In Case Of Failure

Article 504 of the National Electrical Code (NEC) covers installation requirements for intrinsically safe (IS) systems. Where the circuits installed are IS in accordance with the applicable control drawings, it is acceptable to use any wiring method suitable in unclassified locations, including the cable wiring methods covered in chapters 7 and 8. IS circuits do not depend on the wiring method to prevent ignition of a flammable or explosive atmosphere. If circuits are IS, the energy is maintained at a low enough level that it cannot ignite an explosive atmosphere if the circuit is opened, shorted or faulted to ground at the same time a fault occurs at the source. Therefore, the wiring method is not limited to what would be required for ordinary branch circuits installed in hazardous locations. 

Although the wiring is not an ignition threat, due consideration should be given to circuit operation. If the circuit has a safety function that would be lost as a result of damage, the wiring method could become very important. The purpose is to ensure the electrical system is not an ignition source. 

Control drawings required

Section 504.10(A) requires IS apparatuses, associated apparatuses and other equipment to be installed in accordance with the associated control drawing. It is extremely important to understand the control drawings’ value and requirements for IS circuits and systems. The control drawings are essential for proper installation of these systems. Zener-diode barriers installed for IS systems often reference a particular control drawing. A control drawing is required for these systems to provide specific information and instructions related to wiring methods, circuit lengths and so forth. The control drawings also typically include grounding and bonding information critical to the integrity of the IS system or circuit(s). A supplementary connection to the grounding electrode may be needed for some associated apparatus (e.g., zener-diode barriers) if the control drawing specifies. The main reason relates to the zener-diode barrier shunting to ground as it operates.

Grounding and bonding

NEC Sections 504.50 and 504.60 include the grounding and bonding requirements for IS systems. Section 504.50(A) requires IS apparatues, associated apparatues, cable shields, enclosures and raceways to be grounded if made of metal. The requirement includes the metal raceways and enclosures for such systems. Branch circuits that include an equipment grounding conductor as covered in 250.118 must supply IS systems. The equipment grounding conductor is necessary for grounding the metal enclosure(s) and other metal parts and equipment of the system.

As indicated above, it is not uncommon for the required control drawing(s) to specify a grounding electrode conductor connection in addition to connection to the required equipment grounding conductor. Usually, terminals for both of these grounding connections are located within the system enclosure or control panel. If a grounding electrode connection for the IS system is required, one of the electrodes specified in 250.50 must be used. The choice of grounding electrode is governed by the same means specified for services or separately derived systems as provided in Article 250. The grounding electrode must be as near as practicable to, and preferably in the same area as, the system’s grounding electrode conductor connection.

Identification of IS wiring 

The Code requires IS circuits are required to be identified at intervals not exceeding 7.5 meters (25 feet), and the identification must include the specific wording “Intrinsic Safety Wiring.” This identification requirement applies to raceways, cables, cable trays and other methods used for IS system wiring. Identification labels must be readily visible and easily traceable for the entire length of the wiring, except for any underground portions, which must be identified as they emerge from the earth. Any separate sections of this wiring created by walls, partitions or other enclosures must also be identified. Section 504.80(C) indicates that, where a color code is used for IS circuit conductors or wiring, such as raceways, boxes, etc., the color light blue must be used.

Spacing requirements 

The NEC permits any wiring method to be used for IS circuits. There are also specific spacing requirements that must be maintained between IS circuits and non-IS circuits. The concern here is the possibility of compromising the circuit’s intrinsic safety through inductive or capacitive coupling conditions related to close proximity of other wiring. 

IS circuits are generally restricted from being installed in the same raceway, cable tray or cable with conductors of any non-IS circuit. IS circuit conductors are permitted where they are separated from non-IS circuit conductors by a space of not less than 50 millimeters or by grounded metal partitions or insulating barriers. The identification rules in 504.80 help owners, installers and facilities maintain initial spacing when additional wiring or systems are installed. Refer to the control drawings and see NEC Article 504 for all IS system requirements.

About the Author

Michael Johnston

Executive Director of Standards, NECA
Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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