In electrical engineering, ground or earth is a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth. Electrical circuits may be connected to ground for several reasons. Exposed conductive parts of electrical equipment are connected to ground, to protect users from ground beef chili recipe shock hazard.
If internal insulation fails, dangerous voltages may appear on the exposed conductive parts. Connection to ground also limits the build-up of static electricity when handling flammable products or electrostatic-sensitive devices. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. Long-distance electromagnetic telegraph systems from 1820 onwards used two or more wires to carry the signal and return currents. In the late nineteenth century, when telephony began to replace telegraphy, it was found that the currents in the earth induced by power systems, electric railways, other telephone and telegraph circuits, and natural sources including lightning caused unacceptable interference to the audio signals, and the two-wire or ‘metallic circuit’ system was reintroduced around 1883. 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. This is called “system grounding” and most electrical systems are required to be grounded. NEC and the UK’s BS 7671 list systems that are required to be grounded. Permanently installed electrical equipment, unless not required to, has permanently connected grounding conductors.
The size of power grounding conductors is usually regulated by local or national wiring regulations. Bonding is the practice of intentionally electrically connecting metallic items not designed to carry electricity. This brings all the bonded items to the same electrical potential as a protection from electrical shock. The bonded items can then be connected to ground to eliminate foreign voltages. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Regulations for earthing systems vary considerably between different countries.
A functional earth connection serves more than protecting against electrical shock, as such a connection may carry current during the normal operation of a device. Such devices include surge suppression, electromagnetic-compatibility filters, some types of antennas, and various measurement instruments. Generally the protective earth system is also used as a functional earth, though this requires care. Distribution power systems may be solidly grounded, with one circuit conductor directly connected to an earth grounding electrode system.
Alternatively, some amount of electrical impedance may be connected between the distribution system and ground, to limit the current that can flow to earth. In a polyphase AC system, an artificial neutral grounding system may be used. A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs. NGR to limit the fault current to 25 A or less. They have a continuous rating, and are designed to operate with a single-ground fault.
This means that the system will not immediately trip on the first ground fault. If a second ground fault occurs, a ground fault protection relay must trip the breaker to protect the circuit. On an HRG system, a sensing resistor is used to continuously monitor system continuity. Where the danger of electric shock is high, special ungrounded power systems may be used to minimize possible leakage current to ground. Examples of such installations include patient care areas in hospitals, where medical equipment is directly connected to a patient and must not permit any power-line current to pass into the patient’s body.
Medical systems include monitoring devices to warn of any increase of leakage current. AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards. This is especially common in schemes with submarine cables, as sea water is a good conductor.
Buried grounding electrodes are used to make the connection to the earth. The site of these electrodes must be chosen carefully to prevent electrochemical corrosion on underground structures. A particular concern in design of electrical substations is earth potential rise. When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to points distant from it. This is due to the limited finite conductivity of the layers of soil in the earth of the substation. Many electronic designs feature a single return that acts as a reference for all signals.