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For the January issue of EC&M Magazine
Extracted from my Grounding versus Bonding book.
The grounding and bonding requirements contained in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.
The purpose of the National Electrical Code is the practical safeguarding of persons and property from hazards arising from the use of electricity [90.1(A)]. In addition, the NEC contains provisions that are considered necessary for safety. Compliance with the NEC, combined with proper maintenance, shall result in an installation that is essentially free from hazard [90.1(B)].
No other article can match Article 250 for misapplication, violation, and misinterpretation. In fact, we commonly see Article 250 violations as requirements! For example, many industrial equipment manuals require violating 250.24(A)(5) as a condition of warranty. The manuals insist on installing an "isolated grounding electrode," which is an electrode without a low-impedance fault-current path back to the source neutral, other than through the earth itself. That means the ground-fault current return path to the source neutral (utility transformer) is on the order of several ohms rather than a fraction of an ohm that the typical NEC-compliant installation would provide.
If you apply basic physics and basic electrical theory, you can clearly see Article 250 is right and equipment manuals that require isolated grounding are wrong and other references agree. IEEE-142 and Soares Book on Grounding use the same physics and electrical theory as Article 250. This article isn't a "preferred design specification." As with the rest of the NEC, it serves the purpose stated in Article 90 to be sure the installation is, and remains, SAFE!
Article 250 covers the requirements for providing paths to divert high voltage to the earth, requirements for the low-impedance fault-current path to facilitate the operation of overcurrent protection devices, and how to remove dangerous voltage potentials between conductive parts of building components and electrical systems.
Over the past two Code cycles, this article was extensively revised to make it better organized and easier to implement. It's arranged in a logical manner, so it's a good idea to just read through Article 250 to get a big picture view-after you review the definitions. Then study the article closely so you understand the details. The illustrations will help you understand the key points.
Why Grounding is Often Difficult to Understand
The reason it's difficult to understand the rules contained in Article 250 - Grounding, is because many do not understand that this article applies to both grounding and bonding. In addition, the proper definitions of many important terms such as "bond, bonded, bonding, ground, grounded, grounding, and effectively grounded," and their intended application is not understood or improperly used. So before we get too deep into this subject, let's review some important definitions contained in Articles 100 and 250.
When the NEC uses the term "grounding" where the intent is connected to the earth, this column will use the term "grounding (earthing)." If the term grounding used in the NEC is actually intended to be used for bonding, then this column will use the term "grounding (bonding)" to help the reader have a better understanding of the application.
It's unfortunate that the term "grounding" is used interchangeable by the industry (including the NEC) for both grounding and bonding, because this often results in the misapplication of the NEC when one grounds (earths) metal parts, when it should have been bonded.
Bonding (Bonded). The permanent joining of metallic parts together to form an electrically conductive path. Such path must have the capacity to conduct safely any fault current likely to be imposed on it. Figure 250-02 def 06 bonding 01 100.cdr
Note: Graphics are not included in the this Newsletter.
Author's Comment: Bonding is accomplished by the use of bonding conductors, metallic raceways, connectors, couplings, metallic-sheathed cable with fittings, and other devices approved by the authority having jurisdiction as recognized for this purpose [250.118].
Bonding Jumper. A reliable conductor that is properly sized in accordance with Article 250, to ensure electrical conductivity between metal parts of the electrical installation. Figure 250-02 def 07 bonding jump 01 100.cdr
Effectively Grounded. The intentional connected to earth through a connection of sufficiently low impedance.
Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance conductive path designed to carry fault current from the point of a ground fault on a wiring system to the grounded (neutral) point at the electrical supply source. Figure 250-02 def 01 effective CC250-01.cdr
The effective ground-fault current path is intended to facilitate the operation of the circuit overcurrent protective device, or the ground-fault detector on a high-impedance grounded system. Figure 250-02 def 02 effective CC250-02.cdr
Equipment Grounding (Bonding) Conductor. The low-impedance fault-current path used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the grounded (neutral) conductor and equipment grounding (bonding) conductor at service equipment or at the source of a separately derived system.
Author's Comment: The purpose of the equipment grounding (bonding) conductor is to provide the low-impedance fault-current path necessary to facilitate the operation of overcurrent protection devices in order to remove dangerous voltage potentials between conductive parts of building components and electrical systems [250.4(A)(3)].
Author's Comment: According to 250.118, the equipment grounding (bonding) conductor must be one or a combination of the following: Figure 250-02 def 14 250-s118 100.cdr
Ground (Earth). An intentional or accidental connection to the earth. Figure 250-02 def 10 ground 100.cdr
Danger: Because the resistance of the earth is so high, very little current will return to the power-supply neutral if the earth is the only ground-fault return path. If a ground rod is the only ground (bonding) connection, then the earth is being used as the sole ground-fault current path, in violation of 250.4(A)(5). The result is that the circuit overcurrent protection device will not open and metal parts will remain energized at a lethal level waiting for someone to make contact with them and the earth. Therefore, a ground rod cannot be used to lower touch voltage to a safe value for metal parts that aren't bonded to an effective ground-fault current path.
To understand how a ground rod is useless in reducing touch voltage to a safe level, let's review the following:
Touch Voltage - The IEEE definition of touch voltage is "the potential (voltage) difference between a bonded metallic structure and a point on the earth 3 ft from the structure."
Hazardous Level - NFPA 70E, Electrical Safety Requirements for Employee Workplaces, cautions that death and/or severe electric shock can occur whenever touch voltage exceeds 30V.
Surface Voltage Gradients - According to ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1], the resistance of the soil outward from a ground rod is equal to the sum of the series resistances of the earth shells. The shell nearest the rod has the highest resistance and each successive shell has progressively larger areas and progressively lower resistances. The following table lists the percentage of total resistance and the touch voltage based on a 120V fault.
Don't worry if you do not understand the above statement; just review the table below with Figure 250-04A5 05 250-54 grounding touch potential un250-31.cdr.
Distance from Rod Resistance Touch Voltage
Many think a ground rod can reduce touch voltage and create a safer installation. However, as the above table shows, the voltage gradient of the earth drops off so rapidly that a person in contact with an energized object can receive a lethal electric shock one foot away from an energized object if the metal parts aren't bonded to an effective ground-fault current path.
Scary as it might be, the accepted grounding (earthing) practice for street lighting and traffic signaling for many parts of the United States is to use the ground rod as the only fault-current return path. That is, the metal pole of a light fixture or traffic signal is grounded to a ground rod and an effective ground-fault current path isn't provided! Preliminary studies indicate that about half of one percent of all metal poles probably has dangerous touch voltage. This is one reason so many people get killed by touching street lighting and traffic signal poles in the United States. For case studies on this subject, visit http://www.MikeHolt.com/news/archive/html/master/04-29-2002-7-48.htm
Ground Fault. An unintentional connection between an ungrounded conductor and earth or metallic parts of enclosures, raceways, or equipment. Figure 250-02 def 03 ground fault un250-18.cdr
Author's Comment: Ground fault (line-to-case faults) aren't always the low-impedance type; they might be the high-impedance arcing type. These are difficult to clear before the arc destroys the equipment or starts a fire. High impedance, in this case, occurs where improper bonding techniques have been used. This is a particular problem for 480V solidly grounded systems, and that is why the NEC requires equipment ground-fault protection for larger installations [230.95]. Another way to reduce this hazard is by using current-limiting fuses or installing high-impedance neutral systems [250.36]. Current-limiting fuses will be discussed briefly in other parts of this textbook, but high-impedance neutral systems are beyond the scope of this textbook.
Ground-Fault Current Path. An electrically conductive path from a ground fault to the source neutral.
Author's Comment: The NEC uses the term "Ground-Fault Current Path," however, fault current isn't traveling to the earth, it's traveling to the source neutral of the power supply.
FPN: The fault-current path could consist of bonding conductors, metallic raceways, metallic cable sheaths, electrical equipment or other electrically conductive materials, such as metallic water or gas piping, steel-framing members of a building or structure, stucco wire mesh, metal ducting, reinforcing steel, or shields of communications cables. Figure 250-02 def 04 FPN ground-fault current path.cdr
Author's Comment: The difference between an "effective ground-fault current path" and " fault-current path" is that the effective ground-fault current path is "intentionally" made to provide the low-impedance fault-current path to the source neutral for the purpose of clearing a fault. A fault-current path is simply a path that fault current flows on its return to the power-supply neutral during a ground fault. In reality, there are almost always multiple paths along which fault current flows. The intent of the NEC is to maximize the current carried by the "effective ground-fault path" and to minimize the current on all others.
Grounded (Earthed). Connected to earth.
Grounded (Neutral) Conductor. The conductor that is intentionally grounded to the earth. Figure 250-02 def 13 grounded conductor 100.cdr
Grounding (Earthing) Conductor. A conductor used to connect equipment to a grounding (earthing) electrode.
Author's Comment: A grounding (earthing) conductor can be used to connect a supplementary grounding (earthing) electrode to a piece of equipment [250.57] Figure 100-grounding conductor 250-54.cdr
Grounding (Earthing) Electrode. A device that establishes an electrical connection to the earth. Figure 250-02 def 15 grounding electrode NEW 100.cdr
Author's Comment: See 250.50 through 250.70
Grounding (Earthing) Electrode Conductor. The conductor used to connect the grounding (earthing) electrode(s) to the equipment grounding (bonding) conductor, to the grounded (neutral) conductor, or to both, the service [250.24(A)], each building or structure supplied by feeder(s) or branch circuit(s) [250.32(A)], or the source of a separately derived system [250.30(A)] in accordance with 250.142. Figure 250-02 def 16 def GEC 100.cdr
Main Bonding Jumper. A conductor, screw, or strap that bonds the equipment grounding (bonding) conductor (service disconnecting means) to the grounded (neutral) conductor in accordance with 250.24(B). For more details, see 250.24(A)(4), 250.28, and 408.3(C).
Solidly Grounded. The intentional electrical connection of one system terminal to the equipment grounding (bonding) conductor in accordance with 250.30(A)(1).
Author's Comment: The industry calls a system winding that has one system terminal bonded to it's metal case a solidly grounded system. Figure 250-02 def 12 grounded solidly NEW 100.cdr
System Bonding Jumper. The conductor, screw or strap that bonds the equipment grounding (bonding) conductor (metal parts of a separately derived system) to one of the system conductors or terminal in accordance with 250.30(A)(1). Figure 250-02 def 09 bonding jump sys 01 NEW 100.cdrAuthor's Comment: The system bonding jumper provides the low-impedance fault-current path to the source neutral for fault current. For more information see 250.4(A)(5), 250.28 and 250.30(A)(1).
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