By Dan Hebert, Senior Technical Editor
Installing electrical and automation panels in hazardous areas requires use of one of three protection methods: explosion-proofing (Exd), purge and pressurization (Exp) or intrinsic safety (IS). Exd contains the pressure of the explosion, and then cools it through a critical flame path to a level that will not ignite the surrounding environment.
Exp protects the environment by segregating hazardous material from the ignition source before equipment is powered. First, explosive mixtures are purged from the enclosure; then a positive pressure is maintained inside the enclosure to insure hazardous gasses do not propagate back in during operation.
IS protection limits the energy entering the hazardous area to a level incapable of igniting the easiest ignitable concentrations of gas/air mixtures under fault conditions.
"None of the protection methods can provide absolute certainty of preventing an explosion," observes Robert Schosker, product manager for intrinsic safety and HART at Pepperl+Fuchs (www.pepperl-fuchs.com). "However, statistically, the probabilities are so low that not even one incident of an explosion has been verified when a standardized protection method has been properly installed and maintained."
Joe Zullo, IS business unit manager for the Americas at Cooper Crouse-Hinds MTL (www.cooperindustries.com), adds that, "The common methods of IS are Exia (safe under two faults) and Exib (safe under one fault). Though the United States and Canada do have zone classifications on the books, most installations fall under the division concept, which requires Exia equipment."
IS is feasible only when total power requirements are relatively low. Electrical distribution panels aren't candidates for IS, nor are most analyzers and some instruments, all because of their relatively high power requirements. But, if the components in the panels have low powerneeds, then IS is often the best solution.
"IS is the only method permissible in all classes, divisions and zones—and the only method accepted for Zone 0. No special wire, cable, enclosures, conduits, seal-offs or alarms are required. Hot maintenance is allowed. The flexibility and ease of installation and maintenance make it very effective for process control and measurement," explains Zullo.
Despite its advantages, IS isn't widely used in some parts of the world. "IS is very popular in Europe and Asia, but not so much in the United States or the Middle East," notes Rob Stockham, general manager of Moore Industries-Europe (www.miinet.com).
Many observers chalk up the relatively low use of IS to nothing more than greater familiarity with other protection methods. If so, this prevents an opportunity for money saving in these areas by thinking outside the explosion- proof and purged-enclosure boxes.
"Relative to maintenance costs, intrinsic safety is the most advantageous because it allows live maintenance with no need for plant shutdown. Intrinsic safety is also more reliable because of the use of de-rated components as prescribed by the standards," says Schosker. "Comparing the three most widely used protection methods, intrinsic safety is preferred for safety and reliability reasons. Intrinsic safety is also the most economical for installation and maintenance. The use of intrinsic safety provides the best mix of an affordable system and safety requirements."
Stockham seconds Schosker's opinion, but adds some caveats. "With IS, there is never enough energy to initiate an explosion, so conventional panels, wiring and glands can be used. The costs of devices and installations can be less, and the devices can be accessed and worked on live," explains Stockham. "However, IS systems have to be planned and designed using certified components matched to meet safe electrical system parameters. Not all devices can be made IS, and some site policies may prohibit mixing of protection methods to avoid confusion."