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What is nonincendive equipment? Section 500.7(F) of the National Electrical Code explains that it is a protection technique for equipment installed in Class I, Division 2, hazardous (classified) locations. With the advent of more stringent environmental regulations, intentional release of flammable materials is greatly restricted and, with the resulting changes in containment techniques, many areas that previously were classified as Division 1 (continuously flammable) have been reclassified as Division 2 (infrequently within the flammable range).

This allows the installation of nonincendive equipment in applications previously limited to Division 1 protection techniques, such as explosionproof or intrinsic safety. Nonincendive equipment tends to be less costly and less complicated to maintain than Division 1 equipment. The obvious economic benefits will mean that this protection technique will see an expanded usage in the years to come.

The same concept can be applied in Class II and Class III, Division 2, locations; but, in those locations, the enclosures of the nonincendive equipment must also prevent the entrance of dust.

Three NEC definitions apply:

• Nonincendive Equipment
Equipment having electrical/electronic circuitry that is incapable, under normal operating conditions, of causing ignition of a specified flammable gas-air, vapor-air, or dust-air mixture due to arcing or thermal means.

• Nonincendive Circuit
A circuit, other than field wiring, in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under specified test conditions, of igniting the flammable gas-air, vapor-air, or dust-air mixture.

• Nonincendive Component
A component having contacts for making or breaking an incendive circuit; the contacting mechanism is constructed so that the component is incapable of igniting the specified flammable gas-air or vapor-air mixture. The housing of a nonincendive component is not intended to exclude the flammable atmosphere or contain an explosion. ¹

Nonincendive equipment is a combination of nonincendive circuits and nonincendive components.

Nonincendive equipment may be listed by a independent nationally recognized test laboratory, or inspected and approved on-site by the authority having jurisdiction [AHJ]. Inspection and approval by an AHJ tends to be limited to relatively simple types of equipment. Although it is certainly feasible for an AHJ to examine more complicated equipment, access to schematics, bills of materials and component specifications are needed, and these may prove difficult to obtain. To facilitate the installation of the more complicated equipment, most manufacturers generally seek listing by an independent nationally recognized test laboratory, so additional inspection of the construction of the equipment is not required by the AHJ at the time of installation and can serve as a basis for approval.

Related to the protection technique nonincendive equipment is the wiring method, nonincendive field wiring, permitted by 501.4(B)(3) or 502.4(B)(3). Nonincendive equipment is required to be wired using the methods defined in 501.4(B)(1)/(2) or 502.4(B)(1)/(2) as these wiring methods provide a degree of physical protection to the wiring, and no "faults" in the wiring are considered likely.

Nonincendive field wiring provides an alternative wiring method based on limitation of energy in the wiring during possible wiring faults, similar to intrinsic safety wiring, but specifically limited to Division 2. This wiring method is generally limited to low-power apparatus such as 4-20 mA instrumentation circuits. The wiring employed may be any of those types permitted in unclassified locations, as nonincendive field wiring assumes that the wiring can be damaged in normal operation, but is not considered a source of ignition due to the limitation of the energy available in the wiring during possible wiring faults. In the examination of nonincendive field wiring, faults such as opening, shorting and grounding of the wiring are considered, and the resulting energy release is verified to be incapable of igniting a surrounding flammable atmosphere. Therefore, the wiring can be considered as suitably protected and not likely to cause a fire or explosion.

In addition to the wiring, there are two other integral parts to the installed nonincendive field wiring system: the associated nonincendive field wiring apparatus (power supply), and the nonincendive field wiring apparatus (instrument), the installation of which is defined on a control drawing.

Three additional NEC definitions apply when nonincendive field wiring is concerned:

• Nonincendive Field Wiring Apparatus
Wiring that enters or leaves an equipment enclosure and, under normal operating conditions of the equipment, is not capable, due to arcing or thermal effects, of igniting the flammable gas-air, vapor-air, or dust-air mixture. Normal operation includes opening, shorting or grounding the field wiring.

• Associated Nonincendive Field Wiring Apparatus
Apparatus in which the circuits are not necessarily nonincendive, but affect the energy in nonincendive field wiring circuits and are relied upon to maintain nonincendive energy levels. Associated nonincendive field wiring apparatus may be either of the following:

(1) Electrical apparatus that has an alternative type of protection for use in the appropriate hazardous (classified) location

(2) Electrical apparatus not so protected that shall not be used in a hazardous (classified) location

FPN: Associated nonincendive field wiring apparatus has designated associated nonincendive field wiring apparatus connections for nonincendive field wiring apparatus and may also have connections for other electrical apparatus.

• Control Drawing:
A drawing or other document provided by the manufacturer of the intrinsically safe or associated apparatus, or of the nonincendive field wiring apparatus or associated nonincendive field wiring apparatus, that details the allowed interconnections between the intrinsically safe and associated apparatus or between the nonincendive field wiring apparatus or associated nonincendive field wiring apparatus.²

The wiring method, nonincendive field wiring, requires the user to align the output parameters of the associated nonincendive field wiring apparatus with the input parameters of the nonincendive field wiring apparatus in accordance with the control drawing(s). When aligned correctly, the voltage and current limitation of the associated nonincendive field wiring apparatus combined with the limited energy storage (inductance and capacitance) of the nonincendive field wiring apparatus limits the potential release of energy in the wiring to levels below that required to initiate an explosion.

Additional information on examination and testing of nonincendive equipment and the alignment of nonincendive field wiring parameters can be found in ANSI/ISA – 12.12.01-2000, "Nonincendive Electrical Equipment for Use in Class I and II, Division 2, and Class III, Divisions 1 and 2, Hazardous (Classified) Locations," available from – The Instrumentation, Systems, and Automation Society (ISA), or visit their Web site at http://www.isa.org.

© 2002 FM Global Technologies LLC. All rights reserved. Permission granted one-time use international rights.

1 NEC 2002, National Fire Protection Association, Article 100, Definitions, p. 70-37.

2 NEC 2002, National Fire Protection Association, 500.2, Definitions, p. 70-339–340.

Bill Lawrence is a senior engineering specialist with the Hazardous Location group of FM Approvals, a U.S. nationally recognized testing laboratory. Bill has been with FM Approvals for nineteen years and is involved with the examination and testing of electrical equipment rated for use in hazardous (classified) locations. Before joining FM Approvals, he was employed by a test instrument manufacturer where he also worked in product safety. He received his Bachelor of Science in Electrical Engineering from the Pennsylvania State University and currently serves on various National Fire Protection Association, ISA, and International Electrotechnical Commission committees for hazardous location electrical equipment.

Nick Ludlam is a technical team manager with the Hazardous Location Group of FM Approvals, a U.S. nationally recognized test laboratory. Nick has been with FM Approvals since 1996 and is involved in the examination and testing of intrinsically safe and nonincendive electrical equipment for use in hazardous (classified) locations. Before joining FM Approvals, he spent 13 years with Sira in the United Kingdom. Sira is a European Notified Body for the certification of electrical equipment for use in hazardous locations and was the laboratory manager for this business area. He was also a member of the advisory group for hazardous location certification also known as the Heads of Test Laboratories or HoTL. Nick currently serves on various Instrumentation, Systems and Automation society standard committees, is the Chair of ISA SP12.02 for intrinsic safety and ISA SP12.12 for Division 2 / Zone 2 electrical equipment. He is a member of IEC MT19 for Type of protection ‘n’ and sits on the NFPA’s panel 14 for the Hazardous (Classified) Location section of the National Electrical Code.

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