What Is an Earth Fault Current? Definition, Flow Path, Calculation, Value

Earth fault current (UK and IEC) or ground fault current (US): current flowing to earth due to an insulation fault [defined in the IEC 60050-195-2021 [1]].

In a fault condition, a live part of an electrical installation of building may be short-circuited to an extraneous-conductive-part of the building. There will be an electric current flowing from the live part to an extraneous-conductive-part, which is also an earth-fault current.

In an emergency mode of an electrical installation of building, due to a fault in the insulation of a live part, the resistance between this live part, on the one hand, and the exposed conductive part of the electrical equipment of class I and the protective conductor connected to it, extraneous-conductive-parts, as well as the earth, on the other hand, is sharply reduced.

This dramatically increases the value of the electric current flowing from the live part to the exposed conductive part of class I equipment and the protective conductor connected to it, to extraneous-conductive-parts, to the earth, and to the conductive parts connected by protective conductors to the earthing arrangement of the electrical installation of building and to the earthed live part of the power source.

Such an electric current flowing under single or multiple fault conditions is referred to in international standards as earth fault current.

Earth Fault Current Path

The path that a earth-fault current can take in an electrical distribution system depends on the type of system earthing.

Consider the most common electrical distribution system, which is an electrical installation of building connected to a low-voltage electrical distribution network consisting of a step-down transformer substation and an overhead or cable distribution line.

TT System

If the basic insulation of a class I hazardous live part of electrical equipment is faulted and a circuit occurs to an exposed conductive part, then in an electrical installation of building, corresponding to the TT system earthing type (Figure 1), the earth-fault current flows from the live part to the exposed conductive part [2].

From the exposed conductive part, an electric current flows to the local earth via the protective conductor, the main earthing terminal, the earthing conductors and the eart electrode.

Through the earth, the earth fault current flows to the earth electrode of the earthing arrangement of the power source, which is a transformer installed in a 10/0.4 kV transformer substation.

The live part of the power source, namely the neutral of the step-down transformer, is connected to this earthing arrangement.

Earth-fault current path in the TT system
Figure1. Earth-fault current path in the TT system

IT System

In an electrical installation of building, corresponding to the type of earthing of the IT system, the earth-fault current flows from the live part to the exposed conductive part. From the exposed conductive part, the electric current then flows to the local earth via the protective conductor, the main earthing terminal, the earthing conductors and the earth electrode. Since in an IT system, the neutral of a transformer installed in a transformer substation is usually isolated from the earth, the earth-fault current flows through the earth and the full resistances of the phase conductors against the earth to the under voltage phase conductors.

The figure shows: 1 – earthing arrangement of the power source; 2 – earthing arrangement of the electrical installation of building; Iefc – earth fault current.

For TT and IT systems, we can talk about the “classic” path of the “classic” earth fault current, namely, from the live part to the earth.

TN System

If the electrical installation of building is of the TN-S, TN-C or TN-C-S type of system earthing (Fig. 2), the earth-fault current from the live part flows into the exposed conductive part of the class I equipment and the protective conductors of the electrical installation of building connected to it.

Further, the predominant part of the earth-fault current flows through the PEN conductors of the low-voltage distribution network to the transformer’s earthed neutral point. At the same time, a small fraction of the earth-fault current flows in parallel to the PEN conductor of the distribution line between the earthing arrangements of the electrical installation of building and the power source.

This path of earth-fault current should be considered as the characteristic path of earth-fault current in the TN-C-S system. In the TN-S and TN-C systems, the earth-fault currents follow similar pathways, differing only in the type of protective conductors used in the electrical installations of buildings and the distribution lines corresponding to the TN-S and TN-C types of system earthing.

Thus, the basic paths by which earth-fault currents flow in TN systems are drastically different from the “classical” paths in TT and IT systems. However, these artificial conductive paths are specifically designed to multiply the earth-fault currents in TN systems, as compared to those in the TT system, and even more so in the IT system.

[2]
Earth-fault current path in the TN-C-S system
Figure 2. Earth-fault current path in the TN-C-S system

Figure 2 shows the path of the earth-fault current in the TN-C-S system. The symbols are the same as in Figure 1.

The earth fault currents of class 0, II and III emergency electrical equipment follow less defined conductive paths than those of class I electrical equipment, e.g. through the conductive sheathing of the electrical equipment into the earth or through extraneous-conductive-parts. And part of this conductive path may be the body of a human holding hand-held electrical equipment or in electrical contact with accessible conductive parts of mobile or stationary electrical equipment of classes 0, II and III.

[2]

Earth fault current can flow through floors, walls, and other building components if they have little resistance or if for some reason (such as increased humidity) their resistance has decreased dramatically, or through other previously unknown conductive paths.

Value

The value of the earth-fault current depends on the type of system earthing to which the electrical installation of building corresponds. The smallest earth-fault currents (typically up to 1 A) occur in IT systems in which the live parts of power sources are isolated from earth or some live parts are connected to earth through high resistances.

Significantly higher earth-fault currents (up to several tens of amperes) occur in TT systems. The earth-fault current here is approximately equal to the quotient of the nominal phase voltage divided by the sum of the total resistances of the earthing arrangements of the power source and the electrical installation of building.

Very high earth-fault currents (up to several thousand amperes) can occur in electrical installations of buildings corresponding to the TN-C, TN-S and TN-C-S types of system earthing. The earth-fault currents in TN systems are comparable to single-phase short-circuit currents, as the phase conductor is shorted to a protective conductor, a PEN conductor, or a combination of these conductors connected to an earthed live part of the power source.

Calculation of Earth Fault Current

To calculate earth-fault currents, consider the TT system (Fig. 3) and TN-C-S system (Fig. 4), which are shown in Figs. 1 and 2, respectively.

In the substitution diagrams, the total resistances of the power source and the earth are not shown, since they have little influence on the estimation of the earth-fault currents.

Since, in the following, short-circuits to earth are considered, transient resistances between phase conductors and protective conductors of electrical installations of buildings are not shown in the schemes.

Simplified substitution diagram of the TT AC system
Figure 3. Simplified substitution diagram of the TT AC system

Figure 3 shows:

  • ZL DL is the total resistance of the phase conductor of the distribution line from the low-voltage switchgear of the transformer substation to the input terminals of the electrical installation of building;
  • ZL EIB is the total resistance of the phase conductors of the distribution and final electrical circuits from the input terminals of the electrical installation of building to the earth fault point;
  • ZPE EIB is the total resistance of the protective conductors of the distribution and final electrical circuits from the main earthing terminal of the earthing arrangement of the electrical installation of building to the earth fault point;
  • ZEA PS is the total resistance of the power source earthing arrangement;
  • ZEA EIB is the total resistance of the earthing arrangement of the electrical installation of building;
  • IEFC – earth fault current.

In the TT system, the earth-fault current generated by the power source flows in a closed loop formed by the total resistances of the phase conductors of overhead lines or underground cable, the phase and protective conductors of the electrical circuits of the electrical installation of building, and the total resistances of the earthing arrangements of the power source and the electrical installation of building.

The sum of the total resistances of the ZEA PS power source earthing arrangements and the ZEA EIB electrical installation of building is usually many times greater than the sum of the total resistances of the phase and protective conductors of the distribution line and the electrical installation of building. Therefore, the earth-fault current in the TT system can be approximated by the formula:

IEFC ≈ Uo / (ZEA PS + ZEA EIB) ,

where Uo is the nominal voltage AC or DC of the line conductor relative to earth.

If, for example, the nominal voltage of the electrical installation of building is 230/400 V, the total resistance of the transformer neutral earthing arrangement installed in the transformer substation is 4 Ohm, and the total resistance of the earthing arrangement of the electrical installation of building is 10 Ohm, then the maximum earth-fault current will be approximately equal:

IEFС ≈ 230 В / (4 + 10) Ом ≈ 16,4 А, where 230 V is the nominal phase voltage.

If any live part of the power source in an IT system is earthed through a resistance, the earth-fault current flows along a similar conductive path. Its value can be calculated using the above formula. If all live parts in the IT system are insulated from earth, the earth-fault current from earth flows to the phase conductors through their capacitive resistances against earth.

Simplified substitution diagram of the TN-C-S AC system
Figure 4. Simplified substitution diagram of the TN-C-S AC system

To the designations described for Figure 3, the following has been added in Figure 4:

  • ZPEN DL is the total resistance of the PEN conductor of the distribution line from the low-voltage switchgear of the transformer substation to the input terminals of the electrical installation of building.

In the TN-C-S system, the predominant part of the earth-fault current flows in a closed loop formed by the total resistance of the overhead line phase conductors or underground cable, the phase and protective conductors of the electrical circuits of the electrical installation of building, and the total resistance of the PEN conductor of the distribution line. The sum of the total resistances of the power source earthing arrangements and the electrical installation of building is many times the total resistance of the PEN conductor of the distribution line. Therefore, the portion of the earth-fault current which flows through these two resistances, connected in parallel with the PEN conductor resistance, can be neglected.

Since the cross-sections and distances of the neutral and protective conductors of the distribution and final electrical circuits from the input terminals of the electrical installation of building to the earth fault point are generally equal, the total resistances of these conductors are also equal. The maximum earth-fault current in a TN-C-S system is therefore equal to the single-phase short-circuit current between the phase and neutral conductors in AC electrical systems, or to the single-pole short-circuit current between the pole and the mid-point conductor in DC electrical systems.

If the electrical installation of building is located close to a transformer substation and is connected to it by a distribution line with large cross-section conductors, or if the transformer substation is built into the building, the transformer resistance must be taken into account when calculating earth-fault currents.

Protective Measure

To protect against electric shocks in electrical installations of buildings, a special protection measure is used – automatic disconnection of supply, oriented to the recognition of earth fault currents and disconnection of electrical circuits in which earth faults have occurred. Depending on the type of system earthing to which the electrical installation of building corresponds, disconnection of electrical circuits with class I emergency electrical equipment is carried out by means of various protective devices.

  • In TN-S, TN-C and TN-C-S systems with very large earth-fault currents, automatic disconnection is usually provided by overcurrent protection devices – circuit breakers and fuses;
  • In TT systems with small earth-fault currents, automatic disconnection is generally provided by residual current devices (RCDs);
  • IT systems usually use insulation monitoring devices that indicate the occurrence of the first earth fault.

Electric current flowing through the human or animal body into the earth or conductive parts electrically connected to earth when a live or exposed conductive part is touched is an earth-fault current. Detection and disconnection of such a current is possible by means of a residual current device. Almost all protective devices which disconnect earth-fault currents directly or indirectly protect people and animals against electric shocks under fault conditions.

FAQ

What Causes Earth Fault Current?

There are many potential causes of earth fault current. Some of the most common include: damaged or loose electrical wiring, faulty electrical equipment, poorly maintained electrical systems, wet or damp conditions, lightning strikes.

Earth fault current can cause a variety of problems, ranging from sparks and fires to damage to electrical equipment. It is important to be aware of the potential causes of earth fault current in order to prevent accidents and keep your home or business safe.

What Is Earth Fault in Electrical Circuit?

An earth fault is a condition that occurs when the current flowing through an electrical circuit deviates from its intended path and flows into the earth. This can happen due to a number of reasons, such as a break in the insulation of the conductor or a faulty appliance. Earth faults can be dangerous as they can lead to electric shocks or fires.

What Is Over Current and Earth Fault?

An over current is a condition where there is too much current flowing through an electrical circuit. This can be caused by a number of things, but the most common cause is a short circuit. When there is too much current flowing through a circuit, it can cause the wires to overheat and eventually fail.

An earth fault is a condition where there is a loss of connection between the live (energised) conductor and the earthing (grounding) conductor. This can happen for a number of reasons, but the most common cause is damage to the insulation surrounding the live conductor. When this happens, the current will take the path of least resistance and flow through the earth instead of back to the source. This can cause serious problems as it can lead to an electrical shock.

What Is the Difference Between Ground Fault and Earth Leakage?

Ground fault and earth leakage are two terms that are often used interchangeably, but there is actually a difference between the two. A ground fault occurs when there is a break in the continuity of the grounding conductor, which can allow current to flow through the earth instead of being safely grounded. An earth leakage, on the other hand, happens when current flows through the earth and into example water or other conductive materials, which can pose a shock hazard.

So, while both ground faults and earth leakages can be dangerous, they are different phenomena. It’s important to be aware of the difference so that you can take appropriate precautions to protect yourself and your property.

References

  • IEC 60050-195-2021
  • Kharechko Y.V. Concise Terminological Dictionary of Low Voltage Electrical Installations. Part 3 // Appendix to the journal “Library of the safety engineer. – 2013. – № 4. – 160 c.;
  • Kharechko Y.V. Earth fault current // Electrica. – 2013. – № 8.