Earthed System: What it Is, Examples, Features

Earthed system is an electrical system in which at least one live part is earthed directly.

Note 1 – Three-phase four-wire and single-phase three-wire AC electrical systems must be earthed. In three-phase three-wire and single-phase two-wire AC electrical systems where there is no neutral, the phase conductors must be earthed.

Note 2 – In a three-conductor DC electrical system, the live mid part must be earthed. In a two-wire DC electrical system where there is no live mid part, the pole conductor must be earthed.

Thus, the quoted definition unambiguously identifies all earthed electrical systems, and the notes to it provide comprehensive explanations of the most common types.

The term “earthed system” only loosely defines an electrical system and its elements. A more specific characteristic of a low-voltage electrical system is established by means of the types of system earthing, the requirements for which are set forth in IEC 60364-1, subsection 312

The IEC standards widely use other terms to define earthed electrical systems, such as: “solidly earthed neutral system”, “impedance earthed neutral system”, “resonant earthed system”, “isolated or not effectively earthed system”, etc. These terms are used to identify low-voltage and high-voltage electrical systems having power sources whose neutrals are earthed directly or through some impedances and reactors, i.e. these electrical systems are characterized by having at least one earthed live part.

The term ‘solidly earthed neutral system’ is officially defined within the IEC 60050-195 as:

system in which at least one neutral is earthed directly.

IEC 60050-195

The term ‘impedance earthed neutral system’ is officially defined within the IEC 60050-195 as:

system in which at least one neutral is earthed through a device having an impedance designed to limit the line-to-earth short-circuit current.

IEC 60050-195

The term ‘resonant earthed neutral system’ is officially defined within the IEC 60050-195 as:

system in which at least one neutral is earthed through a device having an inductance designed to compensate approximately for the line-to-earth capacitances in case of a single-line-to-earth fault.

IEC 60050-195

Note 1 – The following are examples of a “resonant earthed system”: arc-suppression-coil earthed system, peterson coil earthed system and earth fault compensation system.

The term ‘isolated or not effectively earthed system’ is officially defined within the IEC 60050-195 as:

electrical system in which all live parts isolated from the Earth, or one live part earthed through a high impedance.

IEC 60050-195

The definitions provided describe AC electrical systems with at least one neutral point directly earthed. Such electrical systems can be three-phase four-wire electrical systems whose power sources are star-connected and therefore have neutral points, or single-phase three-wire electrical systems whose power sources also have neutral points.

Since three-phase three-wire electrical systems whose power sources are connected in a triangle and single-phase two-wire electrical systems do not have neutral points, the definitions under consideration cannot be used for them. In addition, the definitions under consideration do not apply to DC electrical systems. That is, they characterize only specific AC electrical systems.

Examples of Earthed Systems

According to the requirements of IEC 60364-1 to earthed systems include electrical distribution systems that have TN-S, TN-C-S, TN-C and TT types of system earthing.

Figures 1-9 below show examples of 3-phase 3-wire and 4-wire and single-phase 2-wire earthed AC electrical systems, and figures 10-13 show examples of 3-wire earthed DC electrical systems.

Earthed AC electrical systems:

TN-S system 3-phase, 4-wire with separate the neutral conductor and the protective conductor throughout the distribution system
Figure 1 – TN-S system 3-phase, 4-wire with separate the neutral conductor and the protective conductor throughout the distribution system
TN-S system 3-phase, 3-wire with separate the earthed phase conductor and the protective conductor throughout the distribution system
Figure 2 – TN-S system 3-phase, 3-wire with separate the earthed phase conductor and the protective conductor throughout the distribution system
TN-C-S system 3-phase 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation
Figure 3 – TN-C-S system 3-phase 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation
TN-C system with neutral and protective conductor functions combined in a single conductor throughout the system
Figure 4 — TN-C system with neutral and protective conductor functions combined in a single conductor throughout the system
TT system 3-phase, 4-wire with the neutral conductor throughout the distribution system
Figure 5 – TT system 3-phase, 4-wire with the neutral conductor throughout the distribution system
TN-S system single-phase, 2-wire with separate the earthed line conductor and the protective conductor throughout the distribution system
Figure 6 – TN-S system single-phase, 2-wire with separate the earthed line conductor and the protective conductor throughout the distribution system
TN-C-S single-phase two-wire system with separation of PEL conductor into an earthed line conductor and protective conductor for part of the installation
Figure 7 – TN-C-S single-phase two-wire system with separation of PEL conductor into an earthed line conductor and protective conductor for part of the installation
TN-C system single-phase, 2-wire with earthed phase conductor
Figure 8 – TN-C system single-phase, 2-wire with earthed phase conductor
TT system single-phase, 2-wire with the earthed phase conductor throughout the distribution system
Figure 9 – TT system single-phase, 2-wire with the earthed phase conductor throughout the distribution system
TN-S DC 3-wire system
Figure 10 – TN-S DC 3-wire system
Figure 31J – TN-C d.c. system
Figure 11 – TN-C DC system
TN-C-S DC three-wire system
Figure 12 – TN-C-S DC three-wire system
TT DC three-wire system
Figure 13 – TT DC three-wire system

Features

The electrical installation of a building connected to the most common low-voltage general electrical distribution network, with three phase conductors and a PEN conductor, is generally of the TN-C-S type of system earthing. The TN-C system is used less frequently, since IEC 60364-1 forbids the installation of residential and public buildings, commercial establishments and healthcare facilities with the TN-C type of system earthing.

When connected to the specified electrical distribution network, it is possible to perform the electrical installation of a building with the TT type of system earthing. In this case, the PEN conductor of the distribution line will only have the function of the neutral conductor.

If the electrical installation of a building is connected to a low voltage electrical distribution network with three phase conductors, a neutral conductor and a protective conductor, or if a transformer substation is built into the building, the electrical installation of a building must be of the TN-S type of system earthing.

The electrical installations of buildings are generally low-voltage AC electrical installations. However, some electrical installations of buildings may have parts that function on DC current. These parts are usually made with the TN-S and TT types of system earthing.

In most cases, the electrical installations of buildings are part of electrical distribution systems that have directly earthed neutrals or other live parts of power sources. In other words, the electrical installations of buildings are parts of earthed electrical systems. For this reason, the following AC and DC voltage ranges, as specified in IEC 60449 for earthed electrical systems, are generally applicable for electrical installations of buildings:

Table 1: AC voltage bands:

Bands Earthed systems
Phase to earth1, V Between phases, V
I U≤ 50 U≤ 50
II 50<U≤600 50<U≤1 000

U = nominal voltage of the installation.

Note 1: Phase-to-earth voltage is the voltage between a phase conductor and a reference earth at a given point in an electrical circuit.

Table 2: DC voltage bands:

Bands Earthed systems
Pole to earth2, V Between poles 3, V
I U≤ 120 U≤ 120
II 120<U≤900 120<U≤1 500

Note 2 – The pole-to-earth voltage is the voltage between the pole conductor and the reference earth at a given point in an electrical circuit.

Note 3 – Pole-to-pole voltage is the voltage between two pole conductors at a given point in an electrical circuit.

U = nominal voltage of the installation.

References

  1. IEC 60050-195
  2. IEC 60449
  3. IEC 60364-1

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