Effective bonding is important for safety in general locations, but in hazardous areas, there are increased concerns about establishing an effective ground-fault current path that will not create an explosion or fire if a ground fault occurs. These requirements apply regardless of the voltage. This article takes another look at these rules.

During a ground-fault event, the amount of fault current is significant until the fault is cleared by an overcurrent protective device (OCPD). The reason for enhanced or strengthened bonding is to address concerns about arcing and sparking at any fitting terminations. The more restrictive bonding required in these locations ensures an effective path for ground-fault current to facilitate fast operation of the OCPD protecting the circuit and equipment, which 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.

Article 250

Part V of Article 250 contains general bonding requirements. Section 250.90 includes a requirement that bonding be provided to ensure electrical continuity and adequate capacity for any fault current. Section 250.100 provides general requirements for bonding in hazardous locations and applies regardless of the system or circuit’s 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 in 250.92(B)(2) through (B)(4). This enhanced bonding for metal wiring methods is required even if an equipment grounding conductor of the wire type is installed.

Standard locknuts or bushings are not permitted to do the bonding required for wiring in a hazardous location. Where metal 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 in 250.92(B)(2) through (B)(4).

In the event of a ground fault, heavier current levels will be present in the ground-fault current path for the time it takes the OCPD to open the faulted circuit. As mentioned before, strengthened bonding methods are required for wiring within the hazardous location and required for the entire metal raceway system, even the sections not in the hazardous area.

The bonding requirements in Chapter 5 clarify that the strengthened bonding applies to all metal raceways and enclosures in the hazardous location and metal raceways and enclosures of the same circuit(s) run, extending to the grounding point of the applicable service or derived system. The point of grounding is typically where the service main bonding jumper is installed or 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.

The NEC recognizes wiring methods such as flexible metal conduit (FMC), liquidtight flexible metal conduit (LFMC) and liquidtight flexible nonmetallic conduit (LFNC) in Division 2 locations under restrictive conditions, as in 501.10(B) and 502.10(B). LFMC and LFNC must be used with listed fittings. LFMC and LFNC are permitted in Class III locations, as indicated in 503.10(A)(3)(2). Where FMC, LFMC or LFNC is installed for flexibility in hazardous locations, a wire-type equipment bonding jumper is usually required in accordance with 250.102. 

Bonding jumpers are often installed external to the flexible conduit installation using suitable fittings that provide an attachment lug. The equipment bonding jumpers can be installed inside or outside the conduit. The conduit alone is not a suitable bonding means.

Recalling that Section 90.3 indicates that the rules in Chapter 5 modify or amend the general rules in chapters 1–4, the .30(B) sections of articles 501–503, 505 and 506, provide the amendatory and more restrictive requirements.

Static electricity

There are also concerns about static electricity as ignition source(s) in hazardous locations. Static electricity problems are typically addressed using equipotential bonding and grounding techniques that reduce potential differences in the hazardous location. In some cases, specifically in indoor applications, raising humidity levels can provide an effective solution, but each issue requires its own analysis and solution. 

For information on protection against static electricity and lightning hazards in hazardous locations, see NFPA 77, Recommended Practice on Static Electricity; NFPA 780, Standard for the Installation of Lightning Protection Systems; and API RP 2003-2015, Protection Against Ignitions Arising Out of Static Lightning and Stray Currents.

michael johnston