Hi Jim:
> Returning to the 120Vdc limit in UL1703 and other standards, a quick look at > 61201 says that 120Vdc would cause at least tetanization in most situations, > even in dry conditions. DC does not cause tetanization. Only AC within a specific frequency range causes tetanization. IEC 61202, 2nd, sub-clause 4.1 states: This specification addresses the following physiological effects: – startle reaction from current; – strong involuntary muscular reaction (such as inability to let-go from an electrode for a.c.); – ventricular fibrillation. Best regards, Rich ---- Jim Eichner <[email protected]> wrote: > The horizontal standards on this topic should be required reading for anyone > in the safety compliance field. Only in the last few years have I become > familiar with the details contained in IEC 61201 and IEC 60479, and it not > only explained the basis of the type of discrepancy that started this thread, > but it made me realize how uninformed some standards writing committees may > have been in the past. Admittedly even these horizontal documents have > undergone major revisions in the last 5 years, and it appears to be a topic > where we still have much to learn (except as noted in a previous post, that > electricity is rarely without hazard!) > > To establish a safe touch voltage limit requires analysis and assumptions > around the following parameters: > > - area of contact (fingertip, whole hand, large part of the body, etc.) > - skin condition (dry, wet, salt water wet, immersed) > - path the current will take (hand to hand, one hand to both feet, etc.) > - condition you are trying to protect against (startle reaction, tetanization > (inability to let go), and ventricular fibrillation are all used as the basis > for touch voltages in various well established standards) > - who you are trying to protect: healthy adults, surgical patients, small > children, infirm elderly people, etc. > - the statistical level you are trying to achieve (a given current may be > safe for X% of the healthy adult population and not for the rest - roll the > dice anyone?) > > So the symptom is that even very well established standards have different > voltage limits, but the underlying problem may be lack of awareness of these > factors and the need to analyze the types of equipment and users involved and > make informed decisions on all of these factors. I believe it is also > important for a standard to state clearly what assumptions were made - what > the basis of the touch voltage limits is. A sample statement, as un-nerving > as it may seem at first, would be something along the lines of "The touch > voltage limits in this standard will protect 80% of the population against > tetanization, under dry conditions, when the live part is contacted by an > area no larger than the hand, and the current path is from one hand to either > or both feet". It could go further and state what outcomes can be expected > outside these boundaries: hand to hand current or wet conditions may cause > V-Fib in some portion of the population, etc. If nothing else, it would make > desi! gn! > ers and Certifiers think a bit more before blindly accepting touch voltage > limits. > > By the way, the approaches that I am proposing here are being followed by the > authors of IEC 62477 (TC22, PT5) which is the future "group safety function" > (ie nearly horizontal) standard for safety of all power conversion equipment. > It is to that group that I owe my relatively new understanding of all this. > The users of that standard will be the authors of product standards, who will > be given the choice of either accepting the numbers in 62477 (which will > clearly state the basis for those numbers) or using its approaches for > determining other limits based on different assumption sets. > > Returning to the 120Vdc limit in UL1703 and other standards, a quick look at > 61201 says that 120Vdc would cause at least tetanization in most situations, > even in dry conditions. Only very small contact areas would result in a less > severe situation. Under wet conditions the curves are a little different > (not as much as you might expect) and a hazard arises sooner as you increase > contact area or voltage. I would not call 120Vdc safe on an outdoor product > with metal surfaces large enough to contact with your whole hand, with your > feet on wet ground or on a grounded metal panel mounting bracket, and in fact > the very existence of a Class 2 designation for solar panels was called into > question at the last IEC TC82 (PV) meeting I was at. > > Jim Eichner > Compliance Engineering Manager > Xantrex Technology Inc. > e-mail: [email protected] > web: www.xantrex.com > > Confidentiality Notice: This e-mail 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 > > > > > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of Haynes, Tim > (SELEX GALILEO, UK) > Sent: Friday, July 31, 2009 4:50 AM > To: John Woodgate; [email protected] > Subject: RE: "safe" voltage differences between UL 1703 and IEC 61140 > > Hi All and John! > > Back to basics (as I read them) and a light hearted review of published > information. > > ICNIRP and a few other sources put the threshold of perception as 1mA > a.c. > > Other sources give the resistance of a person as 2kOhms. > > This puts the voltage associated with the perception of an electric > shock at 2V a.c. Death can occur (according to some documents) with a.c. > current above 9mA - that would be 18Va.c. for a 2kOhm person > > Taking a step further, the resistance of a person that has lost the > normal layers of skin is very low - so a 2V a.c. source connected to two > cheese graters could be lethal! > > Of course 2V a.c. has never been safe if the current is unlimited - it > might not shock you, but it can give you a shock to find that the low > voltage source has caused a fire. > > 1.4V d.c (a NiCad cell) put a good burn on the inside right of my right > thigh when I mistakenly put the car keys in the same pocket as the spare > re-chargables for my camera. That gave me a shock! > > Many years ago, I remember being told that an old lady died from a shock > caused by a 9v "radio" battery - but I cannot vouch for the truth of > this tale. > > Seriously, there is a risk with electricity that never quite goes away. > The lower the voltage, the greater is the chance that a person will > survive the shock. Given the range of resistance that exists in people, > it would be a very bad day if someone with 2k resistance came into > contact with 18V and died as a result. > > However, if you want to have a safe product, it is probably not worth > arguing over what voltage is safe, it is better to prevent access to any > voltage - just in case the person has been grating cheese. > > There are some serious thoughts in the above if you look for them. > > Have a good week end > Regards > Tim > > ************************ > SELEX Sensors and Airborne Systems Limited > Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14 > 3EL > A company registered in England & Wales. Company no. 02426132 > ******************************************************************** > This email and any attachments are confidential to the intended > recipient and may also be privileged. If you are not the intended > recipient please delete it from your system and notify the sender. > You should not copy it or use it for any purpose nor disclose or > distribute its contents to any other person. > ******************************************************************** > > - > ---------------------------------------------------------------- > 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]>
