Consulting-Specifying Engineer - July 2008 - (Page 51)

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. Connections 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 50 amp CB 0.2 ohm (wire) Z1 Load being served V = 120 V 0.2 ohm (wire) Z2 50 amp CB 0.2 ohm (wire) Z1 Load being served V = 120 V 1 ohm Z5 1 ohm Z4 1 ohm Z3 0.2 ohm (wire) Z2 Figure 1: Sample calculations based on Ohm’s Law show that 50 amps are required to open the protective device. The top scenario shows a proper equipment grounding, as 300 amps are produced. In the bottom scenario, due to a small amount of impedance, there is a limited amount of fault current that prevents the protective device from clearing a fault. The bottom scenario shows that only 35.3 amps are produced. Source: Interstates Engineering 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: • Exothermic welding • Listed pressure connectors • Listed clamps • 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- Consulting-Specifying Engineer • JULY 2008 51

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - July 2008

Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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