La toma de tierra, también denominado hilo de tierra, toma de conexión a tierra, puesta a tierra, pozo a tierra, polo a tierra, conexión a tierra, conexión de puesta a tierra, o simplemente tierra, se emplea en las instalaciones eléctricas para llevar a tierra cualquier derivación indebida de la corriente eléctrica a los elementos que puedan estar en contacto ,ya sea directa o indirectamente, con los usuarios (carcasas, aislamientos, etc.) de aparatos de uso normal, por un fallo del aislamiento de los conductores activos, evitando el paso de corriente al posible usuario. La puesta a tierra es una unión de todos los elementos metálicos que mediante cables de sección suficiente entre las partes de una instalación y un conjunto de electrodos, permite la desviación de corrientes de falla o de las descargas de tipo atmosférico, y consigue que no se pueda dar una diferencia de potencial peligrosa en los edificios, instalaciones y superficie próxima al terreno.

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In electrical engineering, ground or earth is the 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. In electrical power distribution systems, a protective ground conductor is an essential part of the safety Earthing system. Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent user contact with dangerous voltage when electrical insulation fails. Connection to ground also limits the build-up of static electricity when handling flammable products or electrostatic-sensitive devices. In some telegraph and power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor (see single-wire earth return). For measurement purposes, the Earth serves as a (reasonably) constant potential reference 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. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard. The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in portable electronic devices such as cell phones and media players as well as circuits in vehicles may be spoken of as having a "ground" connection without any actual connection to the Earth, despite "common" being a more appropriate term for such a connection. This 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 return path for current from many different components in the circuit.

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