NEC Chapter 5 provides requirements for special occupancies and specific rules for electrical equipment installed and operated in hazardous (classified) locations, such as fuel dispensing facilities, chemical plants and bulk fuel-storage facilities.
For equipment grounding and bonding requirements in Class I, Class II and Class III locations, the specific rules are found in 501.30, 502.30 and 503.30, respectively. For hazardous locations using the Zone System, the grounding and bonding rules are in sections 505.25 and 506.25. Grounding and bonding requirements that apply to intrinsically safe systems are provided in sections 504.50 and 504.60, respectively.
Special equipment is required in locations classified as hazardous. This means the equipment needs to be suitable for use in the specific location. Suitability is often established through product certification (listing) and identification. The Code rules for conductive equipment, raceways and other electrical wiring installed in these areas are more restrictive in most cases. In explosive atmospheres, it is essential to control and eliminate all possible sources of ignition, such as an electrical arcing event. A motor controller or switch that has make and break arcing contacts are good examples of electrical equipment that must be installed in suitable enclosures that effectively contain the arcing under normal conditions.
Part V of Article 250 contains general bonding requirements for electrical installations. Section 250.90 provides a performance requirement that bonding be provided to ensure electrical continuity and adequate capacity for any fault current. Effective bonding is important for safety in general locations. In hazardous locations, there are increased concerns about establishing an effective ground-fault current path that will not create an explosion or fire if a ground fault were to occur.
During a ground-fault event, the amount of fault current is significant to the point where special bonding methods are necessary for metallic wiring methods. The reason for enhanced bonding is to address concerns about arcing and sparking at any fitting terminations. The strengthened bonding required in these locations ensures an effective path for ground-fault current to facilitate fast operation of the overcurrent device protecting the circuit and equipment.
Fast opening of an overcurrent device reduces possibilities of a hot spot developing on an enclosure at the point of the ground fault. Otherwise, such hot spots or heated enclosure surfaces could quickly become an ignition source if the arcing event is sustained even for a short period of time. Section 500.8(E) requires installing conduit with five full threads fully engaged and wrench-tight. The purpose is to maintain the explosion-proof integrity of enclosures and conduit system and prevent arcing or sparking at threaded joints if fault current passes over the conduit during a ground-fault event.
Section 250.100 provides general requirements for bonding in hazardous locations and applies regardless of the system or circuit voltage. This rule indicates that the noncurrent-carrying metal parts of equipment, raceways and other enclosures in hazardous locations must be bonded using one of the methods provided in 250.92(B)(2) through (B)(4). This enhanced bonding for metallic wiring methods is required even if an equipment grounding conductor of the wire type is installed.
Standard locknuts or bushings are not permitted to accomplish the bonding required for wiring in a hazardous location. Where metallic wiring connections are made at equipment such as boxes, enclosures, cabinets and panelboards, effective bonding must be ensured around any joints in the fault-current path to prevent sparking and ensure a low-impedance path for any ground-fault current. Standard locknuts and bushings can be used to connect the raceways to the enclosures, but bonding continuity must be ensured around any standard locknut, bushing or combination using one of the methods provided in 250.92(B)(2) through (B)(4).
During a ground fault, heavy amounts of current will be present for the time it takes the overcurrent device to open the faulted circuit. Strengthened bonding methods are required for wiring within the hazardous locations, and they are also required for the entire metallic raceway system that is not within the hazardous location.
The bonding requirements in NEC Chapter 5 clarify that the enhanced bonding applies to all metal raceways and enclosures in the hazardous location and to all metal raceways and enclosures of the same circuit(s) extending to the “point of grounding” of the applicable service or derived system, which is typically where the service main bonding jumper is installed or where the system bonding jumper is installed for a separately derived system. This is also where the grounding electrode conductor connection is made at the service or system.