Down to Earth

I conduct a number of National Electrical Code (NEC)-related seminars each month. Lately, people have been asking about the requirements for grounding and bonding systems together to provide an equipotential plane.

Originally, grounding and bonding was to prevent hazardous conditions from developing in manmade electrical environments. However, in a modern grounding system, further techniques must be employed to protect systems and personnel from lightning strikes, dissipating surges and other unwanted disturbances. Low impedance in the grounding system is essential to reduce telecommunications and electronic noise and protect against transient voltages.

Why ground?

An earth ground is an electrical connection to the earth that establishes a grounding-electrode system for connecting the electrical system and other system sources to it. Systems that are appropriately earth-grounded aid in ensuring rapid clearing of overcurrent protection devices (OCPDs) and proper operation of protective relays. This phenomenon is accomplished by providing fault-paths in the grounding system to rapidly remove any unwanted foreign potential.

Why bond?

Bonding is the permanent jointing of metallic parts to form an electrically conductive path that will ensure electrical continuity and provide capacity to conduct safely any current likely to be imposed. Bonding ensures that the grounding scheme continues. Naturally, bonding jumpers must be sized with enough capacity, and/or fittings must be used with wiring methods that are adequate for handling large available fault-currents and safely opening OCPDs and preventing damaging circuits, components and equipment.

Single-point grounding provides a means to reduce voltage differentials and control surge currents within the facility equipment and circuits. This method reduces voltage differences and surge currents developed from control devices.

Voltage differences are produced between the ends of equipment--grounding conductors (EGCs) and busbars during fast-rising wavefronts associated with electrical surges from utilities and electronic equipment. All electrical systems and sources must be routed to a single-point ground that is connected to a grounding-electrode system that is earth-grounded.

The path to ground for circuits, equipment and conductor enclosures must be permanent and continuous, have capacity to conduct safely any fault-current likely to be imposed on it, and have sufficiently low impedance to limit the voltage to ground. It must also facilitate the operation of the overcurrent protection devices protecting the circuits.

To accomplish this task, 250.4(A)(5) requires a grounding-electrode conductor (GEC) to be used to connect metallic raceways, metal cables and EGCs to the service-equipment enclosure(s) and the grounded conductor to the grounding-electrode system.

An alternating current system that is grounded must have a GEC connected to the grounding-electrode system as required in 250.50, 250.52, 250.53 and 250.66. The secondary side of the utility transformer supplying the electrical system must have the grounding electrode connected to the supply side of the service-disconnecting means at a conveniently accessible point, preferably within the service disconnect and OCPD enclosure.

To ensure an adequate ground-fault return path, all exposed noncurrent--carrying metal parts of fixed or cord-and-plug connected equipment, which are likely to become energized, must be grounded and bonded with a properly sized EGC per Table 250.122. Items such as metallic raceways and cables, enclosures, and other metal parts must be kept a safe distance from lightning rod down conductors to reduce the change of flash over surge currents. All systems in the facility, including the electrical equipment, must be grounded and bonded to a common grounding-electrode system so that an equipotential plane bonds all facility systems together.

Section 250.122 contains requirements that can be used for selecting a grounding scheme. To have a low impedance path, the EGC(s) must be routed with the circuit conductors in the same raceway, cable, cord, etc., per 300.3(B).

Satellite dishes, antenna masts, primary protectors and cables pertaining to communications circuits, radio and television equipment, CATV installations, and systems other than the electrical system must be grounded and bonded to the grounding-electrode system specifically designed and installed for this purpose.

When grounding and bonding is done correctly, the result is a lower impedance that reduces the potential difference between these systems including wiring methods and components during lightning and other surges that are harmful to their operation. NEC references are 800.100, 810.21, 820.100 and 830.100.

Unfortunately, it is not feasible or practical to explain here all of the problems that poor grounding and bonding of the systems can create.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.

About the Author

James G. Stallcup

Code Contributor
James G. Stallcup is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.

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