UL94 for engineering plastics: HB, V-0 and 5VA explained

UL94 V-0 plastic is one of the most requested specifications in the plastics trade, and one of the most misunderstood. A V-0 rating does not make a part fireproof, and it does not apply to a material in the abstract: it applies to a specific thickness of a specific grade, tested in a specific way.

This guide explains what the UL94 test actually measures, how the ratings from HB to 5VA differ, why thickness changes the result, and how to read a supplier’s yellow card without being misled. It is written for buyers and design engineers who have to justify a material choice to a safety reviewer.

What the UL94 test actually measures

UL94 is a small-scale laboratory test. A bar or plaque of the material is held in a controlled flame for a set time, then removed, and the tester records how long it keeps burning, whether it drips, and whether a drip ignites a cotton pad below. The rating is a summary of those observations.

It is worth being clear about what that means. UL94 compares how materials behave in a small, repeatable test; it is not a building-code fire rating and it does not predict how a finished part performs in a real fire. It is one input into a safety case, not the whole of it.

Because the test is small-scale, results are repeatable and cheap to obtain, which is why UL94 appears on almost every datasheet. Its value is in comparing materials on equal terms, not in promising what happens in a real fire.

HB, V-2, V-1, V-0 and 5VA in plain terms

HB is the lowest rating and the most common: the specimen is held horizontally and must burn more slowly than a set rate. The V ratings are vertical tests, where the flame is applied to the bottom of a hanging bar. V-2 and V-1 both allow up to 30 seconds of afterflame, but V-2 permits dripping that can ignite the cotton below, while V-1 does not.

V-0 tightens the afterflame to 10 seconds per specimen and forbids drips that ignite the cotton. 5VA is a more severe vertical test that also checks a plaque; it allows up to 60 seconds and no drip that burns through. In practice a UL94 V-0 plastic is what most electrical enclosures ask for, and 5VA is reserved for the harshest duties.

Thickness dependence: why the same grade changes rating

A material is not simply V-0. A rating is tied to a thickness, often quoted as something like V-0 at 1.5 mm and V-2 at 0.8 mm. Thin sections heat faster, lose material faster and are more likely to drip, so a grade that passes at one thickness can fail at a thinner one.

For a buyer this has a direct consequence. If a drawing calls for a 3 mm wall, the certificate has to cover 3 mm or less. A datasheet that shows V-0 only at 6 mm tells you nothing useful about a thin part, and that gap is a common source of late-stage surprises.

Glow wire and the difference from UL94

Glow wire testing is a separate method, often asked for alongside UL94. Instead of an open flame it uses an electrically heated element pressed against the part at a set temperature, which is closer to a fault inside a connector or a failed component. The usual targets are 750 C, 850 C and 960 C.

The two tests answer different questions, so a good UL94 rating does not guarantee a good glow wire result, and the reverse is also true. A safety reviewer will usually want both if the part sits near a live conductor or a heat source.

Where a part sits well away from any heat source, the glow wire question may never arise, and asking for both tests only adds cost. Where it sits over a live terminal, both are usually mandatory, and the grade choice follows from them.

Which engineering plastics offer V-0 and in what thickness

Most commodity grades, such as unfilled POM, are HB by nature and cannot be pushed to V-0 without a flame-retardant additive. Grades sold as V-0 usually carry that additive, or they are inherently flame-resistant polymers.

For a part that must be UL94 V-0 plastic at a thin wall, the grade list narrows quickly. In practice the engineering plastics that commonly offer V-0 include flame-retardant polycarbonate at thin sections, PPS in both unfilled and glass-filled forms, and several PEI and PEEK grades. The exact thickness at which V-0 holds varies by grade and supplier, so it must be read from the yellow card, not assumed.

The sectors that ask for these ratings most often are switchgear, connector and enclosure makers, and we list them under industries we supply.

Questions buyers ask about UL94 V-0 plastics

Is a V-0 part fireproof?

No. V-0 means the material self-extinguished within the limits of a small lab test. It still burns if the fire is large enough or sustained, and it still produces heat and smoke. The rating is a comparison, not a guarantee of survival in a real fire.

Does colour change the rating?

It can. Pigments and additives are part of the formulation, and a rating is granted to a specific formulation at a specific thickness. A different colour can be a different listing, so a black grade’s V-0 does not automatically transfer to the natural version.

Which industries require UL94 V-0?

It is common in electrical and electronic equipment, switchgear, appliances and anything with an enclosure around live parts. Automotive interiors and rail also use it. The exact requirement comes from the end-product standard, not from UL94 itself.

What should I send to my customer as proof?

The manufacturer’s yellow card for the exact grade and thickness, plus a declaration on our letterhead that ties the delivered lot to that grade. If a reviewer wants more, contact us and we will confirm what can be issued before you promise anything.