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Monday, October 13, 2014

When is Ground not a Ground?

By Richard P. Bingham, Dranetz

Grounding is a term that has many different facets, depending on the application. Some of the reasons for grounding systems and equipment are to: limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltages; stabilize the voltage to earth under normal operation; establish an effective path for fault current that is capable of safely carrying the maximum fault current and with sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions. In addition, the most important application is to increase the protection people and equipment from shock and/or damage.

In the power quality field, grounding plays an important part in the proper operation of "sensitive" equipment. "Of all the power and grounding problems affecting electronic load equipment, almost 90% are caused by electrical power and grounding conditions inside the facility in which the equipment is used....More importantly, almost 75% of the power quality problems inside the facility relate to grounding, which makes it the single most important factor from a facility standpoint, in having reliable equipment operation." [Warren Lewis, "Quality Grounding and Power Quality: Having Your Cake and Eating It, Too", EC&M Power Quality Advisor, February 1998]

More numerous than the reasons for grounding are the terms related to grounding. The Grounding Subcommittee of NFPA70B Electrical Equipment Maintenance Committee has compiled over fifty related definitions. Article 250 of the National Electric Code is a good source for many of the definitions, though one must remember that the primary purpose of the NEC is to protect life and property, and not necessarily ensure that equipment will run properly in a facility. This does not imply that any of the requirements should be compromised for the sake of improved performance, but rather additional efforts may be required.

A grounding system has several key components: the connection to earth (often the grounding electrodes); the grounding conductor (typically called the green wire); the bonding jumper that connects the grounding conductor to the grounded conductor (often referred to as the neutral); and, the connection of the equipment connected to the grounding system. A few of the definitions from the NEC are listed below, to ensure that we are all speaking the same language.

Bonding Jumper -  A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.

Grounding ConductorA conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Grounding Conductor, EquipmentThe conductor used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.

Grounded ConductorA system or circuit conductor that is intentionally grounded.

Grounding Electrode Conductor - The conductor used to connect the grounding electrode to the equipment grounding conductor, to the grounded conductor, or to both, of the circuit at the service equipment or at the source of a separately derived system.

Grounding Electrode A grounding electrode is a conductive body deliberately inserted into earth to make to electrical connection to earth.

The grounding system operates under the same rules as the normal current carrying conductors.

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