It's Only Low Voltage

By Michael Johnston | Feb 15, 2009
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Electrical circuits and systems in hazardous ­(classified) locations present challenges for installers. The National Electrical Code (NEC) indicates that hazardous locations are where explosions or fire hazards are possible due to flammable gases, flammable liquid-produced vapors, combustible liquids, combustible liquid-produced vapors, combustible dusts or ignitable fibers/flyings.

Low-voltage or limited-energy systems present the same ignition hazards as those of 120 volts and greater. The concern is that arcing, even at a low-energy level, could cause ignition of an explosive material.

It is common to think low-voltage and limited-energy systems are less dangerous than higher voltage systems. From a shock and fire hazard standpoint, this may sometimes be the case, but when installed in hazardous locations, these systems cannot be treated any differently than 120-volt systems. The arc causes ignition, so the hazards are the same.

Chapter 5 of the Code provides wiring rules for special occupancies, including all hazardous locations. The key to how the NEC applies to low-voltage and limited-

energy systems in hazardous locations starts with Section 500.1, which indicates that the rules in Articles 500 through 504 apply to electrical and electronic equipment and wiring for all voltages. Taking this a step further, if one were working with a remote control signaling circuit, the basic rules are provided in Article 725. As an example, Section 725.3(D) validates the information provided in Section 500.1 by providing the correlation between Article 725 and the applicable rules for all wiring in hazardous locations covered in Articles 500 through 516 and Part IV of Article 517. Not only does this installation have to meet the general rules in Article 725 and in Chapters 1 through 4 as referenced, it also must be installed in compliance with all applicable Chapter 5 requirements for a hazardous location. Similar rules in other NEC limited-energy articles, such as those covering communications systems and fire alarm systems, appear within each article to refer the reader to important applicable rules to be considered when these systems are located in a hazardous location.

When wiring in hazardous locations, it is critical to not cause ignition of the explosive atmosphere and to not spread the hazardous atmosphere to other areas through the conduit systems. Suitable identified enclosures must be used for equipment, conduit seals must be provided and appropriate wiring methods used. This requires these circuits to be installed in a rigid conduit system or other wiring method suitable for use in a hazardous location as provided in Section 501.10 for Class 1 locations, 502.10 for Class 2 locations, and 503.10 for Class 3 locations. The degree of hazard is what determines the type of wiring method that must be used, even for low-voltage or limited-energy systems. The degree of hazard is provided as a particular division or zone under each classified location. For example, Class 1, Division 1 is a greater degree of hazard than Class 1, Division 2; thus, the wiring methods allowed for one degree of hazard differ slightly from others.

It is important to know how an area is classified—and the extent of the classified area—for correct wiring methods to be installed. Section 500.4 requires all hazardous locations to be properly documented, and the documentation must be available for those authorized to install, maintain, inspect or operate electrical wiring and systems in those areas.

Circuits and systems identified as intrinsically safe are suitable for installation in any hazardous location. These circuits are not capable of causing ignition because their energy levels are so low. Any wiring method can be used, including cable wiring methods provided in Chapters 7 and 8. Intrinsically safe circuits and systems are suitable for Division 1, Zones 0 and 1 locations. A nonincendive circuit is defined in Section 500.2 as one in which any arc or thermal effect produced under intended operating conditions is not capable, under specified test conditions, of igniting the flammable gas/air, vapor/air, or dust/air mixture. Nonincendive circuits and systems are suitable for Division 2 and Zone 2 locations, the difference being the degree of hazard involved. When installing these systems, specific control drawings are necessary. These limited-energy systems are suitable for installation in hazardous (classified) locations only when installed according to the drawings. The manufacturer of nonincendive systems and intrinsically safe systems provides the applicable control drawings. While a circuit or system may be low voltage or of limited energy, the requirements for these circuits and systems in hazardous locations are generally the same as those for higher voltage circuits and systems. Unless intrinsically safe circuits or systems or nonincendive circuits or systems are installed per the applicable control drawings, the wiring methods and equipment specified in Chapter 5 for the particular classified area must be used.

It’s only low voltage, but it must be safe and Code-compliant.

JOHNSTON is NECA’s executive director of standards and safety. He is former director of education, codes and standards for IAEI; a member of the IBEW; and an active member of the NFPA Electrical Section, Education Section and the UL Electrical Council. Reach him at [email protected].

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected]


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