General Differences between UL and IEC Standards for Small Household Kitchen Appliance (Part 2)
Rating for Motor-Operated Appliance
Rated input is marked on IEC 60335 compliant appliances by means of showing either rated power or rated current. However, cord-connected UL Listed appliances rated more than 50 W have to be marked with rated current once the full-load power factor is 80% or less. Marking of rated power is optional.
Non-Metallic Materials Evaluation
Except where included in the end-product standard, the additional standard UL 746C, Polymeric Materials — Use in Electrical Equipment Evaluations, is required for evaluating of non-metallic enclosure materials. Typically, investigation includes checking the rating of flammability, relative temperature index (RTI), thickness and pigment. In addition, the enclosure is subjected to the Mold Stress Relief Distortion, Impact, Strain Relief after Oven, Severe Conditions, Input after Oven and Abnormal Operation Tests to determine the acceptability of a non-metallic enclosure material.
In accordance with UL 746C, if the material is located within 0.8 mm of uninsulated live parts or 12.7 mm of arcing parts, the material must have adequate properties for Comparative Tracking Index (CTI), Hot-Wire Ignition (HWI) and High Current Arc
Resistance to Ignition (HAI). The HWI and HAI ratings reflect resistance to ignition from electrical energy sources, but UL does relate them to flammability classification. There is a trade-off between resistance to ignition from electrical sources versus ignition from a flame. The greater resistance to one, the less needed for the other. The table shown as follows can help to determine the acceptance of material.
For IEC requirements, ball pressure test, glow-wire test, and needle-flame test are three significant tests for verifying compliance of plastic materials used in appliance.
The following are the main differences between the investigation of nonmetallic materials for UL and IEC.
a. Ball Pressure Test vs. Mold Stress Relief Distortion Test
The IEC Ball Pressure Test is a material test which tests for softening of the material via a 1-hour oven test while balancing the Ball Pressure Apparatus on a sample of the material. The UL Mold Stress Relief Distortion Test is an end-product test which tests for stress points in the molded part resulting in cracking or warping of the enclosure via a 7-hour oven test.
b. Required Flammability Rating
IEC 60335 requires all nonmetallic materials likely to be ignited or propagate a flame to be subjected at a minimum to the Glow-Wire Test at 550°C or be rated HB40. The IEC requirements then require Glow-Wire Testing at higher temperatures for parts in close proximity to or supporting connections, depending on the use of the product and generally do not recognize lammability ratings except where the Needle Flame Test is applicable. For UL, the required flammability rating of the enclosure is dependent upon the use of the product. For example, the enclosure material of a portable attended use product must be rated at least HB, while portable unattended products require V-2 minimum rated materials. While a material classified as HB is allowed to have a burn rate of 80 mm per minute in material thicknesses under 3 mm, HB40 requires a burn rate of less than 40 mm per minute. HB40 and HB classified materials are both allowed a burn rate of 40 mm per minute for thicknesses greater than 3 mm.
c. Glow-Wire Test vs. Hot Wire Ignition (HWI)
The UL HWI Test is a material test, while the Glow-Wire Test is an end-product test. UL 746C allows the use of the Glow-Wire Test as an alternative to having a material with an adequate HWI rating.
d. Direct Contact with Live Parts or Support of Connections
UL 746C includes requirements for parts in direct contact with or close proximity to uninsulated live parts. These consist
primarily of determining if the material has the required HAI, HWI and CTI. As indicated previously, HAI and HWI are dependent upon the flame rating of the material. For IEC, only parts supporting or in close proximity to connections are required to be further evaluated by means of Glow-Wire Test. The temperature of the Glow-Wire Test is dependent upon the use of the end-product and the current carried in the connection. These materials may also make use of the Glow-Wire Ignition Temperature (GWIT) and Glow-Wire Flammability Index (GWFI).
e. Close Proximity
For UL, this term is applicable to parts within 0.8 mm of an uninsulated live part in accordance with UL 746C. For IEC, it is applicable to parts with 3 mm of parts supporting connections.
Another major difference between the UL and IEC standards is the probe used to determine accessibility of live parts and parts capable of causing injury, the UL Articulated Accessibility Probe and the IEC Test Finger as shown below:
1. UL Articulated Accessibility Probe
2. IEC Test Finger (Test Probe B of IEC 61032)
While both of these probes are applied to openings without appreciable force, if the IEC Test Finger does not enter an opening, the probe is applied in the straight position with a force of 20 N to determine if accessibility through the opening. The UL Articulated Accessibility Probe is applied to openings with a minor dimension of less than 1 in. (25.4 mm). When the minor dimension of the opening is greater, accessibility is determined by requiring no uninsulated live parts be located within a distance of five times the diameter of the largest rod which can be inserted through the opening.
3. IEC Test Pin
In addition to the IEC Test Finger, for IEC 60335, the IEC Test Pin (Test Probe 13 of IEC 61032) is used to determine accessibility of live parts through openings in products without a means for earthing. It is also applied to products with a means for earthing, through openings in non-metallic enclosures and metal enclosures with a non-conductive coating.
4. IEC Test Probe
For products with visibly glowing heating elements, such as toasters, the IEC Test Probe (Probe 41 of IEC 61032) is used to
determine accessibility of these live parts. For UL, accessibility of an open wire heating element for all products is determined by use of the articulated accessibility probe except for an automatic toaster, since in operation the open-wire element of an automatic toaster is radiating heat and light, and is regarded as visibly glowing. In addition, the automatic toaster is required to have a double pole switching device to disconnect the heating elements from all conductors of the supply circuit when they are in an “unenergized” state.
(For “Part 1”, refer to Asia On the Mark, Issue 26, Summer 2008)
by Cherie Ip, Engineer, Conformity Assessment Services, Hong Kong