The International Electrotechnical Commission (IEC) documents use a totally different nomenclature to describe the electrical potential between phase conductors and earth than the National Electrical Code (NEC). The NEC refers to the electrical potential between phase conductors and normally non-current-carrying metal parts of equipment as grounding. However, earthing, which may be generally mistaken as a synonym, is being seen more often since equipment is increasingly being imported from areas of the world where IEC documents are used as the standards for the equipment. Imagine picking up a piece of IEC standardized equipment at the job site and reading “earthing” on its label. You know it is foreign equipment, so you may assume earthing is the same as grounding.
In the United States, electrician apprentices study grounding and bonding requirements and concepts based on Article 250 in the Code. This system is based on either a grounded system or an ungrounded system. Grounding is established as a single point located at the electrical service normally supplied by a utility company or at a separately derived system supplied within a premises. At this single point, the grounded and the grounding conductors are connected together with separation of the grounded conductor from the grounding conductor once past this point.
In an ungrounded system, there isn’t a grounded conductor to separate downstream. Both grounded and ungrounded systems are required to be connected to a grounding-electrode system at the point of service or at the point of the separately derived system. Downstream from this point, all normally non-current-carrying metal parts of equipment will be connected to an equipment-grounding conductor to provide an effective grounding path for fault current.
While we all need to become more familiar with the term earthing, there are good reasons to keep the relatively simple system for grounding nomenclature in the NEC versus the complex earthing system. Never was this difference more apparent than during the 2005 NEC process when there were multitudes of proposals to change grounding to earthing throughout the NEC. The suggested change seemed reasonably easy to do on the surface, but there was much more to the earthing system that needed to be studied and implemented before a change of such magnitude could occur.
For example, earthing defines the electrical potential of the conductors of an electrical system relative to that of the Earth’s conductive surface. The IEC has both a protective earth (PE) connection and a functional earth connection. The protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the Earth to avoid the risk of electrical shock should a person touch a device in which an insulation fault has occurred. The functional earth connection serves a purpose other than providing protection against electric shock: A functional earth connection may carry current during normal operation of the equipment.
In addition, there are three different types of earthing arrangements with two letter codes to describe the different functions of the arrangements, such as TN, TT or IT. The first letter indicates the connection between earth and the power supply equipment, which could be a generator or a transformer. The second letter indicates the connection between earth and the electrical device being supplied. The “T” indicates a direct connection to earth. The “I” stands for isolation and is an arrangement where no point in the system is connected to earth. However, the isolation type can be connected through a high impedance device. The second “T” is the direct connection with earth, which is independent of any other earth connection in the supply system. The second “N” is the connection to earth via the supply network.
Then there are additional letters added to the two letter codes, such as “S” for separate protective earthing, “N” for neutrals, “C” for “combined” protective earth and neutral, and then a combination of “TN-C-S” that stands for direct connection to earth with “combined” but “separate” connections.
As can be seen by the possible different letter codes for the different systems and the possible combinations, this system, used in many countries outside the United States, is not compatible with the grounding and bonding concepts covered in Article 250. An entire rewrite of the NEC, as well as all related electrical documents throughout the U.S. electrical industry, would be necessary to change to the earthing concept. In addition, a theoretical change in the understanding of the earthing concepts would be necessary for anyone trying to apply earthing techniques to installations in the United States.
In the end, the two systems are very different, but since foreign equipment is becoming very common, an understanding of both systems will be necessary in the future to ensure proper connection of the equipment to the grounding and bonding system. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at email@example.com.