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 Safety of Polymeric Materials -- Short-Term Evaluations. The recognized component directory tabulates the results of the small-scale tests conducted on the materials.

The HWI test indicates a material's resistance to ignition when exposed to abnormally high temperatures resulting from a component failure such as a conductor carrying far more than its rated current. HWI performance is expressed as the mean number of seconds required to ignite a specimen when wrapped with an energized non-chrome resistive wire that dissipates a specified level of energy.

The HAI test determines the material's ability to withstand electrical arcing either directly on or just above the surface of the plastic material. This can occur in the presence of open switch contacts or in the event of the failure of an electrical connection. HAI performance is expressed as the number of arc rupture exposures -- using standardized electrode materials, geometry and electrical supply circuit -- required to ignite a specimen when the arc occurs directly on the surface or a specified distance above the test specimen.

The HVTR for a material is expressed as the rate (in inches per minute) that a tracking path can be produced on the surface of the material under standardized test conditions. This test relates to establishment of an electrically conductive path on the surface of a solid, insulated material as a result of electrical stress.

Another ignition test can be applied to measure a material's resistance to ignition property. This test is the glow-wire ignitability test and is also described in UL 746A and 746C, the Standard for Safety of Polymeric Materials -- Use in Electrical Equipment Evaluations. The method is based on a test procedure that is documented in IEC 60695 and specified in numerous IEC end-product specifications including IEC 60335-1. The test is somewhat similar to the HWI test in that it measures a material's resistance to ignition on application of a heated non-flaming source.