If circuits are intrinsically safe (IS), the energy is at a low enough level that ignition of an explosive atmosphere will not occur if the circuit is opened, shorted or faulted to ground at the same time a fault occurs at the source. IS circuits do not depend on the wiring method to prevent ignition of flammable or explosive atmospheres. 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 the operation of the circuit. Should the circuit have a safety function that would be lost if the circuit were damaged, then the wiring method chosen is very important. Installation requirements for IS systems are covered in Article 504 of the National Electrical Code . IS apparatuses and associated apparatuses must be listed.
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. A control drawing provides specific information and instructions related to wiring methods, length of circuits, and so forth. It is extremely important to understand the value and requirements of the control drawing for IS circuits and systems. The control drawing(s) are essential for proper installation of these systems and also necessary for making an installation. Zener-diode barriers installed for IS systems often reference a particular control drawing.
These control drawings also typically include grounding and bonding information that is critical to the integrity of the IS system or circuit(s). A supplementary connection to the grounding electrode may be needed for some associated apparatuses, e.g., zener-diode barriers, if specified in the control drawing. The main reason relates to the zener-diode barrier shunting to ground as it operates. Follow the control drawing.
Grounding and bonding
Sections 504.50 and 504.60 include the grounding and bonding rules for IS systems. Section 504.50(A) requires IS apparatuses, associated apparatuses, cable shields, enclosures and raceways, if made of metal, to be grounded. This 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.
It is common 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, the grounding electrode(s) shall be as specified in 250.52(A)(1), (2), (3) and (4). The grounding electrode(s) specified in 250.52(A)(5), (7) and (8) shall not be used unless the electrodes in 250.52(A)(1), (A)(2), (A)(3) and (A)(4) are not present. Metal parts and raceways for IS wiring must be bonded in accordance with Section 504.60.
Identification of IS wiring
The NEC requires IS circuits to be identified at intervals not exceeding 7.5 m (25 feet) and must include the specific wording “intrinsic safety wiring.” This identification requirement applies to raceways, cables, cable trays and other wiring methods used for IS system wiring. Identification labels must be located in a way that is readily visible and easily traced 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 and so forth, the color light blue must be used.
The NEC recognizes that any wiring method can be used for IS circuits. There are also specific spacing requirements that must be maintained between IS and non-IS circuits. The concern is the possibility of compromising the intrinsic safety of the circuit through inductive or capacitive coupling effects related to close proximity to 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. Conductors of IS circuits are permitted where they are separated from non-IS circuit conductors by a space of not less than 50 mm (2 in.) or by grounded metal partitions or insulating barriers. The identification rules in 504.80 help owners, installers and facilities managers maintain initial spacing when additional wiring or systems are installed. Always refer to the control drawings and see Article 504 for all NEC requirements that apply to IS circuits and systems.