I read in !emc-pstc that cherryclo...@aol.com wrote (in <43.47bb025.29689...@aol.com>) about 'EMC-related safety issues', on Sat, 5 Jan 2002: > The "one in a billion" John refers to sounds very dramatic and difficult.
More dramatic than you 'infant daughter' and '40 mph past a school'? I explained in VERY GREAT DETAIL the effect of cumulative probability in requiring very low probability events to be taken into account. In principle, as the probability goes down, the number of risk scenarios increases *combinatorially*. There is no Olber's Paradox in this area, the 'night sky is infinitely brighter than the Sun'! > > So it may be helpful to refer to IEC 61508 which is a recently-published > 'basic safety publication' covering "The functional safety of electrical / > electronic / programmable safety-related systems" > > IEC 61508 uses the concept of the Safety Integrity Level (or SIL) to help > design safety-related systems which have quantified failure probabilities. > > The SILs for average probability of failure to perform design function on > demand are: > SIL level 1: up to 10^ -2 > SIL level 2: 10^ -2 to 10^ -3 > SIL level 3: 10^ -3 to 10^ -4 > SIL level 4: 10^ -4 to 10^ -5 or even lower levels > > The SILs for average probability of dangerous failure per hour of > operation > are: > SIL level 1: up to 10^ -6 > SIL level 2: 10^ -6 to 10^ -7 > SIL level 3: 10^ -7 to 10^ -8 > SIL level 4: 10^ -8 to 10^ -9 or even lower levels > > The standard describes how to select the SIL level for a particular > safety-related application, and we find that SIL4 is required where a > failure of the safety system could result in the deaths or serious > injuries > of large numbers of people. Yes, my 10^-9 figure was in the context of your 'relatives sobbing all over the courtroom'. > > Most safety-related applications that most practising engineers will be > involved in will be SIL1 or 2, maybe even SIL3, and hence require very > much > lower reliability than one in a billion. You are neglecting cumulative probability, in spite of quoting my whole text on it! SIL2, if it is applied to individual risk scenarios, is a recipe for disaster if you are putting many thousands of units, such as PCs or TVs, into the field. If is it applied, as it should be, to the cumulative probability of ALL risk scenarios, then *each one* needs to be constrained to that 10^-9 probability, preferably well below it. 100 scenarios at 10^-9 each gives a cumulative of 10^-7, after all. > -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
