INTRINSIC SAFETY In Tech • www.isa.org • June 2001 51 A universal approach for hazardous-area classifications By Jim Peterson Intrinsic safety is cost effective everywhere. The adoption by the National Electric Code of the International Electrotechnical Commission standard allowing use of zones, an alternative to the traditional division method of area classification, was added to the 1996 code and greatly expanded in the 1999 code. The availability of two area classification methods increases flexibility when choosing the electrical equipment used in hazardous areas. Though the traditional, North American method of area classification in North America is still useful, engineers can now use the zone method for new installations or reclassifying existing facilities. The zone method allows designers to use a wider variety of equipment and techniques than the traditional method in all but Zone 0 applications, where intrinsic safety (IS) is the only method that may be used. The only technique allowed in all area classifications worldwide is IS. The designer’s choice of a technique depends on the type of equipment required. Low-power signal and process control equipment is readily available and easily applied in all area classifications using IS, which is by far the safest standard, regardless of the area classification. IS systems are not only safest but also very cost effective. Further, IS is a truly universal approach because devices certified for a higher classification can be used in lower classifications of a similar gas group and temperature rating. Intrinsic safety’s advantages When choosing a method of protection, evaluate the following: Are adequate field and interface devices available for the application? The number of IS field devices and interfaces is large and steadily growing. The devices now available include transmitters, valve positioners, electropneumatic valve actuators, displays, currentto-pneumatic converter and pneumatic-tocurrent converter devices, proximity sensors, and many more. Can designers use general-purpose devices in a hazardous area? The use of “simple apparatus” devices that will neither store nor generate more than 1.2 volts, 25 milliwatts, or 20 microjoules of energy, as defined in NEC Article 504-2, is an advantage of the IS method. Some examples are light-emitting diodes, remote terminal displays, thermocouples, and switches. These devices require no certification when used with suitable “associated apparatus” (intrinsically safe barriers). Are live maintenance (powered loops) and troubleshooting permitted in the hazardous area? INTRINSIC SAFETY 52 June 2001 • www.isa.org • In Tech Zone-based standard approaching adoption By Nick Ludlam The standard expected to form the basis for IS zones in U.S. practice is undergoing review and balloting, and a final version may be available to practitioners as early as next year. ISA’s SP12.02 committee is responsible for developing a number of standards related to the hazardous (classified) location type of IS protection. The committee has prepared three standards: ISA-12.02.01–1999, Electrical Apparatus for Use in Class I, Zones 0, 1 & 2 Hazardous (Classified) Locations – Intrinsic Safety Requirements; ISA-RP12.2.02–1996, Recommendations for the Preparation, Content, and Organization of Intrinsic Safety Control Drawings; and ISA-TR12.2–1995, Intrinsically Safe System Assessment Using the Entity Concept. ISA-12.02.01–1999 resembles IEC 60079-11 (1988) but introduces differences to comply with American practice and to agree with the requirements of ANSI/NFPA 70, National Electrical Code (NEC). The ISA standard differs from the International Electrotechnical Commission (IEC) standard in its requirement for an input voltage of 250 volts (V) for stand-alone associated apparatus, 120 V or 250 V for non-stand-alone associated apparatus, and the addition of a section on shunt diode zener barriers. Work on the standard started after publication of the 1996 NEC, which introduced the zone concept for hazardous (classified) locations in a new Article 505. ISA is presently planning to adopt a revised version of IEC 60079-11 (1999) as a standard. This work is nearing completion, and the draft is currently being balloted by the subcommittee. Once finalized, it is the subcommittee’s aim that this document will form the basis for IS for zones in the U.S. One apparent problem is the existence of ANSI/UL 913–1997, Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II and III, Division 1 Hazardous (Classified) Locations, and ANSI/UL 2279–1997, Electrical Apparatus for Use in Class I, Zone 0, 1 and 2 Behind the byline Jim Peterson is a senior applications engineer at MTL, Inc. in Hampton, N.H. His address is email@example.com. Hazardous (Classified) Locations. ANSI/UL 913 is the IS standard referenced in the 1999 edition of the NEC. ANSI/UL 2279 is one of the documents listed in the NEC in Article 505 for zone concept. The differences among the standards appear to be considerable. The ISA document emulates IEC 60079-11 and lists major technical deviations in an annex. UL 2279 provides only the deviations from the IEC document. This is because ISA has the exclusive U.S. rights to publish the IEC text. ISA and Underwriters Laboratories (UL) are working together, however, to harmonize the technical requirements of the two U.S. versions of the IS zone standards, and a correlating committee has been set up by ISA and UL for this specific purpose. A co-published ISA/UL document will be available sometime in the future. Although UL 913 covers a concept similar to both ISA-12.02.01 and UL 2279 Part II, UL 913 is for Class I, II, and III, Division 1 hazardous (classified) locations. At this time, the major differences between the draft ISA12.02.01 standard and IEC 60079–11 (1997) standard include the addition of a requirement for live maintenance and a section on control drawings. The committee is now rewriting ISA-RP12.2.02 and ISA-TR12.02.03. Work on these two documents is at an early stage, but the subcommittee is hoping to have the documents ready for publication in early 2002. The ISA SP12.02 committee is working closely with ISA SP12.06 to develop installation guidelines for intrinsically safe circuits in both Class I, II and III, Division 1 hazardous (classified) locations and Class I, Zone 0 and 1 hazardous (classified) locations. If everything goes according to plan, the three documents will be available early next year. Behind the byline Nick Ludlam is a senior engineer with Factory Mutual Global in Norwood, Mass. His address is firstname.lastname@example.org. The design of IS loops allows field devices and cables to be safely serviced without shutting down power. The low-power and fault-tolerant circuitry associated with IS provides added safety for personnel. Is there an acceptable level of safety for both personnel and property? By choosing IS, you are afforded the safest proven method available. Is mounting space a problem? IS devices, by nature of their low-power design, are smaller than other hazardous-area devices. The IS interfaces can be the high packing density type, which saves control-room real estate. Designers can mount devices in Class 1, Division 2/ Zone 2 areas in general-purpose enclosures, thus limiting expensive cable runs. Further, incorporating IS eliminates requirements for rigid sealed conduit because IS cable installation can use any method approved for ordinary areas. Owners realize cost savings with installation, access to cables for maintenance and repair, and overall cost of ownership. Is adequate, clear, and concise technical support of the chosen method readily available? IS is a well-understood, universally accepted method of protection with well-defined design, certification testing, and installation practices. Are the field devices and interfaces compatible with major control systems? A wide variety of devices is available for interfacing IS field devices with controllers, ranging from applicationspecific devices geared to the large distributed control system and programmable logic controller manufacturers to custom solutions for any conceivable controller configuration. Traditionally, engineers use intrinsically safe systems in only the most hazardous environments. IS need not be restricted to these areas, however. The extremely high degree of safety, low-cost maintenance, installation ease, increasingly broad availability of components, and worldwide acceptance make IS the most sensible choice for any hazardousIT area application.