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