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Approvals and certifications of the Instrument IndustryAnother sad fact: only a few years ago Washburn mill wascompletely burned down for the second time in 1991.The reason was very simple again, which did not lead toany new physical theories. Around the machines inside thebuilding, flammable plastic and wooden materials wereused in the hazardous area. Now the ATEX Directive prohibitsthe usage of all flammable materials in Dust-Exzones.At NIVELCO we achieveincreased safety - in orderto avoid catastrophes - byapplying active and passivecircuit protectioncomponents. In manycases these methodsmake the system toocomplex, eventually theycould be the obstacles to reliable functionality.This is almost the same situation as in the case of thehousehold electrical miniature circuit breakers or fuses:if they shut down the power frequently, the power supplywill be unstable, if we disable the protection, we are endangeringour personal and material safety.The Washburn millon 2nd of May in 1878 (source: wikipedia.org)Learning from the experiences of similar accidents, safetyregulations for hazardous explosive atmospheres arechanging every year and getting stricter and stricter in orderto prevent future disasters. At NIVELCO we are using therevised versions (MSZ EN 60079-0:2010, MSZ EN60079-25:2011, MSZ EN 60079-31:2010) of the 60079numbered multi-volume standard-collection, done by theIEC. This standard has been revised recently in 2010 and2011 by CEN/CENELEC (European Committee forElectrotechnical Standardization).Another conformance mark, similar to CE is the so-calledUL sign used in the USA. In contrast to CE, this does notmark hundreds of notified bodies, but only one certificationinstitute. This institute is the Underwriters Laboratories establishedin 1894. UL has developed remarkable amountof safety and measurement standards in many specificareas during the past 120years. Most of these standardsare required by thelaws, one of these is UL94which is binding all overthe world. The UL94 standardis about the classificationand determinationof plastics flammability.The IEC (International Electrotechnical Commission)created a multi-volume standard-library numbered61508 in order to define the categories of operationalsafety as well as summarise the measures for achievingadequate safety. The related application-technologydescription can be found in the 61511 numbered standard.These standards provide detailed definition of relatedconcepts including hazard, risk, hazardous and safefailure, probability of failure, allowable frequency offailure, statistical life expectancy. Usually there are complexprobability-calculating algorithms likewise testing andmeasuring techniques for the determination of theseconcepts. The question of operational safety is mainlyapproached by mathematical and statistical methods inthe standard. In a huge system it is not enough to determinethe “weakest link” or fix it, because all instrumentscarry the probability of failure.The devices are divided into 4 categories according totheir probability of failure. These categories are the socalled„SIL” (Safety Integrity Level) classes. In case of thelowest (SIL1) class the statistical expected failure-less periodis approximately 100 years, the highest (SIL4) classequipments are able to operate flawlessly up to 100000years! Of course if we want to use 5000 pieces of independentSIL4 rated instruments the statistical time periodof flawless operation is only 20 years. It is easy to justifywith the help of mathematics that using “redundant” devices- two devices for one application - the probabilityof failure can be halved.22