I don’t know of a specific NRTL decision that a jacketed cord is considered fault tolerant as a rule, but that is generally the practice. The cord will regularly be exposed to ungrounded metal in residential and commercial environments. It will regularly be exposed to human contact. As such, it should be fault tolerant. Cord gets stepped on, twisted and abused in a myriad of fashions and it still needs to provide protection. No cord can be perfectly fault tolerant, but the proper cord type should be fault resistant.
Depending on the cord type, a jacketed cord will typically have two layers of insulation. If you can determine that the minimum thickness of one of the layers is at least 0.4 mm, it can be considered supplementary or reinforced insulation under 60950-1 clause 2.10.5.2. With that thickness, there would be no issue. For jacketed cable, reference NFPA 70 (National Electrical Code) table 400.4. You will find that most cables have a nominal jacket thickness of at least 0.76 mm. Vacuum cleaner cord is the exception at 0.38 mm thick. It is generally thinner to allow the flexibility needs for the vacuum cleaner cord reels. A thickness of 0.76 mm will be considered reinforced insulation and does not need to have a clearance to SELV or ungrounded metal. The same argument can be applied even to single-insulated wires. I have tested products with internal line voltage wiring carried by THHN wiring. NEC table 310.13(A) allows me to see the thickness of the wire’s insulation. The wires had to be restrained away from SELV or needed additional insulation when using 14 or 12 AWG THHN. These sizes are only guaranteed to have 0.38 mm thick insulation. However, when 10 AWG was used, no extra precautions were required because the insulation thickness was 0.51 mm thick and therefore considered reinforced insulation. Both UL and VDE accepted this construction technique. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. From: Jim Eichner [mailto:[email protected]] Sent: Thursday, May 14, 2009 11:24 PM To: [email protected] Subject: Re: NRTL Mark vs. NEC Inspector The criteria that determines if an accessible conductive part requires grounding is generally whether or not a single fault can make that part come into contact with a hazardous live part. The single fault may be failure of functional or basic insulation, reduction of a functional or basic clearance, a component fault, etc. >From your statement that the only risk would be if the power supply cord “near” the heatsink becomes damaged and contacts the heatsink, it sounds as if you have determined that the internal construction is such that there is no risk of the heatsink becoming live from contact with internal live parts under normal or single fault conditions. If you have not determined that, you need to, because the inspector might be onto something and the NRTL might have missed something. It happens. Assuming that we are indeed left with only the p/s cord to consider, then 2 things have to happen as you stated: damage to the cord, and cord contact with the heatsink. If the p/s cord has nothing securing it in position away from the heatsink, I think you have to assume it can or will contact that heatsink, without that constituting a fault. In that case, the question becomes: can the Listed power supply cord have an exposed live conductor as the result of a single fault? I would say yes, except that if it is a jacketed cord, you would have to damage both the jacket, and the conductor insulation. That could easily happen from a single action, but I do not think it counts as a single fault, since 2 layers of insulation are involved. In fact it would seem logical that if it did count as a single fault, a cord like that would not be allowed to be exposed to users. Does anyone have an opinion or know of an official NRTL rule on this? Is an NRTL approved jacketed cord officially considered single fault tolerant? Finally, if the portion of the p/s cord that is near the heatsink is external to the projector (ie if it is accessible), then if it is damaged in such a way that it could energize the heatsink, it is itself a hazard regardless of whether or not it touches the heatsink. Jim Eichner, P.Eng. Compliance Engineering Manager Xantrex Technology Inc. e-mail: [email protected] web: www.xantrex.com <http://www.xantrex.com/> Confidentiality Notice: This email message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. From: [email protected] [mailto:[email protected]] On Behalf Of Christine Rodham Sent: Wednesday, May 13, 2009 7:38 AM To: [email protected] Subject: Re:NRTL Mark vs. NEC Inspector List Members, We have an interesting problem. We sold an OEM product ( very high end Home Theater Movie Projector , better than a movie theater quality) which was listed by a well known NRTL. ( We modify and enhance the SW for high end performance) The projector was installed in a public place and the installation was evaluated by a NEC ( National Electrical Code ) inspector. The projector has a metal ungrounded heat sink attached to the chassis that was added for cooling but was evaluated and approved by the NRTL. The NEC inspector will not sign off on the installation stating non-current carrying exposed metal parts that may be accidentally energized must be grounded to the chassis. Grounding the heat-sink to the chassis will be difficult due to the design of this special aluminum heat-sink. Here are my questions: * What is the criteria to determine if a metal part can become accidentally energized? The only way it could happen in this case is if the power cord that is near the heat-sink is damaged and then touches the heat-sink. The power cord is UL approved and properly rated. * How many fault conditions ( single vs multiple ) are considered to determine if a metal part can become accidentally energized. * What would be our best option in arguing this ruling. Should we request another inspector or get the NRTL involved? Note that this unit is ceiling mounted and only trained service people would have access to it after the initial installation. Thank you! Christine Rodham - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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://www.ieeecommunities.org/emc-pstc 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]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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://www.ieeecommunities.org/emc-pstc 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]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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://www.ieeecommunities.org/emc-pstc 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]>
