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Glossary · Energy industry & energy transition

Earthing (Grounding)

earthing protection technology electricity grid Electrical Engineering safety neutral point treatment

Earthing refers to the conductive connection of electrical installations, items of equipment or conductive parts of buildings to the ground. It does not simply mean a wire "into the ground". It is a planned electrical connection through which potentials are fixed, fault currents are conducted and dangerous voltages are limited. Earthing is a fundamental element of electrical safety and at the same time part of the operation of networks.

Earth as a reference potential

Electrical engineering uses the earth as a reference potential. This is not because soil conducts particularly well, since its resistivity is very high compared with copper. What matters is the enormous cross-section through which currents can spread. The reference is "remote earth", that is, an area far enough away from an earth electrode. This potential is not the same everywhere, however. When current flows into the ground, a voltage cone forms around the earth electrode. A metal rod in the ground therefore does not by itself make an earthing system effective. What matters are the earth spreading resistance and earthing impedance, soil conditions and soil moisture, the shape and position of the electrodes, conductor cross-sections, corrosion protection and coordination with the protective devices. Measurement is in ohms, amperes and volts: the resistance or impedance of the earthing path, the magnitude and duration of possible fault currents, and the voltages that people or animals can bridge.

Types and functions of earthing

  • Protective earthing: Touchable conductive parts, such as enclosures, are earthed so that they do not retain a dangerous voltage in the event of a fault and so that disconnection is triggered.

  • Operational or system earthing: A point of the operating circuit is earthed, usually the neutral point of a transformer or generator. How this is done is governed by neutral point treatment.

  • Functional earthing: It serves not personal protection but the function of devices, for example electromagnetic compatibility, filters or measurement technology.

  • Lightning protection earthing: It discharges lightning currents into the ground. As a rule it is connected to the rest of the earthing system.

  • Working earthing: Isolated parts of installations are earthed and short-circuited before work begins. This is one of the five safety rules for working on electrical installations. It prevents accidental re-energisation, induction or residual charges from becoming dangerous.

Earthing in the low-voltage network: TN, TT and IT

The simplest safety case: a metal enclosure that can be touched should not permanently carry a dangerous voltage with respect to earth. If a line conductor touches the enclosure, the resulting fault current must be large enough for a fuse, a miniature circuit breaker or a residual current device (RCD) to disconnect in time. The path this current takes depends heavily on the network system.

In the TN system, the neutral point of the local network transformer is earthed. The exposed conductive parts of the customer installation are connected to this neutral point via the protective conductor (PE) or the combined PEN conductor. In the event of a fault to exposed conductive parts, the fault current therefore flows back to the transformer mainly through this metallic conductor loop and not through the ground. This produces a large current that quickly trips overcurrent protective devices. The loop impedance is decisive. In the TT system, the customer installation has its own installation earth electrode, separate from the operational earthing of the network operator. Here the fault current actually flows through the ground. It is usually so small that disconnection is, as a rule, performed by an RCD. In the IT system, the live parts are either not earthed or earthed only via a sufficiently high impedance, whereas the exposed conductive parts are earthed. A first fault to earth then does not necessarily lead to disconnection. It must, however, be signalled, for example by insulation monitoring, and be eliminated quickly.

In Germany, disconnection times are governed by DIN VDE 0100-410. For final circuits up to 32 A at 230 V to earth, the maximum is 0.4 s in the TN system and 0.2 s in the TT system. Longer times are permitted for distribution circuits, namely 5 s in the TN system and 1 s in the TT system. The shorter times in the TT system result from the different fault current conditions. The values apply to low-voltage installations under this standard. Different requirements exist for special locations and other voltages.

Distinctions: protective conductor, equipotential bonding, neutral conductor

Earthing is often equated with the protective conductor. That is imprecise. Within an installation, the protective conductor connects the touchable conductive parts to the protection system. The earthing system, by contrast, comprises the earth electrodes themselves, that is, foundation, ring and deep earth electrodes or entire earthing grids, as well as the earthing conductors and the main earthing terminal. Equipotential bonding connects conductive parts such as water, gas and heating pipes, building structures and cable screens with one another. This prevents dangerous voltage differences between them. It is connected to the earthing but is not the same thing.

The neutral conductor is not earthing either. It is an active conductor and carries current in normal operation. The protective conductor must not carry operating current in normal operation. Neutral and protective conductors are connected to each other at certain points, for example at the earthed neutral point or in the PEN conductor of a TN-C section. They are nevertheless not interchangeable. Downstream of the splitting of the PEN conductor, N and PE must not be reconnected. If they are swapped or interrupted, in particular the PEN conductor, touchable parts can become live or protective devices can fail.

Earthing in high- and medium-voltage installations

In the power grid, earthing serves to protect people and installations, to control faults and for electromagnetic compatibility. In a substation, a meshed earthing grid lies in the ground. It distributes high fault currents and keeps touch and step voltages within permissible limits. Overhead line pylons are earthed to discharge lightning and fault currents. Cable screens are earthed to control fields and to enable protective functions. For high-voltage installations above 1 kV AC, the standard DIN EN 50522 (VDE 0101-2) applies in Germany, currently in the edition of 2023-10. The predecessor edition of 2011 could be applied in parallel only until January 2025. Such an earthing system is dimensioned mainly according to the magnitude and duration of the fault current and the properties of the soil. Earth electrodes in the ground must be made of corrosion-resistant material.

"Earthed" does not mean "voltage-free". During a fault, the potential of an entire earthing system relative to remote earth can rise by many hundreds or thousands of volts. This earth potential rise can be carried to remote locations via pipelines, fences, rails or cable screens. This is called potential transfer. What counts for the hazard is therefore not the absolute potential of a point, but the voltage that the body bridges and how long it is applied. The step voltage is present between two points on the ground at stride distance, the touch voltage between a conductive part that is touched and the person's own standing position. The shorter a fault lasts, the higher the touch voltages that are permissible. Earthing design and protection times are therefore directly linked.

Relationship with neutral point treatment and protection technology

Closely connected with earthing is neutral point treatment. The neutral point of a transformer or generator can be solidly or low-impedance earthed, compensated via an arc suppression coil (Petersen coil), or remain isolated. This choice determines how large the current becomes in an earth fault, how protection relays detect the fault and whether a network section may initially continue to run after a single-pole fault. In German medium-voltage networks, compensated neutral point treatment is widespread because it limits capacitive earth fault currents. In cable networks, low-impedance neutral point earthing is also used. In the extra-high-voltage network at 220 and 380 kV, the neutral point is usually low-impedance earthed so that faults are disconnected quickly and the insulation is stressed only to a limited extent.

Earthing is thus part of protection technology. Protective devices must reliably detect a fault and disconnect the affected part of the network with the best possible selectivity, that is, only the faulty section. For this, the path of the fault current must be sufficiently well known. If the impedance is too high, a fault current may remain too small for reliable tripping. If it is very low, large short-circuit or earth short-circuit currents can place heavy thermal and mechanical stress on installations. What is sought is therefore not the "strongest" possible earthing, but one that suits the network configuration, protection concept and use.

Common misconceptions

Earthing does not simply lead current "away" harmlessly. Current does not disappear into the ground; it always flows back to its source in a closed circuit, that is, to the transformer or generator. The return path can run via protective conductors, PEN conductors, the ground, pipelines, cable screens or parts of buildings. If these paths are unfavourable or unplanned, safety risks, corrosion, stray currents, electromagnetic interference and potential transfers arise.

Earthing and lightning protection are closely related but are not the same thing. External lightning protection consists of air-termination systems, down conductors and an earthing system and must observe separation distances. Internal lightning protection comprises lightning protection equipotential bonding and surge protection. Good earthing reduces risks but does not replace a complete lightning and surge protection concept.

Buildings: foundation earth electrode and earthing system

In Germany, new buildings generally receive an earthing system in accordance with DIN 18014. It is required through the installation standards and through the technical connection conditions (TAB) of the network operators. The 2023 edition governs the entire earthing system and no longer just the foundation earth electrode in the base slab. It provides, for example, for a ring earth electrode in the ground where waterproof concrete or perimeter insulation beneath the base slab prevents contact with the ground. It also applies to retrofits. The Federal Network Agency (Bundesnetzagentur, Ruling Chamber 6) does not consider it abusive in principle if distribution system operators require an earthing system for new buildings in their TAB. Equivalent alternative designs should, however, be permitted.

Legally, standards are not laws. However, under Section 49 of the German Energy Industry Act (Energiewirtschaftsgesetz), it is presumed that the technical requirements are met if the VDE provisions are observed. The TAB apply by virtue of the connection relationship with the network operator. For existing installations, the state of the art at the time of construction generally applies. In the case of substantial changes or extensions, an adaptation may become necessary.

Significance for the energy transition

With the energy transition, more photovoltaic systems, battery storage systems, heat pumps, charging parks and new transformer stations are being built. The interfaces between customer installation and network grow accordingly. Each of them needs a coordinated protection and earthing concept. An inverter with its filters causes leakage currents and, in the event of a fault, can generate DC fault currents. This influences the choice of RCD type and the design of the equipotential bonding. In the event of a fault, inverters also supply considerably less short-circuit current than rotating generators. This affects the disconnection conditions. For large ground-mounted PV plants and battery storage systems with their own medium-voltage station, the earthing systems of the station, mounting frame and fence must be compatible.

Responsibilities are clearly allocated. The network operator is responsible for the earthing of its equipment and sets the connection conditions. Electrical contractors plan and install the customer installation in accordance with the standards, including the foundation earth electrode or ring earth electrode. Operators and owners are responsible for ensuring that the installation remains in proper condition. Inspectors measure and assess earth resistance, protective conductor continuity, loop impedance and disconnection conditions. Weak points typically arise at transitions: when new technology meets old buildings, when earth electrodes are missing or corroded, or when protective measures of different network systems are mixed.

Earthing is thus the controlled definition of electrical potentials and fault current paths. It protects reliably only if it is planned together with the network system, protective devices, equipotential bonding, equipment and responsibilities. It makes electrical faults manageable before they turn into dangerous touch voltages, damage to installations or unnecessary disconnections.

Sources

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Edited by eCMS on 11.11.2026 19:47

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