Thanks Rich and others for your input on this. The discussion so far has been regarding what level of protection is required for various types of hazards but what I was after was a system of classification for the protection systems themselves. In the meantime I came across a reference to EN 954-1 and it has what I was looking for: a regime of "categories" that describe different levels of protection.
Roughly, its categories are: B -a single fault can defeat the protection 1 - a single fault can defeat the protection, but the likelihood of a single fault is reduced through selection of reliable parts and proven safety principles 2 - a single fault can defeat the protection, but the functionality of the protective system is checked at regular intervals (usually automatically) so loss of protection is detected at the next check 3 - a single fault is not allowed to cause loss of protection, and wherever practicable the fault should be detected by the system; an accumulation of faults may cause loss of protection 4 - a single fault is not allowed to cause loss of protection and is detected "before the next demand on the safety function" or if that is not possible, an accumulation of faults may not cause loss of protection Interesting stuff. Typically I'd say product safety standards use category 3 without demanding self-checking of the protection (ie taking advantage of the "wherever practicable" allowance). For example a grounded metal box with basic insulation to live parts is category 3 in that a single fault does not result in a hazard, but most products don't regularly measure their own insulation resistance and would not be capable of measuring their own ground continuity so a busted ground wire will go undetected and a 2nd fault will cause loss of protection. Does anybody familiar with this standard care to add comments? Does my summary reasonably accurately paraphrase the requirements? _______________________________________ _____________________________________________ Jim Eichner | Schneider Electric | Renewable Energies Business | CANADA | Compliance Engineering Manager Phone: +1-604-422-2546 | Mobile: +1-604-418-8472 Email: [email protected] <mailto:[email protected]> | Site: www.schneider-electric.com/renewable-energies <http://www.schneider-electric.com/renewable-energies> | Address: 8999 Nelson Way, Burnaby, BC, V5A 4B5 *** Please consider the environment before printing this e-mail From: Richard Nute <[email protected]> To: [email protected] List-Post: [email protected] Date: 10/29/2010 12:27 PM Subject: Re: [PSES] Protection system fault-tolerance hierarchy? ________________________________ > I don't want to re-invent the wheel, but I'm looking to find a simple > way to communicate the idea that some systems don't need protection, > others need protection, others need protection that will still protect > even with a single-fault present anywhere, and so-on. Somewhere somebody > must have codified this idea for example with type 0, type 1, type 2, > etc. I'm sure there are standards out there for this - can anyone point > me to one or more? > > Thanks, > > Jim Hi Jim: The new IEC 62368-1 standard does exactly what you wish for. The standard asserts that injury (or fire or whatever) is due to a transfer of energy (from an energy source) to a body (or other susceptible part). Energy sources are classed as: Class 1: Little or no perceptible effect on the body (or other susceptible part) even in the event of a single fault. No safeguards are required. The energy source is inherently safe. Class 2: Sensation to pain on a body part but no injury or equivalent on a susceptible part) even in the event of a single fault. At least one safeguard, e.g., a basic safeguard, must be interposed between the energy source and a body part (or other susceptible part). Class 3: Severe pain or injury to a body part or equivalent susceptible part). At least two safeguards, e.g., basic plus supplementary, or a reinforced safeguard must be interposed between the energy source and a body part (or other susceptible part). The energy source is inherently hazardous. The only difference between what we do today and this new standard is the inclusion of the Class 2 energy source criterion. Class 2 was included because most telephone circuits fall between Class 1 and Class 3. The standard applies this concept to electric shock, electrically-caused fire, thermal injury, mechanical energy, radiated energy (acoustic, electromagnetic, and optical), and chemically-caused injury. All of these energy sources have continuous gradations from low (marginally detectable) to high (injurious). Most energy sources have well-defined and researched limits for "low" (non-hazardous). Best regards, Richard Nute Product Safety Consultant San Diego - ---------------------------------------------------------------- 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 <http://www.ieeecommunities.org/emc-pstc> Graphics (in well-used formats), large files, etc. can be posted to that URL. 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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://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]>
