60 s, no burn-through of the plaque |
The step from V-1 to V-0 lies mainly in the afterflame time: 10 instead of 30 seconds per specimen. The step to 5VB and 5VA additionally requires the material to withstand a much larger flame. These highest levels are demanded for housings that must remain a barrier in a fire.
Test methods: horizontal and vertical
UL 94 uses two basic set-ups. In the horizontal test (HB) the bar lies flat and a 20 millimetre flame is held to the end for 30 seconds. What is measured is how fast the flame travels between two marks. If the rate stays below the limit or the material extinguishes first, HB is passed.
For V-2, V-1 and V-0 the specimen stands vertically. A 20 millimetre flame is applied twice for ten seconds each to the lower end. Afterwards the afterflame time, the afterglow and whether burning drips ignite a cotton pad below the specimen all count. Five specimens must meet the limits. The 5V tests use a larger flame of about 125 millimetres, applied five times for five seconds each, and for 5VA also check a flat plaque for burn-through.
Link to thermal class and material choice
UL 94 and the thermal class describe two different things that are often confused. The thermal class to IEC 60085 (Y, A, E, B, F, H and higher) says which continuous temperature an insulation material withstands over its life. UL 94 says how it reacts in a fire. A material of thermal class H at 180 degrees can still reach only HB in UL 94, and a well flame-retarded plastic can have a low thermal class. Anyone who needs both checks both values separately. The thermal grouping runs through the insulation classes, the fire grouping through UL 94.
For material choice this means: first read the required flammability class from the device standard, then look for a material that safely reaches this class at the planned thickness. Not every electrically insulating plastic holds V-0. Many standard types reach only HB without a flame-retardant additive.
Typical insulation materials by class
The behaviour can be sorted roughly by material family, even though the exact class always depends on the type, the additives and the thickness.
- Only HB without flame retardant: many standard thermoplastics such as polyethylene, polypropylene and untreated ABS.
- V-0 with flame retardant or by nature: flame-retarded polyamide, PBT and PC/ABS, plus high-performance plastics such as PEI, PPS and PEEK.
- Often V-0 from the material itself: many thermosets based on phenolic or epoxy, such as laminated fabric and glass-fibre reinforced epoxy laminates.
- Rated separately as film: thin films often run through the VTM classes, the equivalent of V-0 to V-2 for very small thicknesses. Which electrical insulating films reach VTM-0 and which only VTM-2 is shown in the film comparison.
For insulation parts in switchgear or e-mobility, customers usually demand V-0, because arcs and hot contacts can occur in a fault. We fix the right insulation material and the thickness together with the customer before any tool is made.
GOBA Takeaway
UL 94 sorts plastics by their fire behaviour, from HB to 5VA, and V-0 is the class most often demanded for insulation parts in practice. The class always applies to a specific thickness and must be kept separate from the thermal class, which only describes the continuous temperature. Fixing both values early avoids late material changes and approval loops. GOBA has been processing insulation materials from polyester film to laminated fabric since 1959 and knows the UL 94 rating of the common materials from production. Choose the right material with us from our insulation materials, send us your requirement with the required class and you will receive a technical quote.
