Grounding and bonding practices for hazardous areas
Within certain industrial facilities, there are areas of atmospheric conditions that may be inherently harmful to humans. If an ignition source is present within those areas, an explosion or fire can take place, causing damage to equipment, and injury or death to personnel.
Within certain industrial facilities, there are areas of atmospheric conditions that may be inherently harmful to humans. If an ignition source is present within those areas, an explosion or fire can take place, causing damage to equipment, and injury or death to personnel. Flammable gases or vapors, combustible dusts, or ignitable fibers and flyings produce hazardous locations and represent distinctly different fire or explosion hazards.
Such conditions exist in many places, but the ignitable or combustible mixtures are usually not present in quantities that can create a fire or explosion hazard. These ignitable and combustible materials must exist in concentrations high enough to warrant a hazardous location.
In North America, a Class and Division system has been adopted for hazardous locations. Hazardous locations are divided into three Classes with each Class subdivided into two Divisions. The Classes are based on the type of hazard potential and the explosive nature of the materials, while Divisions are based on the probability of an explosion occurring.
Class I locations (defined by National Electric Code [NEC] Articles 500 and 501) have flammable gases or vapors (or have the potential for such gases or vapors) in concentrations within the atmosphere that may create the potential for fire or explosion. Class I subdivides into Division 1 and Division 2 categories based on the probability that a gas or vapor exists in the atmosphere. Division 1 areas are at a much higher risk, whereas Division 2 areas are at a lower risk but still constitute a hazard under abnormal operating conditions or catastrophic failure.
For example, a Class I, Division 1 location could be inside a vessel that has flammable gas around its vent(s), where flammable vapors can be present. A Division 2 location may be the area immediately surrounding the tank that does not normally contain sufficient quantities of gas or vapors but could in the event of rupture or leak.
Class II locations (NEC Articles 500 and 502) have combustible dusts (or have the potential) in high enough concentrations within the atmosphere that the potential for fire or explosion exists. In Class II, Division 1 areas, combustible dusts normally are suspended in the air. Examples of Class II, Division 1 areas are inside an elevator leg or a grain receiving area with no aspiration. In Class II, Division 2 areas, the concentration of combustible dusts is normally not present within the atmosphere, but it could be under adverse conditions, such as mechanical breakdown or abnormal equipment operation. The interior of a typical feed mill would be an example of a Class II, Division 2 area.
Class III locations (NEC Articles 500 and 503) contain ignitable fibers or flyings that are not likely to be suspended in air but are in concentrations high enough to produce a fire or flash explosion. The Class III location categories are divided differently from those in Class I and II. Class III, Division I locations primarily contain fibers and flyings due to manufacturing materials such as rayon or cotton, while Class III, Division II locations contain ignitable fibers and flyings that are handled and stored but are not part of the manufacturing process. While Class I and II locations are subdivided into several groups based on their material composition, Class III locations are not.
Various sources of ignition can prompt fire or explosions due to hazardous locations within industrial and commercial facilities. Mechanical breakdown of equipment, open flame, cigarettes, or sparking from sources such as dropping tools, excessive temperatures, and electric arcing or sparking are all potential sources. For the purpose of this article, attention will be focused on electric arcing or sparking.
Industries that typically are at risk for hazardous classified locations include, but are not limited to, occupancies such as gasoline dispensing and service stations, paint-finishing process plants, petroleum and petrochemical processing plants, wood processing facilities, agricultural grain handling and processing facilities, coal pulverizing plants, textile mills, and facilities where metal dusts and/or powders are processed.
With the guidelines for hazardous “classified” locations covered in summary detail, the next step is to define grounding and bonding and discuss how they are used in hazardous and nonhazardous areas.
Grounding and bonding
There are many different types of grounding systems. Equipment (safety) grounds, grounding for lightning protection, static grounds, system grounds, and electronic system grounding are all examples. Equipment grounding consists of three primary considerations: personnel safety, proper operation of protective devices, and mitigation of electromagnetic interference (EMI). In regard to grounding and bonding in hazardous locations, engineers mainly are interested in equipment grounding, and within equipment grounding, engineers primarily are concerned with personnel safety and operation of protective devices. Therefore, EMI and the other types of grounding systems mentioned above will not be covered in this article.
In order to understand grounding and bonding, one must know the differences between them. Grounding is the physical connection of conductive material to earth in order to limit the voltage imposed by lightning or unintentional contact with a higher voltage line, and also to limit the voltage to ground on normally noncurrent carrying materials. An example is the green wire pulled back from a light fixture to a panel board.
Bonding, on the other hand, is defined in Article 250 of the NEC as “The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.” An example of bonding would be bare copper wire connecting a processing pipe to nearby building steel.
Why is bonding necessary? Article 250 of the NEC requires that all conductive material be electrically connected (bonded) together to limit the potential difference between one piece of equipment and another.
As mentioned above, personnel safety and proper operation of protective devices are two primary concerns with equipment grounding. Each of these is attained through the proper use of an equipment grounding conductor (EGC). The EGC is defined by Article 100 of the NEC as “the conductor used to connect the noncurrent carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.” Noncurrent carrying metal parts are bonded together and then tied to the ground (grounding) using an EGC.
A properly designed and installed equipment grounding system can prevent fire and explosions in hazardous areas. The EGC, or fault return path, aims to accomplish three tasks. First, the EGC must be electrically permanent and continuous from any point in the system back to the source of power for that circuit. Second, the EGC must be sufficiently sized to handle any fault current that may be imposed. If not, an improperly sized grounding conductor will overheat, and its insulation and the insulation on the phase conductors may deteriorate over time. Finally, the fault current return path must have a sufficiently low impedance from the source of the fault back to the power supply. This limits the voltage to ground so the overcurrent protective device operates quickly and prevents damage to equipment, eliminating a shock hazard to personnel.
It is critical in both hazardous and nonhazardous areas that the protective device operates quickly. The amount of fault current that flows through the circuit and the time/current characteristics determine a protective device's operating speed. Based on Ohm's Law, how much current will flow is determined by the impedance between where the fault occurs and the source of power, hence the requirement for a low impedance path. If the equipment grounding conductor is not run in the same raceway as the phase conductors, if there is a loose or rusted connection, or if an improperly bonded conduit is present, the impedance of the fault return path will increase.
If a fault occurred under any of these conditions, the impedance of the ground fault return path may have increased and the fuse or circuit breaker will take too long to open or will not open at all. This may result in the fault continuing to burn and may develop into an ignition source if within a hazardous location (see Figure 1). Thus, proper equipment grounding within hazardous areas becomes even more important than in nonhazardous locations.
All of these requirements for the EGC are for personnel safety and, in the case of hazardous areas, can prevent an arc or spark from reaching the hazardous atmosphere. Additionally, the arc flash level and associated personal protective gear can be affected by the length of time required to open an overcurrent protective device during a ground fault event.
What must we consider “above and beyond” normal grounding and bonding in hazardous locations? In hazardous locations previously defined, all potentially conductive equipment, raceways, and other enclosures connected by permanent wiring methods—regardless of voltage—must be bonded by one or more of the following methods according to NEC Article 250.92:
Listed pressure connectors
Threaded couplings or threaded bosses with the wrench installed tightly
Threadless couplings and connectors installed tightly for metal raceways and metal-clad cables
Other listed means such as bonding-type locknuts, bushings, or bushings with bonding jumpers.
This must be done for motors, raceway, junction boxes, liquid-tight flexible connections, lights, receptacles, PLCs, motor control centers, and similar devices so that the removal of any equipment does not interfere with or interrupt the grounding continuity.
In hazardous classified locations, all equipment must be grounded and bonded as specified in NEC Article 250. However, do not depend on locknut bushings and double locknuts for bonding raceways to enclosures. A bonding locknut with a bonding jumper, threaded bosses, or other listed means must be installed at any point on the conduit system from the hazardous location back to the source of power (see Figure 2).
In hazardous classified locations, liquid-tight, flexible-metallic conduit cannot be relied on to make the required ground connection. It must be bonded with a bonding jumper, either inside or outside the flexible connection as required by Article 501.30(B), 502.30(B), and 503.30(B), respectively. The bonding jumper must be routed with the flexible raceway and should not be wrapped around the flexible connection, as this can create an inductive choke. In Class I, II, and III, Division 2 locations, there is an exception to the rule. The bonding jumper may be eliminated if the flexible connection is less than 6 ft, fittings listed for grounding are used, the protective device feeding the circuit is less than 10 amps, and the load is not used for power utilizations. A good standard to follow in all Class I and II locations (regardless of the Division) is to install the bonding jumper on the outside of the flexible raceway, because it is common for flexible connections to break or crack. This also allows maintenance personnel to quickly check the continuity of the bonding connection.
Methods of grounding and bonding in both hazardous and nonhazardous areas help reduce the risk that an ignition source will exist. This is accomplished by attempting to eliminate static charge and dangerous voltage potentials between normally noncurrent metal parts of equipment and allowing the protective device to operate quickly in order to clear faults.
Heinemann has been an electrical engineer for Interstates Engineering since 2005. He graduated with an electrical engineering degree from the University of Manitoba in Winnipeg, Canada, and has his EIT certification in Iowa. In his current position, Heinemann designs, specifies, and coordinates industrial electrical facilities. Vande Hoef is a senior project engineer for Interstates Engineering and has been with the company since 2002. Vande Hoef's duties include oversight of technical design, client relationship management, and specification of electrical equipment and installation procedures. He is a licensed PE in Iowa and is a member of the Society of American Value Engineers, NFPA, and the Assn. of Energy Engineers.
The 2005 Code Digest, Cooper Crouse-Hinds,Syracuse, N.Y.
IEEE Green book—Recommended Practice for Grounding of Industrial and Commercial Power Systems. 2007. IEEE, Los Alamitos, Calif.
NFPA 70: National Electrical Code. 2008. NFPA, Quincy, Mass.
Bossert, John A. 1986. Hazardous Locations: A Guide for the Design, Testing, Construction, and Installation of Equipment in Explosive Atmospheres. Canadian Standards Assn., Mississauga, Ontario.
McPartland, Joseph F. 1968. How to Design Electrical Systems. McGraw-Hill, Columbus, Ohio.
Ramhorst, Darrel. 2005. Shock-proof plan for hazardous areas. InTech with Industrial Computing, 05/01.