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. Grounding In an electrical system, grounding is required to protect building occupants and electrical equipment. Grounding an electrical system begins with a ground, an electrode in direct contact with the earth itself. The grounding conductor is a continuous conductor that connects the ground to the neutral bus bar and the grounding conductor bus bar in the service equipment/main panelboard. The grounding conductor does not normally carry current. Instead, it links ground to the metal frames or housings of appliances and motors and the metal boxes containing out lets and switches. If needed, the grounding conductor can safely carry current to ground in the event of a lightning strike or in cases of damage or defect in the circuiting, appliances, devices, or equipment. Grounding of an electrical branch circuit enables current to take an alternate path back to the overcurrent protection de vice if an electrical device or appliance short-circuits. It re quires an additional, supplemental wire, called the grounding conductor, which is connected to the appliance cabinet or housing and provides an additional grounding path, in addition to the grounded conductor. Without this additional grounding path, current could flow through a user that was touching a faulty appliance. For example, assume a damaged ungrounded (hot) conductor of an appliance (i.e., refrigerator) has contacted the appliance's metal housing, making the housing hot. See Fig.8. === Fig.8 --- When the ungrounded (hot) conductor contacts the cabinet of an electrical device or appliance, the cabinet becomes "hot." If a person contacts the cabinet, a ground fault occurs and the person feels a shock because current flows through the person to ground. This dangerous condition is typically referred to as a ground fault. SHOCKED! FAULTY LOAD (ENERGIZED CABINET) VOLTAGE ACROSS UNGROUNDED (HOT) AND GROUNDED CONDUCTORS PATH FOR CURRENT THROUGH SOIL OR FLOOR ==== Fig.9 Grounding of an electrical branch circuit enables current to take an alternate path back to the overcurrent protection device (i.e., circuit breaker or fuse) if an electrical device or appliance short circuits. This condition causes the overcurrent protection to shut off the circuit so the unsafe condition does not exist. PATH FOR CURRENT THROUGH GROUNDING NO POWER (CABINET NOT ENERGIZED) CIRCUIT BREAKER TRIPS NO VOLTAGE ACROSS UNGROUNDED (HOT) AND GROUNDED CONDUCTORS NO SHOCK ==== Grounding Requirements In building electrical systems, there are two types of grounding: system grounding and equipment grounding. These are discussed as follows. System Grounding System grounding is that part of a building electrical system that provides protection against electrical shock, lightning, and fires. A lightning strike near the building or a high-voltage transmission line contacting the service entrance conductors can introduce high voltage to a building electrical system. A properly grounded electrical system reduces danger and minimizes damage to the wiring and appliances from such an occurrence. System grounding relates to bonding (connecting) all building electrical system components at the service entrance equipment at the neutral bus of the main panelboard or switch board. A grounding system must be connected to some or all of the following elements if available on the building premises: • An underground metal water (not gas) pipe in direct contact with the earth for no less than 10 ft (3 m); the metal building frame where it’s effectively grounded • An electrode made of at least 20 ft (6 m) of electrically conductive steel reinforcing bars (No. 4 AWG or greater) or bare copper wire no smaller than No. 2 AWG that is encased in at least 2 in of concrete that is part of a foundation or footing in direct contact with the earth • An electrode made of a steel or iron plate that is at least 1/4 in thick or copper plate that is at least 0.06 in thick with at least 2 ft 2 (0.2 m^2) of the plate surface in contact with exterior soil • An electrode made of a grounding ring of bare copper wire no smaller than No. 2 AWG that encircles the building at a depth no less than 2.5 ft (0.75 m) below grade • The structural metal frame of the building where the frame is effectively grounded In systems where only a connection to an underground metal water pipe in direct contact with the earth is the only means of grounding, a supplemental electrode is required. A metal pipe, rod, or plate driven or placed into the earth are acceptable as a supplemental electrode. Equipment Grounding Equipment grounding refers to a grounding conductor or grounding path that connects the noncurrent-carrying metal components of equipment. This may be accomplished by in stalling an additional grounding conductor in all circuits or by permanently bonding (joining) metal components such as metallic conduit in a circuit to form a good conductive path. Equipment grounding extends from the outlets to the neutral bus bar at the service entrance equipment. The equipment-grounding conductor is a bare conductor or a green-colored, insulated conductor that connects (bonds) the outlet boxes, metallic raceways, other enclosures and frames on motors, appliances, and other electrical equipment. All receptacles must be of the grounding type (with supplementary grounding slot) and must be connected to the equipment grounding conductor. If properly bonded, a metal raceway (conduit) or armored cable system can serve as a means of equipment grounding so an equipment-grounding conductor is not actually needed. Equipment-grounding and system-grounding electrodes must be bonded. Bonding is accomplished by installing an additional grounding conductor or by permanently joining metal components in a circuit. When joining components to form a good bond, special connections called bonding jumpers may be required to ensure a good connection between the metal components. Bonding jumpers are required in instances when flexible conduit is used. Tables available in Code indicate the minimum size equipment-grounding conductors for grounding raceways and equipment. They are used to size the equipment-grounding conductor. Equipment-grounding conductors should be routed in the same raceway, cable, or cord as the circuit conductors. Where more than one circuit is installed in a single raceway, one equipment-grounding conductor can be installed in the raceway, but it must be sized for the largest overcurrent protection device serving conductors in that raceway. The equipment grounding conductor is never required to be larger than the circuit conductors. When conductors are run in parallel in more than one raceway, the equipment-grounding conductor should also be run in parallel. Double Insulation Double insulation of an appliance or power tool protects the user from electric shock by creating a nonconducting barrier between the user and the electric components inside the appliance or tool. An appliance or tool that is double insulated has two levels of insulating materials between the electrical parts of the appliance and any parts on the outside that can be touched by the user. If the first layer of insulation fails, the second layer provides protection. Small appliances and power tools with double insulation are not required to have a grounding conductor- that is, they are allowed to have a two-prong plug. The primary difference between an appliance with a three-prong plug and an appliance with a two-prong plug is the appliance casing. If an appliance casing is a good conductor (e.g., made of metal), then it must have a grounding conductor and three-prong plug. Many of newer household electrical appliances and tools are double insulated. Examples include coffee makers, blow dryers, electric drills, and other similar small power tools and appliances. Prev: Grounded and
Ungrounded Conductors |