f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Earth fault

earth fault short circuit protection technology electricity grid fault condition grid protection

An earth fault is an electrical fault. In it, a live conductor acquires a conductive connection to earth, to an earthed part of an installation or to a protective conductor. It can affect, for example, a line conductor of an overhead line, a conductor in a cable, a busbar in a switchgear installation or a winding in a transformer. The connection can be low-impedance, for instance in the case of metallic contact. It can also be high-impedance, for instance via damp or contaminated insulating materials, arcs, vegetation or damaged cable sheaths. High-impedance earth faults are particularly hard to detect because the measured quantities change only slightly.

The term describes not only the location of the fault but an electrical relationship. A conductor that is insulated from earth in normal operation acquires an unintended connection to earth potential. This can give rise to fault currents, touch voltages, voltage displacements and protective disconnections. How strong these effects are depends on the grid configuration, the voltage level, the neutral-point treatment and the design of the protection technology.

Distinction from short circuit, earth short circuit and exposed-conductive-part fault

An earth fault is not the same as a short circuit between live conductors. A short circuit is a connection between conductors at different voltages, for instance between two line conductors or between a line conductor and the neutral conductor. In an earth fault, the faulty connection leads to earth or to earthed parts. High-voltage engineering terminology makes an additional distinction: an earth short circuit requires an earthed neutral point. A large current then flows, similar to that of a short circuit. An earth fault in a compensated or isolated grid, by contrast, is expressly not an earth short circuit, because with the first fault only a comparatively small current flows there.

A related term is the exposed-conductive-part fault (fault to frame). This usually means a fault in which a live conductor touches the conductive enclosure of an item of equipment. If the enclosure is earthed via the protective conductor, the exposed-conductive-part fault acts like an earth fault or a fault to the protective conductor. In low-voltage installations this distinction matters for protective measures. Fuses, miniature circuit breakers and residual current protective devices each detect different fault patterns.

An earth fault is also not an operational earthing. Many parts of an installation are deliberately earthed, such as transformer neutral points, metal enclosures, protective conductors, cable screens or lightning protection systems. Earthing is a planned connection with a defined purpose. An earth fault is an unintended connection. It alters normal operation and triggers a protective reaction or a fault search.

Why the neutral-point treatment determines the fault current

Current flows only in a closed circuit. The magnitude of the earth fault current therefore depends on the path by which the current can return to the source. The decisive factor is the neutral-point treatment, that is, whether and how the neutral points of the transformers are connected to earth. If the neutral point is earthed with low impedance, the current can return to the neutral point via earth, protective conductor, neutral conductor or earthed equipment. Large fault currents then often flow, which protective devices must disconnect quickly. In Germany, all 220 kV and 380 kV grids are operated with low-impedance, direct neutral-point earthing.

If the neutral point is isolated or compensated via a coil, a single-phase earth fault does not produce a large short-circuit current. The fault current then flows mainly via the capacitive leakage currents of the healthy conductors to earth. An arc suppression coil, also called a Petersen coil, generates an inductive counter-current and largely cancels the capacitive component. The current does not disappear entirely. A residual current remains, made up of an active component, detuning and harmonics. If this residual current is small enough, an earth fault arc often extinguishes by itself. In German practice, the quenching limit for the residual current is cited as about 60 A in medium-voltage grids and about 120 A in 110 kV grids. This limit also restricts how large a grid operated with arc suppression may become.

Earth fault compensation is not an exception in Germany. A 2001 survey of grid operators covered 1,653 grid groups from 6 to 110 kV. In it, compensation was the most common form of neutral-point treatment. Short-time earthing and current-limiting neutral-point earthing occurred only rarely. However, this finding is older and does not reflect the current situation. For operation in the medium-voltage grid there is a practical reason: in overhead line grids, many earth faults have temporary causes, such as branches, animals, contamination or thunderstorms. If every single-phase earth fault immediately disconnected an entire line, there would be significantly more cases of supply interruption.

Protection technology, location and disconnection

Earth faults are detected using various methods. In low-voltage installations, a residual current protective device (RCD) compares the outgoing current with the returning current. If part of the current is missing, this fault current flows off via earth, protective conductor or a person. The device trips when the differential current exceeds a specified value. In Germany, DIN VDE 0100-410 (edition 2018-10) governs protection against electric shock, including by automatic disconnection of the supply. For certain circuits in TN and TT systems, the standard additionally requires RCDs with a rated residual operating current of no more than 30 mA, for example for socket-outlets up to 32 A for use by ordinary persons. In a TN system with 230 V to earth, a maximum disconnection time of 0.4 s typically applies to final circuits. The exact cases of application are set out in the standard itself. In the IT system, which also occurs in low voltage, the first earth fault by contrast is deliberately not disconnected but is reported by insulation monitoring.

In medium- and high-voltage grids, earth fault protection functions work with zero-sequence quantities, displacement voltage, residual current measurement, direction determination or transient signals at fault inception. In compensated grids, locating the fault is more difficult because the steady-state fault current is small. Grid operators therefore use earth fault directional relays, the active residual current method, pulse location, short-time neutral-point earthing, measurement data from secondary substations or switching tests. In this way they narrow down the faulty feeder. With short-time low-impedance neutral-point earthing, the earth fault is deliberately converted into a measurable earth short circuit, which the protection can disconnect selectively.

The protection philosophy has to balance two goals. People, equipment and installations must not be endangered. At the same time, a single, manageable fault should not disconnect more customers from the grid than necessary. Earth faults are therefore treated differently depending on the grid level. In final low-voltage circuits of TN and TT systems, the focus is on fast automatic disconnection. In compensated medium-voltage grids, operation can initially continue during the fault search, provided the safety conditions are met. This also applies to many 110 kV grids, because the high-voltage level in Germany is operated partly with low-impedance earthing and partly with earth fault compensation. Only in the low-impedance earthed grids, above all at 220 and 380 kV, does every earth fault in principle lead to fast selective disconnection.

Typical misconceptions

  • "The current simply flows away into the earth": Current does not disappear into the earth. It flows only in a closed circuit. Earth, earthed parts of the installation, cable screens, protective conductors, conductor capacitances and neutral-point connections can be parts of this return path. Without the return path, neither the magnitude of the current nor the protective effect can be assessed.

  • "A small earth fault current is harmless": Even with a limited current, dangerous touch and step voltages can arise at the fault location, at poles, fences or cable screens. In isolated or compensated three-phase grids, the neutral point also shifts. The two healthy conductors rise relative to earth to about the line-to-line voltage, that is, to about 1.73 times the normal value. This additionally stresses the insulation.

  • "An earth fault remains a single fault": If, during an existing earth fault, another conductor at a different location also makes earth contact, a double earth fault arises. It acts like a two-phase short circuit with a high current via earth and must be disconnected by the short-circuit protection.

  • "Continued operation means the fault is of secondary importance": Temporary continued operation is a controlled operating mode with conditions attached. It buys time for locating the fault and switching over, but does not replace repair. The quality of grid operation shows in how quickly a fault is narrowed down, assessed and rectified.

Significance in the power system

With the earth fault, technical safety, grid availability and economic decisions meet. Several factors together determine how often faults lead to interruptions and how laborious the search is: the neutral-point treatment, the sensitivity of the protective devices, the quality of the insulation, the extent of the grid and the measurement equipment. Cables have a significantly higher capacitance to earth than overhead lines. As distribution grids are increasingly cabled, the capacitive earth fault current therefore grows. Larger or controllable arc suppression coils then become necessary. Where the residual current exceeds the quenching limit, a change to low-impedance or current-limiting neutral-point earthing may also become necessary.

For grid operators, the earth fault is an everyday but demanding fault case. It affects maintenance, vegetation management, switching planning, occupational safety, grid automation and the statistics of supply interruptions. Customers usually notice the fault only when protective devices trip, the voltage dips or an area is disconnected. For the energy transition the term is important because distribution grids are more heavily loaded and monitored more densely by measurement technology. More cables, more distributed generation plants with power electronics and more automated switching points change the fault currents and the methods for fault detection. Even in a high-voltage grid, the control of arc suppression coils remains a current development topic.

The earth fault is therefore not a marginal case of electrical engineering but a basic category of grid operation. Electrical faults cannot be assessed by their cause alone. What matters are the return current path, the grid configuration, the protection rule and the permissible mode of operation. The consequences of an earth fault result from how the specific grid is earthed, monitored and operated.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:47

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.