O, Mr. Rezabek, there's a problem with your column in May...

In May, we published a provocative column by John Rezabek (see "On the Bus" May 2008) that asked the question, "Is Intrinsic Safety Obsolete Yet?" We got lots of mail. As John said earlier this week when I saw him in Amsterdam, "I'm still pulling the spears out of my back!" But the fact is, his question, and his argument, were compelling enough that even though I didn't agree with him, I was more than willing to let him write it. Yesterday, I received (too late for inclusion into the June issue) a letter from Chris Towle, founder of MTL, one of the pillars of the IS industry. He also sent us a fascinating white paper, which we have put on ControlGlobal.com, in which he reviews the future of Intrinsic Safety. Here's Chris' letter, and remember, you can start the conversation here by clicking on the comment button... Letter to Control Magazine I read with interest the recent note, which raised the question “Instrinsic Safety Obsolete Yet?” [The temptation to reproduce the spelling mistake was irresistible.]. The note may reflect the current attitude of the North American refining industry, which has never been the most enthusiastic user of intrinsically safe systems, but possibly fails to take into account some other factors. For example many plant designs aim to be such that they can be reproduced in any part of the world with the minimum of modification to suit local requirements. These plants frequently use intrinsically safe instrumentation because it is the only explosion proof technique that is universally acceptable. The increasing recognition of IEC Ex certification ensures that this preference will grow. Area classification is changing and Zones 1 are becoming smaller, but Zones 0 continue to exist and intrinsic safety is the preferred solution in that situation. The effect of the risk analysis approach to plant safety implicit in the European ATEX directive and being introduced in the IEC standards may have an impact on the choice of instrument systems. The replacement of ‘nL' systems by the ‘ic’ concept [intrinsically safe in normal operation] in IEC standards extends the use of intrinsic safety to all three Zones. The consequent reduction in spares holding and the need for education in different techniques may prove significant. How far this change will penetrate the North American non-incendive scene is open to debate? The probability is that it will be patchy and will take some years. The increasing use of intelligent instruments makes fault diagnosis from the safe area much easier and reduces the need for complex test equipment in the hazardous area. However the more complex field instruments are potentially less reliable and there are still many simple devices such as switches and thermocouples which are much easier to check when not isolated. Working on a Fieldbus network which has to be isolated to permit replacement or checking is not desirable. There remains a significant advantage in being able to do ‘live maintenance’ without gas clearance certificates and because the probability of making mistakes is reduced it is safer. There are other applications of intrinsic safety such as portable personal communicators and gas detectors where intrinsic safety is the only practical solution. These applications are probably not relevant to the matter of the original note but are a considerable factor in keeping intrinsic safety alive. Similarly there is potentially a significant use of intrinsic safety where the explosion hazard is created by dust. In short to paraphrase a well-known saying ‘ the reports of the obsolescence of intrinsic safety are possibly a little premature’. Chris Towle Founder, MTL Instruments
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  • <p>Perhaps the point of the column was lost in the "senationalism" as Dr. Rothenberg astutely observed. Good engineering practice says, eliminating a risk trumps profilactic measures. IS advocates will say that this is the point of IS. But, if we get to the point where lifting an instrument wire or opening an electronics enclosure under power is made rare enough, the prophylactic measure of current-limiting zener diodes and their ilk will be less critical. We will still need IS, especially for the tablet PC's or PDA's we use to interact with the field devices, as well as for the devices we need that do not inherently meet IS or non-incendive certifications.</p> <p>Thank you to Mr. Towle for sparing me the pointy end of the spear! </p>


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