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Intrinsic safety versus explosion proof

ControlGlobal.com

CONTROL contributing editor, Ian Verhappen, takes a look at why intrinsic safety is so popular in Europe and other areas outside the U.S., and why it has never really caught on yet here in North America.

THE PURPOSE of both Intrinsic Safety (IS) and Explosion Proof is to prevent a malfunction in a piece of electrical process equipment from initiating an explosion or fire through ignition of gases that may be present in the surrounding atmosphere. Both systems do this by keeping the potential energy level below that necessary to start the ignition process.

Intrinsic Safety manages the amount of energy available to a level below which ignition can occur. Explosion Proof on the other hand contains the energy of any possible explosion within the enclosure containing the possible ignition source. This containment is done through careful design of the enclosure so hat the resulting energy is not only contained; it is also dissipated through the large surface of the flanges or threads of the enclosure. Consequently, if the integrity of the enclosure is compromised, either because of a scratch across the flange face or threads or incomplete tightening of the cover, the result is a significant increase in the risk of an explosion. The net result is that Explosion Proof protection has a higher level of required maintenance than an Intrinsically Safe system. (See Figure 1 below).

FIGURE 1: THE LIMITS OF POWER

Classifications
Class 1, Division 1 (Zone 0) areas are defined as having combustible mixtures present routinely or all the time. The probability of a combustible mixture in Div. 1 is defined as 1 to 10-1. This type of probability is a very high risk if arcing devices are used, so Div. 1 electrical design is very conservative.  Div. 1 electrical designs allow for multiple simultaneous failures without the possibility of igniting a flammable mixture.

Generally arcing devices for Div 1 areas are sealed and contained in explosion proof containers. A practical alternate to this approach is to limit power delivery to the extent that it is impossible to generate a spark. This alternate is Intrinsic Safety.

Class 1, Div. 2 (Zone 2) areas are defined as areas where the likelihood of a flammable mixture is low, but it is significant enough that normal electrical activity should not present an ignition source. The probability of a flammable mixture in Div. 2 is defined as less than 10-5. Single failure tolerance with normal activities is much cheaper to design, construct, and maintain than the more redundant designs required for Div. 1. Div. 2 design is the most commonly found area classification in the USA. Unclassified areas have a negligible probability of hydrocarbons. This type of area classification might be found in an office building or control center.

Quick History Lesson
Chris Towle, Secretary to several IEC standards committees and a long time practitioner in the field of electrical safety provides the following bit of background. “Intrinsic safety took off in Europe when transistorized equipment became available,” Towle says, “and computer control became the norm. Possibly the principal factor for the different approach was the area classification. The existence of Zone O, where the flameproof technique was not acceptable, required that intrinsic safety had to be used for some sensors, and since it had to be used for this purpose it became a recognized practice. There was no corresponding requirement imposed by Divisional classification. In 1960 to 1980 most of the investment in Europe was in petrochemicals and pharmaceuticals. This was predominantly done by companies with their own engineering capabilities who were not involved in petroleum refining.”

EPA Says "No!"
In the not too distant past, process leaks were very common. Leaking pump seals were a major culprit and were often tolerated as long as the leaks were small. Consequently, areas around pumps were generally treated as hydrocarbon sources with a moderate likelihood of some hydrocarbon present. If these pumps were in an open area with free air flow and maintenance was not too lax, the envelope around the pump would be classified as Div. 2. If the pump was in an enclosed area, the same envelope would be classified as Div. 1.

Because of these continuous leaks, the EPA made these pump leaks illegal. We now have double sealed or canned pumps, with monitoring programs to make sure ppm leaks are detected, corrected, and reported - far below the threshold of flammability. Average pump seal life has gone from a few months to 7 years and as a result the likelihood of a flammable mixture has dropped a few orders of magnitude over the last 30 years, and for the reasons just described, Div. 1 areas are illegal (by EPA standards) in the USA.