Grounding and Bonding


(محمد سميري) #1

Grounding and Bonding:
Closely Related Terms, But Different Functions
Phil Simmons
[FONT=Times New Roman]In any discussion of grounding and bonding, questions such as the following often arise. What does grounded mean? Does bonded mean the same thing as grounded? If a piece of equipment is grounded, is it also bonded? Can something be grounded and not be bonded? Conversely, can something be bonded and not be grounded?
Changes made to several definitions in Article 100 for the 2008 edition have continued the efforts to clarify grounding and bonding requirements.
In addition, numerous changes throughout the 2008 NEC® were part of a Code-wide effort to clarify and differentiate how these two fundamental principles are intended to be applied. As a result of this effort, there are now clearer definitions of what is intended when a requirement calls for grounding or when a requirement calls for bonding.
Although these terms are frequently used interchangeably because of their close relationship and long history of use, there is a definite difference in their overall objective. The difference is clearly illustrated in the grounding and bonding performance requirements specified in Section 250.4(A)and 250.4(B). This article examines how the 2008 revisions will improve on how these terms are understood and applied.
Definitions
A key to properly applying the NEC requirements on grounding and bonding is to understand the definitions related to these two important safety functions. Several definitions related to grounding and bonding have been changed from the 2005 NEC®. The NEC Technical Correlating Committee created a task group that was directed to review the use of terms related to grounding and bonding. The task group met several times between the 2005 and 2008 NEC revision cycles and reviewed the use of related terms throughout the NEC®. The task group determined that several terms related to grounding and bonding required revision for clarity.
This review resulted in proposals and comments from the task group for many sections in the NEC including Article 250. In these proposals and comments, the task group made an effort to be more prescriptive (state what is required) in a Code rule and to rely less on definitions of terms. The intent was to make the requirements on grounding and bonding easier to understand and the NEC more “user-friendly.” A discussion of changes to the definition of ground, grounded (grounding), bonded (Bonding), and equipment grounding conductor follows.
Ground Thedefinition of this term was changed to its simplest possible form during the processing of the 2008 NEC. Article 100 now defines ground as “the earth.”As described in 250.4(A)(1)and 250.4(B)(1), connections to earth are primarily made for dissipating overvoltages from lightning or mishaps caused by electric utility system higher voltage lines contacting lower voltage lines.
The earth consists of many different types of soil—from sandy loam to rock—which affect the earth’s ability to carry current. Soil resistivity is determined largely by its content of electrolytes—which consist of moisture, minerals and dissolved salts. Soil resistivity also is determined by its ability to retain moisture. Soil with high organic content, such as black loam, is usually a good conductor because it retains higher levels of moisture and thus electrolytes. Sandy soils, which drain faster, tend to have a much lower moisture content and electrolyte level. As a result, sandy soils tend to have a higher resistance. Solid rock and volcanic ash contain or retain virtually no moisture and have high resistivity.
Connections to earth for systems rated 600 volts or less are never made to carry away current. Even soils that are good conductors have too high a resistance to be effective in carrying current.
Grounded (Grounding) The definition of this term was also changed significantly in NEC Article 100. It now reads, “Connected (connecting) to ground or to a conductive body that extends the ground connection.”
The phrase connected to ground is not defined. It is generally accepted that equipment or systems are connected to ground (the earth) through one or more grounding electrodes that make a satisfactory earth connection. The requirements for connecting a service and separately derived system to a grounding electrode (system) are found in Part II of Article 250. The rules for installing or creating a grounding electrode system are found in Part III of Article 250.
Similarly, the phrase a conductive body that extends the earth connection is not defined. Although this phrase can perhaps have more than one meaning, for our purposes it means: A metallic conductive element—such as conduit, cable, or wire­—that extends from the point where an earth connection is made at one or more grounding electrodes to another point on the electrical system where equipment such as a switchboard, panelboard, junction or pull box, or luminaire or grounding-type receptacle is properly connected to it. Such equipment, even if located on an upper floor of a multistory building, is considered to be grounded or connected to earth. It should be fairly easy to follow this grounding path through one or many of the equipment grounding conductors to the grounding electrode (system).
Requirements for providing an effective ground-fault current path are contained in 250.4. These requirements including providing an intentionally constructed, low-impedance, electrically conductive path that is designed and intended to carry current under ground-fault conditions from the point of a fault on a wiring system to the electrical supply source. The current in this path will facilitate the operation of the overcurrent protective device or ground-fault detectors.
Some grounding electrodes also extend the earth connection. A structural metal grounding electrode often extends many stories above the point where it is either bonded to the grounding electrode system or itself makes an earth connection. Separately derived systems can then use the structural metal as a grounding electrode even though located many stories above grade level. Metal water pipe grounding electrodes are also recognized as extending the earth connection for certain occupancies where continuity of the water pipe can be verified.
Bonded (Bonding) The definition of this term was significantly simplified in the 2008 NEC Article 100. It now reads, “Connected to establish electrical continuity and conductivity.” Obviously, values for maximum resistance or impedance or minimum current-carrying capacity of the conductive path are not included in the revised definition. The Code includes many rules that identify where bonding jumpers or conductors are required, how they must be sized, and how they are required to be connected. In its simplest usage, bonding means connecting conductive electrical components together.
Equipment Grounding Conductor This term was also significantly revised in Article 100 to include the concepts of grounding and bonding. Its definition reads, “The conductive path installed to connect normally non–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.” A new Fine Print Note (FPN) clarifies the equipment grounding conductor also performs bonding. A second new FPN points to 250.118, which lists acceptable equipment grounding conductors.
Questions Answered
Let’s revisit the questions asked at the beginning of this article.
Revised definitions of grounded and bonded are found in Article 100, so obviously the terms are different and do not have the same meaning. The equipment grounding conductor serves to both extend the connection to earth and to connect equipment together, and thus performs both a grounding and a bonding function.
Consider a grounding-type duplex receptacle located on the twenty-seventh floor of an office building: Is the receptacle still grounded even though the grounding electrode connection to the service equipment from which it is supplied is located twenty-seven stories below? Is the receptacle also bonded? It is grounded to the grounding electrode though the equipment grounding conductor connection to the service equipment. The equipment grounding conductor and bonding jumpers bond all the metallic components together. It is difficult to imagine a piece of equipment that is grounded to not also be bonded by installation of the equipment grounding conductor. In the case of this twenty-seven story building, as in many other cases, the functions of grounding and bonding become inseparable in the safety system.
Several other changes were made to 2008 NEC definitions related to grounding and bonding of electrical systems or equipment. Becoming familiar with these definitions will significantly enhance understanding of the concepts and help ensure safe electrical installations.

Key Definitions from Article 100 and 250
Bonded (Bonding). Connected to establish electrical continuity and conductivity. [Article 100]
Bonding Jumper, Main. The connection between the grounded circuit conductor and the equipment grounding conductor at the service. [Article 100]
Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems. [250.2]
Ground. The earth. [Article 100]
Grounded (Grounding). Connected (connecting) to ground or to a conductive body that extends the ground connection. [Article 100]
Grounded Conductor. A system or circuit conductor that is intentionally grounded. [Article 100]
Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. [Article 100]
Grounding Conductor, Equipment (EGC). The conductive path installed to connect the normally non−–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. [Article 100]
Grounding Electrode Conductor. A conductor used to connect the system grounded conductor the equipment to the grounding electrode or to a point on the grounding electrode system. [Article 100]
Ground Fault. An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. [250.2]
Ground-Fault Current Path. An electrically conductive path from the point of a ground-fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source. [250.2]

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