All Code references are based on the 2005 National Electrical Code. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.
If someone were to investigate which NEC Article suffers from the most misapplications, violations, and misinterpretations, they'd find that Art. 250 easily claims that distinction. The situation is so dire that Art. 250 violations are sometimes required. For example, many industrial equipment manuals require violating 250.24(A)(5) as a condition of warranty. In particular, they require installing an “isolated grounding electrode.” By this, they mean an electrode without a low-impedance fault-current path back to the source winding (other than through the earth itself). This creates a condition where the ground-fault current return path to the source winding (utility transformer) is on the order of several ohms rather than the fraction of an ohm that would be provided by an NEC-compliant installation.
If you apply basic physics and electrical theory, you can clearly see that Art. 250 is correct and equipment manuals that require “isolated grounding” are wrong. Other standards agree: ANSI/IEEE Standard 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) and Soares Book on Grounding use the same physics and electrical theory as Art. 250. Also, IEEE-142 describes the correct way to provide an isolated ground — and it isn't the method proposed by many equipment manuals.
Art. 250 isn't a “preferred design specification,” as defined in Art. 90. It provides the minimum requirements for a safe installation. These requirements include providing paths to divert high voltage to the earth, providing low-impedance fault-current paths for overcurrent protection devices, and removing dangerous potentials between conductive building components and electrical systems.
Coming to terms with Art. 250. To correctly apply Art. 250, you must understand how the NEC defines specific terms. This 12-part Code Basics series will take that a step further and provide clarification of those terms that can be especially confusing. Where the NEC uses the term “grounding” to mean “connecting to the earth,” “earthing” will follow in parentheses. Where the NEC uses the term “grounding” to mean “connecting to a conductive body for the purpose of providing a low-impedance path to the source winding,” the term “bonding” will follow in parentheses. A variation of this convention will be used for “ground” and “grounded.”
Bonding (bonded). The permanent joining of metallic parts together to form an electrically conductive path. This path must have the capacity to safely conduct any fault current likely to be imposed on it (Fig. 1).
Bonding jumper. A reliable conductor sized per Art. 250 to ensure electrical conductivity between metal parts of the electrical installation.
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 electrical supply source winding (Fig. 2).
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.
Ground fault. An unintentional connection between an ungrounded conductor and earth or metallic parts of enclosures, raceways, or equipment (Fig. 3).
Ground-fault current path. An electrically conductive path from a ground fault to the source winding. The NEC uses the phrase “ground-fault current path,” but fault current isn't traveling to the earth — it's traveling to the source winding of the power supply.
Grounded (earthed). Connected to earth.
Grounding (earthing) conductor. A conductor used to connect equipment to a grounding (earthing) electrode.
Grounding (earthing) electrode. A device that establishes an electrical connection to the earth.
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 in accordance with 250.142.
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 per 250.30(A)(1).
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 per 250.30(A)(1).
Why so much emphasis on wording? If you understand grounding- and bonding-related terminology, you can then comply with Art. 250 requirements and produce safe installations.
An illuminating example. Too often, metal parts are grounded (earthed) instead of bonded. The accepted grounding practice for street lighting and traffic signals in many parts of the United States, in which the ground rod is used as the only fault-current return path, is a good example of how commonly Art. 250 is misapplied. In this case, the metal pole of a light fixture or traffic signal is grounded to a ground rod, but an effective ground-fault current path isn't there.
For this scenario to make sense, you need to understand three concepts: touch voltage, hazardous level, and surface voltage gradients.
The IEEE defines “touch voltage” as “the potential (voltage) difference between a bonded metallic structure and a point on the earth 3 feet from the structure.”
ANSI/IEEE Standard 142, [4.1.1], says 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. This layering of shells results in “surface voltage gradients.”
The Table lists the percentage of total resistance and the touch voltage for the light pole in Fig. 4 above, based on a 120V fault. As the 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 shock one foot away from an energized object if the metal parts aren't bonded to an effective ground-fault current path to remove the voltage by clearing the fault.
Because the resistance of the earth is so high, very little current will return to the power-supply winding if the earth is the only ground-fault return path. If a metal lighting pole is only grounded (earthed) to a ground rod, then the earth is the sole ground-fault current path, which is a violation of 250.4(A)(5). Consequently, the circuit overcurrent protection device won't open and metal parts will remain energized at a lethal level until someone makes contact with them and the earth. Therefore, a ground rod doesn't lower touch voltage to a safe value for metal parts that aren't bonded to an effective ground-fault current path.
If the people involved in street lighting and traffic signaling in these locations where the Code is misapplied understood the terminology of Art. 250, the situation would be very different — and much safer. Sadly, this is only one example of terminology-based misapplication; dozens of others exist.
So you can see the importance of understanding the terminology. But that means taking time to learn definitions. The good news is that task isn't as onerous as it might seem if you can remember the concepts of “earthing” and “bonding.” In the first case, you're connecting to the earth. In the second, you're connecting to a conductive body for the purpose of providing a low-impedance path to the source winding. Many times when the NEC says “grounding,” it's referring to bonding.
As this series progresses, you'll see these terms many times. More importantly, you'll encounter them in your work — where a solid understanding of grounding- and bonding-related terms will allow you to avoid mistakes, such as those in the street lighting example. An old adage says, “Words cannot hurt you,” but when it comes to grounding and bonding, not understanding certain words can hurt everyone.
Stay tuned to Code Basics throughout the year for the rest of this series.