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issue: August 2003 APPLIANCE Magazine

The Open Door
UL 746: Enhancements to Enclosure Flammability & Ignition Requirements

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John Stimitz, senior staff engineer, Conformity Assessment Services, Underwriters Laboratories Inc. (UL)

Last November, Underwriters Laboratories Inc. (UL) published the Fifth Edition of UL 746C, Standard for Polymeric Materials - Use in Electrical Equipment Evaluations. The revised standard features significant enhancements to the enclosure flammability requirements, and manufacturers need to understand the changes to determine the impact on the future development of products. The effective date for the changes is June 1, 2004.

The changes address concerns raised by the Consumer Product Safety Commission (CPSC) and other interested parties with respect to the trend of fires attributed to electrical sources. As a result, recommendations were developed to strengthen the Standard's enclosure material flammability requirements. The revisions to UL 746C were made in conjunction with continued input from the CPSC, the Association of Home Appliance Manufacturers (AHAM), and component industry associations.

The following summarizes the most significant changes to the Standard and provides guidance for end-product manufacturers as well as electrical component and plastic manufacturers.

Fundamental Changes

The fundamental change of the Standard is with respect to the upgrade in the enclosure flammability level to a "V"-rated material for "portable unattended household equipment" and "portable attended and unattended commercial equipment." It will be important to determine the type (attended or unattended, and household or commercial) of equipment of your product in order to know which path and type of enclosure flammability level is required, per Table 4.1 of UL 746C.

The level of attendance is determined by whether or not operator presence is normally required or essential in order for the equipment to perform its intended task or purpose. An attended product, such as a household electric knife, is equipment intended for use where operator presence is required, and would not be affected by the changes to UL 746C. An electric fan would be an example of unattended equipment, and it is affected by the changes. Note that the definition for attended equipment is described in Paragraph 3.4 of UL 746C, and level of attendance is described in paragraph 3.26, with additional guidance provided in Table 4.1 of the Standard. Enclosures for the affected product types mentioned above are required to be:

  • molded of a material rated minimum V-2 (V-1 and V-0 inclusive), OR
  • an enclosure (molded of either a HB or unrecognized material) that must comply with the 12 mm or 20 mm end-product flame test described in Sections 17 and 18 of UL 746C.

Alternate Path

An alternate path for portable unattended household equipment has been added to allow the use of HB enclosure materials, provided that an evaluation of electrical connections associated with internal materials and components comply with the requirements outlined in Section 5 of the Fifth Edition of UL 746C.

The alternate path requires a complete evaluation and analysis of the internal electrical connections located in circuits involving a risk of fire and the surrounding polymeric materials. All materials supporting and located within 3 mm of specified electrical connections are required to meet minimum flammability classifications. All electrical connections, such as wire nuts, splicing wire connectors, quick-connect terminals, terminal connectors, multi-pin, and other forms of wire connectors, are required to comply with their respective component standards. The materials of these connectors shall be rated V-0 or V-1 and shall be so identified.

Identification of the connector requires that the component manufacturer of the device mark the flammability classification of the insulating material on the connector, the smallest unit container, or on an information sheet placed in the smallest unit container. The flammability classification marking of the component material allows better traceability of the materials employed in the manufacture of the component. Other small-scale flammability classifications referred to in Section 5.3 of UL 746C are also acceptable.

A minimum Glow-Wire Ignitability Temperature (GWIT) and a Glow-Wire Flammability Temperature (GWFT) of 750¡C for the material supporting or within 3 mm of the electrical connection is an acceptable alternative, as well as the Glow-Wire End-Product Test for the individual part, as described in Section 75 of UL 746C. Some materials already have this pre-selection criteria available as part of their Recognition for their polymeric material grade.

Electrical component manufacturers may want to consider selecting materials for their components that comply with the new flammability/ignition levels and submitting them for evaluation to the applicable component Standard requirements. Some design alternatives, such as designing the material with a Recognized "V"-rated material or an acceptable Glow-Wire rating, may be considered. Please note that these design alternatives may require additional testing and design considerations.

Another design alternative for a portable attended or unattended product which would allow the use of a HB enclosure material would be to house all insulated and non-insulated live parts that involve a risk of fire in a metal or V-0 sub-enclosure. The outer enclosure would then be subjected to the applicable end-product requirements.

It is important to note that each material selected for the enclosure must comply with all applicable end-product requirements and all of the Material Property Considerations as outlined in Table 8.1 of UL 746C. In addition, the applicable flammability requirements for wire, tubing, sleeving, and tape are also specified in Section 5 of UL 746C.

About the Author

John Stimitz earned a B.S. in Electrical Engineering from Polytechnic University of New York (NY, U.S.). He started his career at UL in 1982 as an engineer and worked in a variety of appliance categories. Mr. Stimitz currently is a senior staff engineer in the plastics and automotive sector.


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