31 May 2001
A universal approach for hazardous-area classifications
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, current-to-pneumatic converter and pneumatic-to-current 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? 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 application-specific 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 hazardous-area application. IT
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Jim Peterson is a senior applications engineer at MTL, Inc. in Hampton, N.H.