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 design! 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 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. 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