Ground and neutral
From Wikipedia, the free encyclopedia
- For uses of the term ground or earth in electricity but outside of mains wiring please see ground (electricity)
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.
- In a polyphase or three-wire AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors, and similar phase spacing. 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 is also referred to as the "neutral". This is formally described in the US and Canadian electrical codes as the "identified" circuit conductor. If the entire system is only single phase then the current carrying conductor that is tied to earth is still a neutral by this definition.
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.
Contents |
[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.
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 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 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. This change to the NEC was implemented because ovens and dryers have components that consume either 120 or 240 volts depending on settings, and thus there is often current on the neutral wire. This differs from the protective earth, which only carries current under fault conditions. A case bonded to neutral is generally safe because the heavier wire that feeds high-power appliances is less likely to break.
[edit] 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. In a TN system where there is a direct connection from the installation earth to the transformer neutral, earthing will allow the branch circuit protection (a fuse 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.
[edit] 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.
[edit] Combining neutral with earth
A neutral is no longer 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 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 core(s) and thicker than normal equipotential bonding must be considered to ensure the system is safe.
[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 case ground. However, in the past, some large white goods appliances did use a combined neutral and earth supplied via a NEMA 10-30 or 10-50 plug but this is no longer accepted; a separate ground is now required.
[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 1-55324-600-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 IEE talking about why bonding metal accessories fed by plastic pipes is a bad idea.
