31 May 2001
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 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 ISA-12.02.01 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.