The other side of the issue is the risk of electric shock. 120 V isn't fun to touch, but the impedance of the human body is enough that contact most commonly is an annoyance, not a risk of injury. As a result, the NEC has very few requirements to reduce the risk of electric shock. The U.S. power system also typically has lower loads on the distribution transformers with the ground connection on the transformer much closer to the ground of a typical house. This make it much easier to get ground fault circuit interrupters (GFCIs) to work well with a trip current of 5 mA. The U.S. has those in areas where the shock risk is higher, such as kitchens, bathrooms and outdoor outlets. They are cheap and effective. Unlike many RCDs, the trip current of a GFCI is low enough to nearly eliminate incidences of electrocutions in residential environments. The vast majority of cases of electric shock in the U.S. come from electrician, construction workers or similar occupations where people ! are directly exposed to higher power circuits.
In Europe, you have to do more to protect against shock. In the U.S., we have to do more to protect against fire. A lot is based on the infrastructure and history. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: John Allen [mailto:john_e_al...@blueyonder.co.uk] Sent: Tuesday, July 29, 2014 1:23 PM To: Ted Eckert; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] NRTL / OSHA interpretation of UL60950 clause 3.2.3 Ted Thanks - that sounds like a reasonable rationale for the NEC rules, even if we "Europeans" might not agree with it (and thanks goodness we don't have to live with it :-)) Regards John -----Original Message----- From: Ted Eckert [mailto:ted.eck...@microsoft.com] Sent: 29 July 2014 21:15 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] NRTL / OSHA interpretation of UL60950 clause 3.2.3 The power system in the United States is based around 110 - 120 V for most consume and commercial products with 208 V or 240 V used for some larger pieces of equipment. It is not uncommon to have fairly high currents. The country has also historically had a lot of fire-prone architecture. There is a lot of wood construction with the use of concrete and stone limited only to larger buildings. The NEC is largely based around fire risks much more than shock risks. Historically, there were a lot of fires started by damaged flexible cords. Over time, flexible cords were restricted to only the applications where they were absolutely necessary. The code even mandates residential outlet placement so that people won't have to run cords long distances across their house. In a kitchen, no place on the countertop can be more than 30 cm from an outlet. For other areas, any point along the wall must be within 2 meters of an outlet. Even in a residential installation, you may have an outlet protected by a 20 A circuit breaker. Depending on temperatures in the breaker panel, the 20 A breaker may take quite a while to trip even at 30 A. I've had rubber cords and plugs get soft on overloaded circuits because the breaker didn't trip fast enough. The risk of fire is further increased because of this. I'm not sure that you would end up with the U.S. power distribution system if you started designing it from scratch. We would probably come up with something completely different. However, because of history, it is what we are stuck with. It carries a higher risk of fire than the European system, so the code is extremely fire averse. There is also a lot of inertia in the development of the NEC. It's hard enough to get something added to the NEC. It is an order of magnitude harder to get something removed from the NEC. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: John Allen [mailto:john_e_al...@blueyonder.co.uk] Sent: Tuesday, July 29, 2014 11:43 AM To: Ted Eckert; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] NRTL / OSHA interpretation of UL60950 clause 3.2.3 Can anyone explain or speculate as to WHY the NEC does not appear to allow cord connection of stationary/fixed equipment because it's a perfectly "legal" way of connection in the UK (and probably other countries), and is regularly done with items like central heating boilers, instantaneous water heaters and so on (provided that the cord outlet is fitted with a manually-operable d/p isolating switch, generally incorporating a fuse, to enable it to be completely isolated )? John Allen W. London, UK -----Original Message----- From: Ted Eckert [mailto:ted.eck...@microsoft.com] Sent: 29 July 2014 14:13 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] NRTL / OSHA interpretation of UL60950 clause 3.2.3 The United States national differences are based off of NFPA 70, also known as the National Electrical Code or NEC. In theory, all NRTLs are obligated to ensure that the products they approve to UL 60950-1 or UL 60950-22 comply with NEC. Not all NRTLs are as strict about it, but it is possible that the NRTL in question is properly reading the NEC. NFPA 70 Section 400 covers flexible cords and cables. Section 400.7 covers permitted uses and 400.8 lists prohibited uses. You may be able to use 400.7(6) or 400.7(8) to argue for a flexible cord on your product. If not, 400.7(10) may be a possibility, but you would need to review section 645, Information Technology Equipment and chapter 8, Communication Systems, to see if there is something that would allow flexible cord on your product. 400.7 Uses Permitted. (A) Uses. Flexible cords and cables shall be used only for the following: (1) Pendants (2) Wiring of luminaires (3) Connection of portable luminaires, portable and mobile signs, or appliances (4) Elevator cables (5) Wiring of cranes and hoists (6) Connection of utilization equipment to facilitate frequent interchange (7) Prevention of the transmission of noise or vibration (8) Appliances where the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance and repair, and the appliance is intended or identified for flexible cord connection (9) Connection of moving parts (10) Where specifically permitted elsewhere in this Code I have run into a few cases where an NRTL has allowed a flexible cord connection on a product based on a liberal interpretation of the NEC, yet the local electrical inspector then prohibited the product from being connected because it did not comply with the NEC. The prohibition of flexible cord connections for many products is a historical artifact of the NEC. There was a safety basis long ago, but I'm not sure that I could argue that the rationale is still valid. I'm not arguing that the NEC is right or wrong. This is only what I have experienced from similar issues. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: John Woodgate [mailto:j...@jmwa.demon.co.uk] Sent: Tuesday, July 29, 2014 2:31 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] NRTL / OSHA interpretation of UL60950 clause 3.2.3 In message <6af0cb17eff94b31971265f87afee...@thhste15d1be4.hs20.net>, dated Tue, 29 Jul 2014, Charlie Blackham <char...@sulisconsultants.com> writes: >I was going off memory, and have had another look at the standard: > >The equipment is "Stationary Equipment" as it is not "movable >equipment" due to it being bolted to a mast or pole. However, >Stationary Equipment" can be pluggable in UL60950-1 - I'm just >wondering whether it has to be permanently connected in UL60950-22 >(which I don't have) I don't see anything in the IEC version of 60950-22 that demands permanent connection. The clause is short but not easy to interpret as it refers to 3.3 of IEC 60950-1 for one case and IEC 60364 for another, but the cases are not easy to distinguish. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Quid faciamus nisi sit? John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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