UL94 is the "Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliance testing" released by Underwriters Laboratories (UL) of the USA.

The standard determines the materials tendency to either extinguish or spread the flame once the specimen has been ignited in various orientations and thicknesses. Materials are categorised from the lowest (least flame retardant) to the highest (most flame retardant):

HB - slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm.

V2 - burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.

V1 - burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.

V0 - burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.

5VB - burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may develop a hole.

5VA - burning stops within 60 seconds on a vertical specimen; no drips allowed; plaque specimens may not develop a hole.