Ground and neutral
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- For uses of the term "ground" or "earth" in electricity but not in the context of mains wiring, please see ground (electricity)
Since the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of ground (earth) conductors and neutral conductors is carefully distinguished in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.
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[edit] Definitions
Ground or earth in a mains (AC power) electrical wiring system is a conductor that exists primarily to provide a low impedance path to the earth to prevent transient hazardous voltages from appearing on equipment; normally a grounding conductor does not carry current. (The terms "ground" (North American practice) and "earth" (most other English-speaking countries) are used synonymously here.) Neutral is a circuit conductor that may carry current in normal operation, and which is usually connected to earth.
In a polyphase or three-wire AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors. By this definition, a circuit must have at least three wires for one to serve as a neutral.
In the electrical trade, the conductor of a 2-wire circuit that is connected to the supply neutral point and earth ground is also referred to as the "neutral". This is formally described in the US and Canadian electrical codes as the "identified" circuit conductor.
The NEC and Canadian electrical code only define neutral as the first of these. In North American use, the second definition is used in less formal language but not in official specifications. In the UK the IET definition is a conductor connected to the supply system neutral point, which includes both these uses.
[edit] Earthing systems
The names for the following methods of earthing are those defined by IEC standards, which are used in Europe and many other regions. For a more detailed explanation, see earthing systems. Different terminology is used in North America, but the basic principles should be the same everywhere.
Different systems are used to minimize the voltage difference between neutral and local earth ground. In some systems, the neutral and earth join together at the service intake (TN-C-S); in others, they run completely separately back to the transformer neutral terminal (TN-S), and in others they are kept completely separate with the house earth having its own rod and the neutral being rodded down to earth within the distribution network (TT). In a few cases, they are combined in house wiring (TN-C), but the dangers of broken neutrals (see below) and the cost of the special cables needed to mitigate this mean that it is rarely done nowadays.
Voltages created in grounding (earthing) conductors by currents flowing in the supply utility neutral conductors can be troublesome. For example, special measures may be required in barns used for milking dairy cattle. Very small differential voltages, not usually perceptible to humans, may cause low milk yield, or even mastitis (inflammation of the udder). So-called "tingle voltage filters" may be required in the electrical distribution system for a milking parlour.
[edit] Combining neutral with earth
Connecting the neutral to the equipment case provides some protection against faults/shorts, but may produce a dangerous voltage on the case if the neutral connection is broken.
Combined neutral and ground conductors are commonly used in electricity supply companies' wiring and occasionally for fixed wiring in buildings and for some specialist applications where there is little choice like railways and trams. Since normal circuit currents in the neutral conductor can lead to objectionable or dangerous differences between local earth potential and the neutral and to protect against neutral breakages, special precautions such as frequent rodding down to earth, use of cables where the combined neutral and earth completely surrounds the phase conductor(s), and thicker than normal equipotential bonding must be considered to ensure the system is safe.
[edit] Fixed appliances on three-wire circuits
In the USA, the chassis/cases of some ovens and clothes dryers were grounded through their neutral wires, as a measure to conserve copper during the Second World War. This practice was removed from the NEC in the 1996 edition, but existing installations may still allow the chassis/case grounding of such appliances to be connected to the neutral conductor. Note that the NEC may be amended by local regulations in each state and city. This practice arose from the three wire system used to supply both 120 volt and 240 volt loads. Because ovens and dryers have components that use both 120 and 240 volts there is often some current on the neutral wire. This differs from the protective grounding wire, which only carries current under fault conditions. Using the neutral conductor for grounding the equipment enclosure was considered safe since the devices were usually permanently wired to the supply and so the neutral was unlikely to be broken without also breaking both supply conductors. Also, the unbalanced current due to lamps and small motors in the appliance was small compared to the rating of the conductors, and so unlikely to cause a large voltage drop in the neutral conductor.
[edit] Portable appliances
In North American practice small portable equipment connected by a cord set may have only two conductors in the attachment plug. A polarised plug is used to maintain the identity of the neutral conductor into the appliance but it is never used as a chassis/case ground. The small cords to lamps, etc., often have one or more ridges or embedded strings to identify the neutral conductor, or may be identified by color.
In places where the design of the plug and socket cannot ensure that a system neutral conductor is connected to particular terminals of the device, portable appliances must be designed as if both poles of each circuit may reach full voltage with respect to ground.
[edit] See also
- Live wire (electricity)
- Neutral wire
- Electrical wiring
- Electrical wiring (UK)
- Electrical wiring (United States)
[edit] References
[1] Rick Gilmour et al., editor, Canadian Electrical Code Part I, Nineteenth Edition, C22.1-02 Safety Standard for Electrical Installations, Canadian Standards Association, Toronto, Ontario Canada (2002) ISBN 1-55324-690-X
[2] NFPA 70, National Electrical Code 2002, National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2002). no ISBN
[3] IEE Wiring Regulations Regulations for Electrical Installations Fifteenth Edition 1981, The Institution of Electrical Engineers, (1981) Hitchin, Herts. United Kingdom
[edit] External links
- http://www.iee.org/Publish/WireRegs/EarthingPlasticPipes.pdf document from Paul Cook of the IET talking about why bonding metal accessories fed by plastic pipes is a bad idea.
