Hello Ryan,
For ITE, you must design the product to be safe even if line and neutral are reversed. IEC 60950-1 does not let you depend on one of the conductors as being a grounded conductor for safety. In some applications, there are advantages to having a polarized plug. The most common example is a light socket. The threaded portion of the socket is connected to neutral and the base of the bulb contacts line. When somebody is replacing a bulb, they have a much higher chance of contacting the threads of either the empty socket or of the bulb once the bulb’s threads engage the socket. If the threads are connected to neutral, and that neutral line is a grounded conductor, then the risk of shock is minimized. I am speaking of neutral as the grounded conductor in the terminology of NFPA 70, the National Electrical Code (NEC). For safety in the light socket application, this means that neutral must be properly derived and bonded to ground at the electrical panel. It also assumes that the outlet is polarized and wired correctly. Finally, it assumes that the light fixture’s plug is polarized. Many regions of the world do not have polarized plugs, so a harmonized ITE standard must assume that either conductor could be live. (For that matter, in some power systems, both could be live.) Ted Eckert Compliance Engineer Microsoft Corporation [email protected] <mailto:[email protected]> The opinions expressed are my own and do not necessarily reflect those of my employer. From: Jayasinghe, Ryan [mailto:[email protected]] Sent: Tuesday, March 22, 2011 10:27 AM To: [email protected] Subject: Line-Neutral orientation with auto-ranging power supplies Dear Group, I understand that maintaining the L-N orientation is regarded as 'good practice' however I cannot find any hard requirement in UL60950 and would like to know the source. We manufacture several 1U high ITE products in the 20-40W range which use several different models of open frame auto-ranging AC power supplies. (100-240VAC to 12VDC). We use a short two wire cable to connect the input AC from a small PCB to the power supply. (Black-Line and White- Neutral). The chassis and power supply are grounded via the Protective Earth yellow/green wire. In some cases the L-N from our AC-PCB does not line up with L-N pins on the power supply, and we ask the manufacturer to reverse their connector. This is not a good approach as it leads to a 'custom' part and other documentation issues for the manufacture. If we were to go in for product safety testing, NRTL, with the L-N orientation reversed on what basis would be rejected. Would we? If a power supply is auto-ranging why would the orientation be important as L-L will appear on both pins in a 220VAC circuit. Most of these power supplies have fuses for both line and neutral. We also have a few power supplies that claim to be auto-ranging but have only one fuse, and I am not sure if these would 'qualify' for a L-N reverse. The other option we have is to provide manufacturing with two sets of cables, but that also can lead to other complications, such as BOMs which are power-supply-model-specific. Your experience and wisdom is greatly appreciated. Ryan Jazz Jayasinghe Compliance Engineer x1198 Canoga Perkins Direct:(818) 678-3898 20600 Prairie Street Company:(818) 718-6300 Chatsworth, CA 91311-6008 e-mail: [email protected] <mailto:[email protected]> www.canoga.com FAX:(818) 678-3798 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
