Safety Information
2"
Horizontal Versus Vertical Positioning
Specimens molded from the plastic material are oriented in either a horizontal or vertical position,
depending on the specifications of the relevant test method, and are subjected to a defined flame
ignition source for a specified period of time. In some tests, the test flame is only applied once, as
is the case of the horizontal burning (HB) test, while in other tests the flame is applied twice or
more.
A HB flame rating indicates that the material was tested in a horizontal position and found to burn
at a rate less than a specified maximum.
The three vertical ratings, V2, VI and VO indicate that the material was tested in a vertical
position and self-extinguished within a specified time after the ignition source was removed. The
vertical ratings also indicate whether the test specimen dripped flaming particles that ignited a
cotton indicator located below the sample. UL 94 also describes a method in which the test flame
is applied for up to five applications, in testing for a 5VA or 5VB classification. These small-scale
tests measure the propensity of a material to extinguish or spread flames once it becomes
ignited.
Difference in Test Methods and Criteria
When looking at the flame ratings for plastic materials commonly molded to fabricate enclosures,
structural parts and insulators found in consumer electronic products (5VA, 5VB, V-O, V-I, V-2
and HB), a material classified as 5VA or 5VB is subjected to a flame ignition source that is
approximately five times more severe than that used in the V-O, V-I, V-2 and lIB tests, and the
specimens may not drip any flaming particles. The three remaining six classifications specified in
UL 94 relate to low-density foam materials commonly used in fabricating speaker grills and
sound-deadening material (HF-l, HF-2, HBF). The remaining three classifications are assigned to
very thin films commonly used in flexible printed wiring boards, generally not capable of
supporting themselves in a horizontal position (VTM-O, VTM-I, VIM-2). A flame rating of VTM-O
rating cannot be considered equivalent to a V-O rating as the test methods are quite different.
Likewise, VTM-l and VIM-2 cannot be considered equivalent to V-I and V-2 respectively.
UL 746A Ignition Tests
In addition to flammability considerations, a material's ability to resist ignition from electrical
sources is another important factor that must be considered in the selection and evaluation of a
material for use in electrical equipment. Possible electrical ignition sources in equipment are:
overloaded (overheated) electrical conductors and components; arcing parts, such as the open
contacts of switches and relays; and arcing at broken or loose connections, e.g., splices or
terminals. Polymeric materials in direct contact with or in close proximity to overloaded or arcing
electrical parts could ignite.
The three basic tests used to evaluate a material's ability to resist ignition are the Hot-Wire
Ignition (HWI) High-Current (or High-Amp) Arc Ignition (HAI): and High-Voltage Arc Tracking Rate
(HVTR). Details of the test criteria can be found in UL 746A, The Standard for Polymeric
Materials - Short-Term Evaluations. The Recognized Component Directory tabulates the results
of the small-scale tests conducted on the materials.