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Ground or '''earth''' in a Mains ( AC power) Electrical Wiring system is a conductor that exists primarily to help protect against faults and which in normal operation does not carry current. The term "ground" is used in Canada and the U.S. ; the term "earth" is used in most of the rest of the English-speaking world. They are used synonymously here. Neutral is a circuit conductor that carries current in normal operation, and which is connected to earth.
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 IEE definition is a conductor connected to the supply system neutral point, which includes both these uses. 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 System s. Different terminology is used in North America, but the basic principles should be the same everywhere.'' Since the neutral point of a supply system is often connected to earth ground, neutral and earth are closely related. Various measures 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. In the USA, the cases of some electric cooking and clothes-drying appliances 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 case ground 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. HOW THE EARTH PROTECTS In a system with a grounded (earthed) neutral, bonding all non-current-carrying metallic parts of equipment to earth ground, will ensure that current due to faults of the insulation will be diverted to earth. Bonding will allow the branch circuit protection (a Fuse (electrical) or Circuit Breaker ) to detect the fault rapidly and interrupt the circuit. In the case of a TT system where the Impedance is high due to the lack of direct connection to the transformer neutral an RCD (Residual-Current Device, sometimes known as a Residual Current Circuit Breaker) must be used to provide disconnection. RCDs are also used in other situations where rapid disconnection of small earth faults (including a human touching a live wire by accident, or damage) is desired. EQUIPOTENTIAL BONDING Equipotential bonding involves joining together metalwork that is or may be earthed so that it is at the same potential to prevent shock from between those pieces of metal as the earth system handles a fault. In the UK, equipotential bonding is done from the Consumer Unit (also known as fuse box, breaker box and Distribution Board ) to incoming Water and Gas services. It is also done in bathrooms where all exposed metal that leaves the bathroom including metal pipes and the earths of electrical circuits must be bonded together to ensure that they are always at the same potential. Isolated metal objects including metal fittings fed by plastic pipe (water in a thin pipe is actually a very poor conductor) are not required to be bonded. In Australia , a house's earth cables must be connected both to an earthing stake driven into the ground and also to the plumbing. Exact rules for this will vary by country. 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 . So-called "tingle voltage filters" may be required in the electrical distribution system for a milking parlour. COMBINING NEUTRAL WITH EARTH A neutral is not permitted to be used as a case ground for equipment downstream of the service entrance switch in North American electrical wiring codes. Combining the ground and the neutral (grounding to the neutral) provides some protection against live shorts to the case, but will produce a dangerous live case if the neutral connection is broken. Combined neutral and earths 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 Tram s. Since normal circuit currents in the neutral conductor can lead to objectionable or dangerous differences between local earth potential and the neutral, special precautions such as frequent rodding down to earth, use of cables where the combined neutral and earth completely surrounds the phase core(s) and thicker than normal equipotential bonding must be considered to ensure the system is safe. 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. However, such a neutral conductor is never used as a case ground. Where an appliance requires a case ground, a third conductor, used only for grounding, is included in the cord set. REFERENCES {Link without Title} 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 155324600X {Link without Title} ''NFPA 70, National Electrical Code 2002'', National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2002). no ISBN {Link without Title} ''IEE Wiring Regulations Regulations for Electrical Installations Fifteenth Edition 1981'', The Institution of Electrical Engineers, (1981) Hitchin, Herts. United Kingdom EXTERNAL LINKS http://www.iee.org/Publish/WireRegs/EarthingPlasticPipes.pdf document from Paul Cook of the IEE talking about why bonding metal accessories fed by plastic pipes is a bad idea. |
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