Appliance Design - February 2008 - (Page 52) CORDS An Interpower flex relief at the equipment end. Other strain relief options can be used at either end of the cord. Strain Relief Materials - Chemical Compatibility Chemicals Acetone Bromine Acetic Acid Detergents Oils, Diesel Toluene Naphtha Vinegar Nylon* Marginal Not Recommended Marginal Recommended Recommended No Info. Marginal Not Recommended PVDF Not Recommended Recommended Recommended Recommended Recommended No Info. Recommended Recommended Brass* Recommended No Info. Not Recommended No Info. No Info. Recommended Recommended Not Recommended Source: Interpower Corp. * Carried by Interpower Corporation as standard product. PVDF = Polyvinylidene Fluoride Typical materials used in strain relief and their resistance to chemicals. the types of chemicals that come into contact with a strain relief, certain metal strain reliefs might be better than plastic. Metal can provide EMI shielding if that is a consideration. In terms of the construction of the strain relief, much more attention is paid to the equipment end than to the plug end, says Oden. “The plug end is not normally as dynamic or frequently moved as the equipment end,” he says, “but you still need a system that provides enough mechanical strength to ensure safety.” According to Oden, the power supply cord supplier usually makes the decision on how the plugs are constructed and the materials used. Typically, these are off-the-shelf products and the appliance project engineer usually has little input with regards to the plug end of the cord, he says. Proprietary plugs are sometimes requested, especially for use in medical equipment or a special tool plug, but that plug still needs to be compliant with standards developed specifically for power supply cords. The cord suppliers are charged with ensuring that they meet these regulations. The plug design is controlled by the UL 817 power supply cord standard from Underwriter’s Laboratories, based in Northbrook, Ill. Cord manufacturers submit the plug design to the UL, who do a variety of tests on the plug and the strain relief including jacket retention tests, flex tests and abrupt pull tests. Typically, these tests require the plugs to survive a 30 lb. pull test and what Oden calls a minimal amount of flexing requirements. For instance, the UL appliance standard 60335-1 52 applianceDESIGN February 2008 requires 20,000 cycles. (UL has a series of standards that have been harmonized with Europe. These standards are named 60335-x, and each “x” standard is for a different product type.) The tests are looking to undercover some of the common types of power cord plug failures found in the field. According to an article by Conductive, Rancho Santa Margarita, Calif, (Appliance Design, Stressing Safety, February 2005, p. 42), these problems, include: Plug/Cord Separation. Plug remains in wall outlet and cord is torn out from the plug. Ground Pin Pull-Out. Plug is pulled out from the wall outlet, but ground pin simply breaks off the plug and is separated from the plug or remains in wall outlet. Plug Blade Pull-Out. Plug is pulled out, but plug blade(s) breaks off or worse yet, remains in wall outlet exposing live current from the wall outlet. Strain relief tests at the equipment end are much more arduous, and they need to be because even if the cord passes the UL 817 standard, it doesn’t mean that it will pass the particular appliance standard. That is why Greg Ruppert, president of Quail Electronics, Livermore, Calif., which makes plugs, cords and cordsets, suggests that OEMs consid- Strain relief can offer damage resistance to cords even when they are sharply bent. Photo from Unicable. er doing their own in-house testing. This includes trying to quantify the force, angle, and number of cycles that the strain relief will be required to endure. “You can bet that the UL will beat the heck out of the (appliance, such as a) toaster and subject the junction of the toaster’s cord to all kinds of abuse to try and get it to fail,” says Ruppert. “In the long run, it will take much less time and money if that kind of testing can be done in house before UL rejects it and it’s ‘back to the drawing board’.” Over the years, UL has developed separate standards for specific appliances, but strain relief is a common thread running across many of the standards, says John Drengenberg, an electrical engineer who serves as UL’s consumer affairs manager. “For most appliances, the strain relief requirements that are applied by UL in its safety standards mandate a 35-lb. pull test on the cord,” says Drengenberg. “For the test, a cord is connected to 35-lbs. of force, eased down and held there for a period of one minute.” The tests checks for: Short circuit between the conductors. Breakage of more than 10 percent of the strands of any conductor. Separation of the conductor from its terminal. Loosening of any cord guard. Damage to the cord or cord guard. Broken strands piercing the insulation. A variation on this test is written into some standards for low-weight products such as hair dryers. In this case, Drengenberg says, if the hair dryer weighs less than 20 lbs., it would be subjected to a 20-lb. pull test. However, a product like the portable hair dryer might also be subjected to a rotational test. “Hair dryers undergo a flexing action that www.applianceDESIGN.com http://www.appliancedesign.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.