In electrical engineering, ground or earth may refer to reference ground – a reference point in an electrical circuit from which voltages are measured, earth ground – an electrically neutral node that has a lot of available charges (e.g. the physical ground of Earth), or common ground – a common return path for electric current (also called neutral in electric power systems). Ground current or earth current is the current that flows through a reference ground, earth ground, or common ground. To ground or to earth an object is to electrically connect the object to a reference ground, earth ground, or common ground. Electrical circuits may be grounded for several reasons. A protective earth conductor (PE conductor) is a conductor that is used for electrical bonding, that is, connecting an electrical equipment from its exposed and conductive but normally unenergized part to common ground. If the conductor is a wire, it is also called ground wire or earth wire. Ground wire is often used for lower voltage systems and is often colored green, or green and yellow, per standard. Electrical bonding can protect users from electrical shock hazards. If internal insulation fails and the equipment is turned on, the exposed conductive parts may have dangerous voltages. Connection of exposed conductive parts to a PE conductor provides a low-impedance path for current to flow back to the incoming neutral. This results in a fault and allows circuit breakers (or RCDs) to interrupt power supply. The current that flows through the low-impedance path is high and will usually trip the breaker or melt the safety fuse if any. Even if the circuit does not break due to failures, the current still only flows mainly through the PE conductor instead of the user upon touching the exposed conductive part. A PE conductor is an essential part of the safety provided by the earthing system. A safety ground is a loose term that refers to a common ground that is used for safety purposes. Connection to earth ground limits the build-up of static electricity. This connection is crucial for safety when handling flammable products or electrostatic-sensitive devices. In a large-scale electrical system, a piece of conducting material, called grounding electrode or earthing electrode, is buried in the soil to act as an earth ground. A grounding electrode conductor or ground conductor is a conductor that connects an electrical equipment to the grounding electrode. In some telegraph and power transmission circuits, the physical ground (an earth ground) itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor (see single-wire earth return and earth-return telegraph). In an electric power system, the neutral is connected to earth ground, close to the point of entry. For measurement purposes, the ground of the Earth, having a reasonably constant potential, serves as a reference ground against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. Circuits in portable electronic devices, such as cell phones and media players, as well as circuits in vehicles, have a reference ground (also called chassis ground in those cases). In electronic circuit theory, a signal ground is the common ground and reference ground for electric signal; and a power ground is the common ground for power current. The signal ground and power ground may be connected. There is usually a large conductor attached to one side of the power supply (such as the "ground plane" on a printed circuit board), which serves as the common ground, the common return path for current from many different components in the circuit. The signal ground and power ground are usually assumed to be an earth ground and idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a physical ground connection has a significant resistance, the approximation of zero potential is no longer valid (no longer a valid reference ground). Stray voltages or earth potential rise effects will occur, which may create noise in signals or produce an electric shock hazard if large enough.
Electric power system
Building wiring installations
Electrical power distribution systems are often connected to earth ground to limit the voltage that can appear on distribution circuits. A distribution system insulated from earth ground may attain a high potential due to transient voltages caused by static electricity or accidental contact with higher potential circuits. An earth ground connection of the system dissipates such potentials and limits the rise in voltage of the grounded system. In a mains electricity (AC power) wiring installation, the term ground conductor typically refers to two different conductors or conductor systems as listed below:
Equipment bonding conductors or equipment ground conductors (EGC) provide a low-impedance path between normally non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source. If any exposed metal part should become energized (fault), such as by a frayed or damaged insulator, it creates a short circuit, causing the overcurrent device (circuit breaker or fuse) to open, clearing (disconnecting) the fault. This action occurs regardless of whether there is a connection to the physical ground; the physical ground itself has no role in this fault-clearing process since current must return to its source; however, the sources are very frequently connected to the physical ground. (see Kirchhoff's circuit laws). By bonding (interconnecting) all exposed non-current carrying metal objects together, as well as to other metallic objects such as pipes or structural steel, they should remain near the same voltage potential, thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. When a conductive system is to be electrically connected to the physical ground, one puts the equipment bonding conductor and the grounding electrode conductor at the same potential (for example, see §Metal water pipe as grounding electrode below).
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