UL 94

UL 94 - Flammability Ratings for Plastics

Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances

How the ratings are tested

UL 94 burns a specimen horizontally (HB) or vertically (V and 5V), measuring how fast it burns, whether it self-extinguishes, and whether it drips. The ratings form a ladder from HB up to 5VA.

Key facts

Full title: Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances
Published by: UL (Underwriters Laboratories)
Harmonized with: IEC 60695-11-10 / -11-20, ISO 9772 and ISO 9773
Common ratings: 5VA, 5VB, V-0, V-1, V-2, HB (plus foam HBF/HF-1/HF-2 and thin-film VTM-0/1/2)
What it measures: Small-flame ignitability and self-extinguishing behaviour only
Verify on: The material's UL Yellow Card, at the actual wall thickness

Horizontal vs vertical burn

HB is a horizontal test: the bar must burn slower than a set rate, or stop before the 100 mm mark. It means slow-burning, not self-extinguishing, and is the lowest rating. The V and 5V ratings are vertical, where the sample must self-extinguish after the flame is removed - a far more demanding condition that also looks at whether the material drips burning particles onto a cotton indicator below.

The 5V tests use a flame roughly five times more severe and add a plaque test: 5VA must not burn through, while 5VB may form a hole.

Choosing a rating

The right rating is set by the end-product standard - IEC 62368-1 for AV/IT equipment, IEC 60601-1 for medical, IEC 60335-1 for appliances - not picked from a datasheet. Parts in or next to current-carrying or heat-generating assemblies usually need V-0 or better; enclosures and structural parts in some categories need 5VA or 5VB; external non-structural parts may only need HB. A higher rating than the standard requires adds cost and can hurt other properties such as impact strength or tracking resistance (CTI).

What UL 94 does not tell you

It is a material-level screening test under controlled lab conditions. It says nothing about smoke density, gas toxicity, heat-release rate or how the part behaves in a real assembly fire, and a high UL 94 rating does not by itself mean the finished product meets its safety standard. Glow-wire and end-product fire tests cover the rest.

UL 94 ratings, least to most flame-resistant

Ratings are thickness-dependent and formulation-specific - always confirm against the UL Yellow Card at the part's minimum wall thickness.
Rating	Orientation	Self-extinguish	Drips
HB	Horizontal	Slow burn (no self-extinguish required)	-
V-2	Vertical	≤ 30 s	Flaming drips allowed
V-1	Vertical	≤ 30 s	No flaming drips
V-0	Vertical	≤ 10 s	No flaming drips
5VB	Vertical + plaque	≤ 60 s	No drips; plaque may burn through
5VA	Vertical + plaque	≤ 60 s	No drips; no burn-through (highest)

Tools that use this standard

UL 94 rating selector Creepage & clearance calculator Enclosure & Safety Standards

Related standards

Standard	Relationship	What it means
IEC 60695-11-10 and -11-20	Harmonized with	The IEC small-flame test methods that mirror the UL 94 horizontal-burn and vertical-burn procedures.
ISO 9772 and ISO 9773	Aligned with	The ISO equivalents for cellular (foam) materials and thin flexible materials.
IEC 62368-1 and UL 746C	Referenced by	End-product safety standards that call up a minimum UL 94 class for enclosure and part flammability.

Source: UL Standards & Engagement. An overview for design reference - verify against the current edition before relying on it for compliance.