Originally Posted By: jtedesco
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|Grounding, Bonding, and Sprinklers
Principal Electrical Specialist
January/February 2000 NFPA Journal
Too often, the distinction among system grounding, grounding electrodes, and the unavoidable grounding connections between electrical equipment and sprinkler piping is misunderstood.
Even when it isn't done intentionally, a building's interior metal piping systems, including its sprinkler piping, become mechanically and electrically interconnected. A sprinkler system's underground water supply main is exposed to circulating currents produced by exposure to the electric utility and premises wiring that grounds the electrical system transformers. Since the sprinkler water main receives its supply from the area's main water supply system, it, too, is in electrical contact. And electrical equipment with an equipment ground is in contact with the metal structure of the building that supports the sprinkler piping, inadvertently grounding it, as well.
Too often, the distinction between system grounding, grounding electrodes, and the unavoidable grounding connections, which occur when electrical equipment comes in contact with other electrical equipment and with the sprinkler piping, is misunderstood. As a result, electrical system grounding, equipment grounding, and bonding as they apply to NFPA 70, National Electrical Code?(r) (NEC(), and NFPA 24, Installation of Private Fire Service Mains and Their Appurtenances, are sometimes misinterpreted. To clear up these misunderstandings, we'll define and describe "grounding" and "bonding."
The reason for grounding
According to Article 100 of the NEC, a ground is "a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth."
There are several reasons for connecting the current-carrying conductor of an electrical system to the earth or to some conductive element effectively connected to the earth. Grounding, or earthing as it's called in some countries, stabilizes voltage in relation to the earth or grounded objects so that the voltage measured between an ungrounded conductor-that is, the live wire-and a grounded object is always at the same potential. The voltage is thus stabilized. Should lightning strike power lines, a current-carrying conductor that's effectively grounded pulls the voltage down to earth potential, reducing the shock and fire hazard. Where contact between high-voltage and lower-voltage conductors is likely, as it is in transformer windings or conductors on poles, grounding the lower-voltage system pulls the voltage down to earth potential when a crossover occurs.
Grounding a system also reduces stress on the system's electrical insulation. In a grounded three-phase, four-wire, 480/277 wye-connected system, for example, the maximum voltage stress is 277 volts relative to earth or a grounded object. If the system were ungrounded, the voltage could be as high as 480 volts or more should a ground fault occur. Grounding also provides an additional ground-fault return path, although it's not the main path for operating the overcurrent device.
An electrical system is grounded by connecting one of the system's current-carrying conductors to a grounding electrode system through a grounding electrode conductor. According to Section 250-50 of the 1999 NEC, the components of a grounding electrode system include a metal water pipe with at least 10 feet (3 meters) of pipe in the ground, the effectively grounded metal frame of a building, or a ground ring that consists of at least 20 feet (6 meters) of No. 2 AWG bare copper wire buried at least 2.5 feet (0.8 meters) deep. Also suitable is a concrete-encased electrode that consists of at least 20 feet of rebar or rods at least 0.5 inches (1 centimeter) in diameter or 20 feet (6 meters) of No. 4 AWG bare copper wire encased in at least 2 inches (5 centimeters) of concrete at the bottom of the foundation footing. If these components aren't available, Section 250-52 identifies other types of electrodes that are allowed.
Equipment grounding and bonding
In grounded systems that serve a structure's premises wiring, the voltage between ungrounded conductors, or live wires, and the ground or grounded objects is predetermined. If the electrical utilization equipment's insulation fails, however, it could produce a ground fault that energizes the metal noncurrent-carrying parts of the system, creating a shock hazard to anyone coming in contact with the equipment while also in contact with earth or a grounded object. To prevent this, install an equipment-grounding conductor in the form of an insulated or bare wire, a metal raceway system, or metal-sheathed cable armor.
The equipment grounding conductor provides a low-resistance path from the ground fault back to the power source in the form of a complete circuit that causes the fuse or circuit breaker to operate, thus removing the voltage from the circuit. The equipment-grounding conductor also serves to maintain a minimum voltage difference between the noncurrent-carrying parts of an electrical installation during a ground fault. The equipment grounding conductor, the grounding electrode conductor, and the electrical system's grounded conductor are connected only at the main service disconnect switch or main service distribution panel board.
Bonding is the "permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any current likely to be imposed upon it," whether it's due to high-voltage crossover between high- and low-voltage systems, to lightning strikes on power lines or induced voltage, or to equipment insulation failures.
Bonding jumpers minimize the voltage, or potential, difference between conductive materials. Metal piping is bonded to establish an equal potential, or zero voltage, between it and other grounded objects should one be energized. Where two different metallic piping systems emerge from the ground, leakage currents can flow through the earth to the piping systems, which can pick them up and become energized. By bonding the two systems with a low-resistance conductor, the voltage between them approaches zero. Voltage equals the product of amperes and resistance. If the resistance approaches zero, so does the voltage.
When proper grounding and bonding perform as intended, lives are saved and fires prevented. Electricity enters a structure from the utility by way of a transformer, which reduces the high voltage of the utility to a voltage that can safely be used in everyday appliances and machinery. These transformers contain primary windings, with voltage that may vary from 2,500 to 14,000 volts, and secondary windings, with voltage of 240/120 volts. These windings are separated by insulation.
If the insulation between the high-voltage transformer primary conductors and low-voltage secondary conductors breaks down, anyone touching electrical equipment while standing on the ground or holding a grounded conductive object could be exposed to elevated voltages. The higher-voltage conductors impress that voltage on the lower-voltage conductor insulation, which breaks down, impressing the higher voltage on the 240/120 volt lines. This higher voltage then travels into the building wiring by way of the electrical service and breaks through the weakest point in the insulation, causing the higher voltage to be present at the electrical appliance or machine.
The electrical systems in your home or office are grounded at the transformer secondary and to the grounding electrode system at the building's service panel board. Voltage is the product of amperes flowing through the conductor, times its resistance. Since the conductor is of low resistance, the voltage will be low, and if the voltage is low, the amount of current will also be low. The low-resistance bonding conductor or equipment grounding conductor is in parallel with the human body, which means that the current will pass through the path of lowest resistance.
To minimize the risk of injury to occupants of buildings in which sprinkler piping is installed, bond the sprinkler piping systems to the electrical grounding system and to the building's metal framework. This raises two questions: Is the sprinkler's main water supply being used as the electrical grounding electrode? And is there an electrical connection between the sprinkler system and the electrical grounding system?
Where an effectively grounded metal structural member of a building or an effectively grounded metal water pipe within 5 feet (1.5 meters) of the building's entrance point are available, Section 250-50 of the NEC requires that they be bonded together to become the grounding electrode system. Where these aren't available, some other electrode specified in Sections 250-50 and 250-52 must be used.
A 6-inch sprinkler main would be a good electrode, but Section 8-3.5 of NFPA 24 prohibits using it as such. Sprinkler systems with a dielectric isolator in the main sprinkler supply wouldn't satisfy the requirements for a grounding electrode, either, although the NEC doesn't prohibit bonding the interior piping to the electrical system. Section 250-104(C) of the NEC requires that piping that may become energized be bonded to the service equipment or grounding electrode system.
In electric fire pump installations, the equipment grounding conductor is a wire or metal raceway. The metal sprinkler piping is connected to the pump's metal case, which is bolted to the electric motor so that the sprinkler piping is in electrical contact with the grounding system. Installing a bonding jumper between the electrical grounding system and the sprinkler piping only allows the connection to decrease the voltage differential under a ground-fault condition, making it safer.
The intentional bonding of all the utilities in a building creates an equipotential ground plane that minimizes the voltage differential between the different systems under both normal and abnormal operating conditions. The result is an environment safer from the hazards of electrocution and fire.
John M. Caloggero is NFPA's principal electrical specialist. This article is his personal opinion, not a Formal Interpretation, since it hasn't been processed in accordance with Chapter V of the NFPA Regulations Governing Committee Projects.
January/February 2000 NFPA Journal
Joe Tedesco, NEC Consultant