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Glossary · Energy industry & energy transition

Fault current

fault current protection technology short circuit earth fault electricity grid Operational safety

A fault current is an electric current that flows because of a fault, above all because of an insulation fault. The International Electrotechnical Vocabulary (IEC 60050-826) defines it as a current that flows at a given fault location as a result of an insulation fault. Typical causes are damaged or aged insulation, moisture in equipment, crushed cables, or a connection of a line conductor to the enclosure of a device (exposed-conductive-part fault), to earth or to earthed parts of the installation. It is therefore not a normal operating current. The fault current takes a path that is not intended, or it flows under conditions for which the installation was not designed.

If current flows through the body of a person or an animal, standardisation calls it touch current. This is a separate term in the same vocabulary. It can be the consequence of a fault, for example when someone touches an enclosure that is live because of an insulation fault. But it can also arise when someone directly touches a live part. Fault current is measured in amperes like any current. However, the current magnitude alone describes the fault only incompletely. The assessment also depends on the current path, the duration, the touch voltage that occurs, the system type, the earthing, the impedances of the fault loop and the type of supply. Even a few tens of milliamperes through the human body can be life-threatening. Fault currents of several kiloamperes can destroy equipment, generate arcs and start fires.

Distinction from short-circuit current, earth fault, overload and residual current

Fault current is often used as a generic term for currents that flow during faults. Standardisation, however, distinguishes several terms from one another: fault current, earth fault current, short-circuit current, overload current, residual current, leakage current and protective conductor current each have their own entries there. In a short circuit, two points at different voltages are connected via a very low impedance. The short-circuit current can become very high because it is limited only by the impedances of the source, transformers, lines and switchgear. An earth fault exists when an active conductor makes a connection to earth or to an earthed part. In networks with an earthed neutral point, this can result in a high fault current. In isolated or compensated networks, the current at the first earth fault is usually much smaller, but the fault remains relevant for operation.

An overload has a different cause than a fault current. In an overload, a circuit carries more operating current than is permissible on a continuous basis. The current nevertheless continues to flow along the intended path. A motor or a line can be overloaded without an insulation fault being present. With a fault current, by contrast, the current path itself is the problem. This distinction is important for the choice of protective devices. Fuses, miniature circuit breakers, circuit breakers, residual current protective devices and protection relays respond to different characteristics of a fault.

In everyday language, fault current is often equated with the tripping current of an RCD switch (known in German as FI-Schalter). Technically more precisely, a residual current protective device (RCD) measures the residual current. It forms the sum of the currents in all active conductors, that is, in line conductors and the neutral conductor. In the fault-free state this sum is almost zero. If part of the current flows away via the protective conductor, earth or a person, a difference arises, and the device trips as soon as it exceeds its operating value. An RCD therefore does not measure the current at the fault location itself, but detects that the current balance in the monitored circuit is no longer correct. It therefore does not detect a short circuit between a line conductor and the neutral conductor, because the balance remains equalised in that case.

Why fault current protection is a systems question

No network and no installation can be built in such a way that faults are ruled out. Cables age, insulation is damaged, moisture penetrates, animals cause short circuits, excavators hit lines. Operation only becomes safe through the interplay of robust design, suitable earthing, protection technology, maintenance and clearly defined disconnection conditions. These include disconnection times, tripping currents and selectivity. Selectivity means that, as far as possible, only the protective device closest to the fault disconnects. A fault in a single outgoing feeder should not de-energise an entire substation or a larger part of the network.

How high a fault current becomes depends strongly on the short-circuit power at the fault location and on the impedance of the fault loop. Close to a large transformer with short lines, very high currents can occur. Protective devices must be able to disconnect these safely, and switchgear must withstand the thermal and mechanical forces. At the end of long lines or in the case of high-impedance faults, on the other hand, the current can be so small that overcurrent protective devices do not trip, or do not trip quickly enough. For low-voltage installations there is a basic rule for this: the product of loop impedance and the disconnection current of the protective device must not exceed the nominal voltage to earth. If this condition is not met, other protection principles are needed, for example residual current protective devices, insulation monitoring or additional protective equipotential bonding.

System type, earthing and protection concept

  • TN system: One point of the power source, usually the neutral point of the transformer, is earthed. The enclosures of the equipment are connected to this point via the protective conductor. In the event of an exposed-conductive-part fault, the fault current flows back to the source via the protective conductor and is usually large enough for disconnection by a fuse or miniature circuit breaker. According to DIN VDE 0100-410, in 230/400 V networks a maximum disconnection time of 0.4 s applies to final circuits up to 32 A. For distribution circuits and larger final circuits it is 5 s.

  • TT system: The enclosures of the equipment are connected to a separate installation earth electrode that is separate from the operational earth of the source. The earthing resistances limit the fault current. The disconnection times of 0.2 s for final circuits up to 32 A and 1 s for other circuits can, in practice and with realistic earthing resistances, usually only be met with residual current protective devices.

  • IT system: The active conductors are insulated from earth or earthed only via a high impedance. At the first earth fault, usually only a small fault current flows. It must be signalled by an insulation monitoring device, but operation may continue. This is used, for example, in medically used rooms or in industrial processes that must not be interrupted. If a second fault occurs on another conductor, a double fault arises, which the protective devices must disconnect.

These differences explain why the same type of fault has different consequences in different installations. A damaged cable can trigger a high short-circuit current in one network and in another initially cause only the insulation monitoring to respond. What matters most is which return path the current takes: on this depends whether a protective device detects the fault at all and how quickly it responds. In medium- and high-voltage networks, the type of neutral-point treatment (solidly earthed, low-impedance earthed, compensated or isolated) shapes the magnitude of earth fault currents and thus the concept of network protection.

Additional 30 mA protection in Germany

For personal protection, DIN VDE 0100-410 in the version of 2018-10 requires, for newly erected TN and TT AC installations in certain circuits, additional protection by residual current protective devices with a rated residual operating current of no more than 30 mA. This concerns socket-outlets up to 32 A that are used by ordinary persons and for general use, final circuits up to 32 A for portable equipment used outdoors, and lighting circuits in dwellings. For special rooms and installations, such as bathrooms or charging points, the 700 series of standards contains further requirements. Existing installations are in principle covered by grandfathering protection. In the case of modifications, extensions or inspections, however, retrofitting may become necessary. The 30 mA protects people; the fire protection effect is limited: RCDs with 300 mA are also frequently used as a fire protection measure.

Fault currents in networks with power electronics

Photovoltaic systems, battery storage, charging infrastructure, wind turbines and drives with frequency converters increase the share of power-electronic equipment, and this changes fault currents on two levels. In the network, synchronous generators briefly supply a multiple of their rated current in the event of a short circuit. Inverters, by contrast, limit their current electronically, usually to about one to one and a half times the rated current. How they behave in the event of a fault depends on the control, grid connection rules and protection parameters. The fault current can therefore be smaller, shorter or different in its phase angle than with rotating machines. Protection concepts that rely on high overcurrents therefore do not automatically work equally well. Network operators must check protection relays, earthing concepts and disconnection conditions accordingly.

In final circuits, converters can also generate smooth DC fault currents or fault currents with higher frequencies. Residual current protective devices are designed for these to differing degrees. Type AC detects only sinusoidal AC fault currents and is not permitted in Germany under DIN VDE 0100-530. Type A additionally detects pulsating DC fault currents and tolerates smooth DC components only up to about 6 mA. Type F additionally detects mixed frequencies up to about 1 kHz. The all-current-sensitive Type B also detects smooth DC fault currents and higher frequencies. Too large a DC component can make a Type A residual current circuit breaker go "blind", so that it does not trip in an emergency.

For charging points of electric vehicles, DIN VDE 0100-722 requires that each connection point receives its own residual current protective device with no more than 30 mA. In addition, it must be protected against smooth DC fault currents. There are two equivalent ways to do this: a Type B RCD, or a Type A or F RCD together with DC fault current detection according to IEC 62955, which disconnects from 6 mA. This detection is already built into many wallboxes.

Typical misunderstandings

Fault current is not a waste of electricity and not an inefficiently used consumption, but a dangerous or undesired current path. To be distinguished from it is leakage current: in many devices it flows away to earth or to the protective conductor even in fault-free operation, for example via interference suppression filters. If many leakage currents add up in one circuit, an RCD can trip without an insulation fault being present. Where the boundary lies between permissible leakage current and a fault depends on the device, the product standard and the protective measure.

Another misunderstanding is that every fuse reliably protects people against electric shock. Fuses and miniature circuit breakers serve above all for the overcurrent protection of lines and installations. They only provide fault protection if the fault loop has a low enough impedance for the disconnection time to be met. They do not protect against currents in the milliampere range through the body. An RCD switch also replaces neither proper earthing nor correct protective conductor routing nor regular inspection of the installation. It is part of a protection concept, not a substitute for it.

The assumption that a high fault current is always worse than a low one is also too crude. High fault currents are destructive and place high demands on the breaking capacity of the protective devices, but they are detected quickly. Small fault currents, by contrast, can remain undetected for a long time and thus create fire or touch hazards. Protection technology therefore assesses current magnitude, time, current path, voltage and breaking capacity together.

Institutional and economic significance

Protection against fault currents is laid down in standards, technical connection conditions and operating regulations, because individual wrong decisions can have effects beyond the installation concerned. An incorrectly dimensioned protective device may not disconnect in the event of a fault. Lack of selectivity can disconnect unnecessarily large areas. An unsuitable RCD type can go blind or trip unintentionally. This gives rise to costs for planning, inspection, documentation and maintenance that are hardly visible in the electricity price but are part of secure supply. For network operators, handling fault currents is part of supply quality, for installation operators part of operational safety and liability, and for manufacturers it influences product design and insulation coordination. With heat pumps, charging points, battery storage and PV inverters in large numbers, this level gains weight in low- and medium-voltage networks.

Fault current thus designates a fault condition that is described through current path, current magnitude and duration. It can be controlled only through the interplay of earthing, protective device, system type and operating regulation. Electrical safety therefore does not arise from faults not occurring. It arises from an installation clearly detecting faults, limiting them locally and disconnecting in time.

Sources

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Edited by eCMS on 11.11.2026 19:47

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