Types of System Earthing for AC and DC Electrical Distribution Systems

Type of system earthing: an integrated characteristic of the distribution system, which assigns availability or absence of earthing of live parts of a power source, availability of earthing of exposed-conductive-parts of an electrical installation or an electrical equipment, availability and performance of electrical connection between earthed live parts of the power source and exposed-conductive-parts [defined in the IEC TS 62257-5-2015].

Requirements

Note 1 to entry: The characteristic «type of system earthing» determines special requirements to all elements of the electrical distribution system. For components of the electrical distribution network this characteristic determines the following requirements:

  • for the power source – presence or absence of electrical earthing of its live parts. If the power source has the earthed live part the additional earthing of conductors connected electrically with the earthed live part of the power source may be provided in the electrical distribution network. If the power source has live parts isolated from the earth the distribution network conductors, as a rule, should be isolated from the earth or, as an exception, some electrical conductor can be earthed through an impedance;
  • for the electrical distribution line – requirements to the arrangement of the protective, neutral, mid-point and earthed line conductors.

For the electrical installations or the electrical equipment this characteristic determines requirements to earthing of the exposed-conductive-parts, and to presence or absence of an electrical connection of the exposed-conductive-parts with the earthed live part of the power source.

The Meaning of the Letters T, N, C, S, I

The first letter determines presence or absence of earthing of live parts of the power source:

  • T – one live part of the power source is earthed. Additional earthing PEN, PEM, PEL conductors and protective earthing conductor (PE) in the distribution network (if any) may be provided.
  • I – all live parts of the power source are isolated from the earth or one live part is earthed through a high impedance. Conductors of the distribution network (if any), as a rule, should be isolated from the earth.

The second letter specifies the earthing of exposed-conductive-parts of the electrical installation or electrical connection presence between the exposed-conductive-parts and the earthed live part of the power source:

  • Т – the exposed-conductive-parts are earthed irrespective of presence or absence of the earthing of any live part of the power source;
  • N – the exposed-conductive-parts have direct connection with the earthed live part of the power source executed by PEN, PEM, PEL conductors or protective earthing conductors (PE).

Subsequent after N letters specify how an electrical connection between the earthed live part of the power source and the exposed-conductive-parts of the electrical installation is performed in the distribution system, and also assign features of the arrangement of conductors which carry out the functions of protective earthing conductor (PE) and the neutral (N), mid (M) or earthed line (LE) conductor in the electrical distribution system:

  • C – the specified connection is provided throughout the distribution system by means of PEN, PEM or PEL conductors. Functions of the protective earthing conductor and the neutral, mid or earthed line conductor are provided throughout the distribution system by means of the single conductor accordingly – PEN, PEM or PEL conductor;
  • S – the specified connection is provided throughout the electrical distribution system by means of the protective earthing conductors (PE). Functions of the protective earthing conductor and the neutral, mid or earthed line conductor are provided throughout the distribution system by means separate conductors – the protective earthing conductor and the neutral, mid or earthed line conductor;
  • C-S – the specified connection is provided in a head part of the distribution system (from the power source) by means of PEN, PEM or PEL conductors, and in other parts of the electrical distribution system – by means of protective earthing conductors (PE). In the head part of the distribution system the functions of the protective earthing conductor and the neutral, mid or earthed line conductor are provided by means of PEN, PEM or PEL conductor, and in other parts of the distribution system are provided by means of separate conductors – the protective earthing conductor and the neutral, mid or earthed line conductor.

Types of System Earthing for AC Distribution Systems

Table 1 shows those types of system earthing for AC distribution systems.

Systems Earthing of live part of the power source Arrangement of neutral, earthed phase and protective earthing conductors Earthing of exposed-conductive-parts
TN TN-C One live part of a power source (neutral or phase) is earthed. Neutral conductor or earthed phase conductor and protective earthing conductor combined in one common PEN-conductor or PEL-conductor. Exposed-conductive-parts of an electrical installation are connected to the earthed live part of the power source by PEN or PEL conductors.
TN-C-S Neutral conductor or earthed phase conductor and protective earthing conductor combined in one common PEN-conductor or PEL-conductor from the power source but kept separate within the electrical installation. Exposed-conductive-parts of an electrical installation are connected to the earthed live part of the power source in a head part of the electrical installation (from the power source) by PEN or PEL conductors, and in the other part of the electrical installation by protective earthing conductors.
TN-S Neutral conductor or earthed phase conductor and protective earthing conductor kept separate throughout the distribution system. Exposed-conductive-parts of an electrical installation are connected to the earthed live part of the power source by protective earthing conductors.
TT One live part of a power source (neutral or phase) is earthed. The protective earthing conductor of an electrical installation is not connected to the earthed live part of the power source. Exposed-conductive-parts of an electrical installation are connected to the earth electrodes of the electrical installation which electrically independent of earth electrodes of the power source by protective earthing conductors.
IT All live parts of a power source (neutral or phase) are isolated from the earth or one live part of the power source is earthed through an impedance. The protective earthing conductor of an electrical installation is not connected to the earthed live part of the power source. Exposed-conductive-parts of an electrical installation are connected to the earth electrodes of the electrical installation by protective earthing conductors.
Table 1 – Type of system earthing in AC distribution systems

AC TN Systems

TN power systems have one point directly earthed at the source, the exposed-conductive-parts of the installation being connected to that point by protective conductors. Three types of TN system are considered according to the arrangement of neutral and protective conductors, as follows:

TN-S System

TN-S system in which, throughout the system, a separate protective conductor is used (see Figures 1, 2 and 3).

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 [based on figure 31A1 IEC 60364-1]
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 [based on figure 31A2 IEC 60364-1]
TN-S system single-phase, 2-wire with separate the earthed line conductor and the protective conductor throughout the distribution system
Figure 3 – TN-S system single-phase, 2-wire with separate the earthed line conductor and the protective conductor throughout the distribution system

TN-C-S System

TN-C-S system in which a single conductor which is carrying out the neutral or earthed line conductor and protective conductor functions is used in a part of the distribution system (see Figures 4, 5 and 6). The PEN and PEL conductors are not allowed to use in the electrical installations of residential premises, public premises, commercial premises, medical locations. The distribution network PEN or PEL conductor should be separated into the neutral or earthed phase and protective earthing conductors at the origin of the electrical installation.

TN-C-S system 3-phase, 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor elsewhere in the electrical installation
Figure 4 – TN-C-S system 3-phase, 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor elsewhere in the electrical installation [based on figure 31B1 IEC 60364-1]
TN-C-S single-phase two-wire system
Figure 5 – TN-C-S single-phase two-wire system
TN-C-S system single-phase, 2-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation
Figure 6 – TN-C-S system single-phase, 2-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation [based on figure 31B3 IEC 60364-1]

NOTE 1. Neutral and protective conductor functions combined in a single conductor in a part of the distribution system.
NOTE 2. Additional earthing of the PEN conductor in the distribution network and of the protective conductor PE in the electrical installation may be provided.

TN-C System

TN-C system in which a single conductor which is carrying out the neutral or earthed line conductor and protective conductor functions is used throughout the distribution system (see Figure 7) . TN-C system is not allowed in the electrical installations of residential premises, public premises, commercial premises, medical locations.

TN-C system with neutral and protective conductor functions combined in a single conductor throughout the system
Figure 7 — TN-C system 3-phase, 4-wire with neutral and protective conductor functions combined in a single conductor throughout the distribution system [based on figure 31C1 IEC 60364-1]

AC TT systems

The TT earthing system has one live part directly earthed at the power source. The exposed-conductive-parts of the electrical installation are connected to earth electrodes electrically independent of the earth electrode of the power source (see Figures 8 and 9).

TT system 3-phase, 4-wire with the neutral conductor throughout the distribution system
Figure 8 – TT system 3-phase, 4-wire with earthed protective conductor and neutral conductor throughout the distribution system [based on figure 31F1 IEC 60364-1]

NOTE. Additional earthing of the protective conductor PE in the electrical installation may be provided.

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

Note. TN-S or TN-C-S system should preferably be used for decentralized rural electrification system, TT system is acceptable.

AC IT Systems

The IT system power source has all live parts isolated from earth or one live part connected to earth through a high impedance. The exposed-conductive-parts of the electrical installation are connected to earth electrodes. This earth electrodes shall be provided in the electrical installation (see Figure 10).
Single-phase equipment connected between a phase and neutral conductors shall be insulated against the line-to-line voltage.

Note. In IT systems with more than one power source, consideration shall be given to the disconnection of a power source resulting in the disconnection of the high impedance earthing from the earthing arrangement.

Diagram 2. IT system 3-phase, 3-wire with all exposed-conductive-parts interconnected by the protective conductor which is collectively earthed [based on figure 31G IEC 60364-1]
Figure 10 – IT system 3-phase, 3-wire with all exposed-conductive-parts interconnected by the protective conductor which is collectively earthed [based on figure 31G IEC 60364-1]

Types of System Earthing for DC Distribution Systems

Table 2 shows those types of system earthing for DC distribution systems.

Types of system earthing for DC distribution systems
Table 2 – Types of system earthing for DC distribution systems

DC TN Systems

Where the following Figures 11 to 15 show earthing of a specific pole of a two-wire d.c. system, the decision whether to earth the positive or the negative pole shall be based upon operational circumstances or other considerations, for example, avoidance of corrosion effects on line conductors and earthing arrangements.

TN-S Systems

The earthed pole conductor LE– in the system of type a) or the earthed mid conductor M in the system of type b) is separated from the protective conductor throughout the electrical installation. (see Figure 11).

TN-S DC 2-wire system
Figure 11 – Type a) 2-wire TN-S DC system [based on figure 31H IEC 60364-1]
TN-S DC 3-wire system
Figure 11 – Type b) 3-wire TN-S DC system [based on figure 31H IEC 60364-1]

TN-C Systems

The functions of the earthed pole conductor LE– and of the protective conductor in the system of type a) are combined in one single PEL conductor throughout the electrical installation, or the earthed mid conductor M and the protective conductor in the system of type b) are combined in one single PEM conductor throughout the electrical installation (see Figure 12).

TN-C system is not allowed in electrical installations of residential premises, public premises, commercial premises, medical locations or their parts.

figure 31J 2-wire TN-C DC system pel conductor
Figure 12 – Type a) 2-wire TN-C DC system [based on figure 31J IEC 60364-1]
Figure 31J – TN-C d.c. system
Figure 12 – Type b) 3-wire TN-C DC system [based on figure 31J IEC 60364-1]

TN-C-S Systems

The functions of the earthed pole conductor LE– and of the protective conductor in the system of type a) are combined in one single PEL conductor in a part of the electrical installation, or the earthed mid conductor M and the protective conductor in the system of type b) are combined in one single PEM conductor in a part of the electrical installation (see Figure 13).

The PEL conductors and PEM conductors are not allowed in the electrical installations of residential premises, public premises, commercial premises, medical locations.

Figure 13 – Type a) 2-wire TN-C-S DC system [based on figure 31K IEC 60364-1]
Figure 13 – Type a) 2-wire TN-C-S DC system [based on figure 31K IEC 60364-1]
Type b) 3-wire TN-C-S DC system [based on figure 31K IEC 60364-1]
Figure 13 – Type b) 3-wire TN-C-S DC system [based on figure 31K IEC 60364-1]

NOTE. Additional earthing of the protective conductor in the electrical installation may be provided.

DC TT Systems

One live part of the power source is earthed. The exposed-conductive-parts of the electrical installation are earthed by connection to earth electrodes electrically independent from the earth electrode of the power source (see Figure 14).

Type a) 2-wire TT DC system [based on figure 31L IEC 60364-1]
Figure 14 – Type a) 2-wire TT DC system [based on figure 31L IEC 60364-1]
Type b) 3-wire TT DC system [based on figure 31L IEC 60364-1]
Figure 14 – Type b) 3-wire TT DC system [based on figure 31L IEC 60364-1]

NOTE. Additional earthing of the protective conductor PE in the electrical installation may be provided.

DC IT Systems

All live parts of the power source are isolated from the earth or one its live part is earthed through a high impedance. The exposed-conductive-parts of the electrical installation are earthed (see Figure 15).

Type a) 2-wire IT DC system
Figure 15 – Type a) 2-wire IT DC system
DC IT system three-wire
Figure 15 – Type b) 3-wire IT DC system [based on figure 31M IEC 60364-1]

1) The system may be connected to earth via a sufficiently high impedance. This connection may be made, for example, at the mid conductor or the pole conductor.
NOTE. Additional earthing of the protective conductor in the electrical installation may be provided.

References

  1. IEC TS 62257-5-2015
  2. IEC 60364-1