Ground (power)

Ground or earth in a mains (AC power) electrical wiring system is a core that exists primarily to help protect against faults.

The term "ground" is used in the US; the term "earth" is used in most of the rest of the English speaking world. They are used synonymously here.

For uses of the term ground or earth in electricity but outside of mains wiring please see Ground (electricity)

Earthing systems

Neutral and earth are closely related. In an ideal world, both should be at zero volts relative to the body of the earth. 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, some appliances were grounded to "neutral" in the past but this was an accident waiting to happen.

It should be noted that the names for the different methods of earthing given above are European. While the names and details may vary the basic principles of each should be the same everywhere.

How the earth protects

When a wire shorts to the earth it should trip some form of fuse or breaker. In the case of a TT system where the impedance is high due to the lack of direct connection to the transformer neutral an RCCB (Residual Current Circuit Breaker, sometimes known as a Residual Current Device) must be used to provide disconnection. RCCBs 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 but the principles remain the same.

Combining neutral with earth

Combining the ground and the neutral (grounding to the neutral) can protect against live shorts to the case. However, this has the additional danger of live cases if there is a fault or break in the neutral wire at any point in the wiring.

Portable appliances

The Americans took grounding to the neutral right into portable appliances. However, people commonly replaced the new polarised plugs with unpolarised ones to fit their existing wiring with the result that in 50% of cases the casing ended up bonded to live! With a separate earth, if there is no provision for earthing, it is much more likely to be left unconnected (which is undesirable but not an imminent danger) than to be connected to live accidentally.

Combining them safely

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 but 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.

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. ja:接地 nl:Aarding