"The housing material is V-0." It is one of the most quoted phrases in electrical product design, and one of the least precise. A UL 94 rating is earned by a specific specimen thickness after specific conditioning. Change the wall thickness, the colour or the filler and you may be holding a different material from the one that was rated.
This article is for laboratories that run UL 94 or its IEC equivalents in-house — for incoming inspection, for compound development, or to check a supplier's claim before a part goes into production.
The ratings, in the order people confuse them
UL 94 groups its small-flame tests into a horizontal test and a vertical test, plus a more severe 5V test with a larger flame.
Horizontal burning (HB). A bar is held horizontally and a 20 mm flame is applied to its free end. The burning rate between the 25 mm and 100 mm reference marks must not exceed:
| Specimen thickness | Maximum burning rate |
|---|---|
| 3 mm to 13 mm | 40 mm/min |
| under 3 mm | 75 mm/min |
A specimen that stops burning before the 100 mm mark also meets HB.
Vertical burning (V-0, V-1, V-2). A bar is held vertically and the same 20 mm flame is applied twice for 10 seconds. A layer of dry cotton below catches any drips.
| Criterion | V-0 | V-1 | V-2 |
|---|---|---|---|
| Afterflame per specimen, each application | ≤ 10 s | ≤ 30 s | ≤ 30 s |
| Total afterflame, set of 5 specimens | ≤ 50 s | ≤ 250 s | ≤ 250 s |
| Afterflame + afterglow after 2nd application | ≤ 30 s | ≤ 60 s | ≤ 60 s |
| Flaming drips that ignite the cotton | not allowed | not allowed | allowed |
| Burning up to the holding clamp | not allowed | not allowed | not allowed |
5VA and 5VB use a 125 mm flame on bars and on flat plaques. The difference between them is whether the plaque is allowed to burn through.
HB is the lowest rating, not an intermediate one. A material that is "HB" has passed a horizontal test only; it says nothing about vertical behaviour.
Thickness is part of the rating
Thin sections burn differently from thick ones. A compound can be V-0 at 3 mm and V-2 — or only HB — at 0.8 mm. That is why a rating is always listed with the thickness at which it was achieved.
For incoming inspection the practical rule is simple: test at the thinnest wall of the part, or at the thickness the supplier's rating states, and write that thickness on the report. A V-0 result at 3.2 mm says nothing about a 1.0 mm rib.
Conditioning is not one step
For the vertical test, specimens are tested in two conditioning sets: one conditioned at standard laboratory atmosphere, and one aged in an oven at 70 °C for seven days and then cooled in a desiccator before testing. The aged set exists because flame retardants can migrate or volatilise; a compound that passes fresh and fails after ageing is not V-0.
Labs short of time often test only the standard-atmosphere set. That is a screening test, not a UL 94 classification, and the report should say so.
Six practical errors
1. Flame calibrated by eye
The 20 mm flame is not just a height. The method fixes the burner, gas and flame confirmation procedure. Our UL 94 cabinet, for example, uses methane of at least 98 % purity and flowmeters set for the 50 W and 500 W flames; whatever cabinet you use, run the flame confirmation the method describes rather than judging height by eye.
2. Draughts in the cabinet
Extraction running during the test leans the flame and the afterflame. Burn in still air; extract between tests.
3. Timing the wrong event
Afterflame and afterglow are separate times. Afterglow — glowing without flame — is easy to miss under bright laboratory lighting. Dim the area around the cabinet and use timers that can be stopped independently for each phase.
4. The cotton indicator
The cotton must be dry, of the specified type and amount, and at the specified distance below the specimen. A thick wad placed too close ignites from radiant heat alone; a thin layer placed too far never ignites. Either error moves a material between V-2 and V-1.
5. Colour and filler changes
Pigments, especially carbon black, and fillers change burning behaviour. A natural-colour rating does not automatically transfer to a black grade. Test the colour you will mould.
6. One failure in five
If one specimen in a set of five fails, the method allows a retest of a second set under defined conditions, and all of that set must pass. Two failures in the first set is a failure. Know which rule you are applying before you start.
UL 94 and IEC 60695
IEC 60695-11-10 describes the 50 W horizontal and vertical tests and IEC 60695-11-20 the 500 W test; ASTM D635 and D3801 cover similar ground. The classifications line up closely with UL 94, but the documents are not word-for-word identical, and a customer specification will name one of them. Report the document that was named.
What to put on the report
- Rating claimed and thickness tested.
- Colour, grade and lot of the material.
- Conditioning sets used, including oven ageing.
- Individual afterflame and afterglow times, drips and cotton ignition.
- Flame confirmation record for the day of testing.
Our UL 94 / IEC 60695 burn tester, with switchable horizontal and vertical programmes, is built in our own workshop in Istanbul. See the UL 94 standard page for the related methods.