What Is a TN-C-S Earthing System? Definition, Meaning, Diagrams

TN-C-S earthing system (or TN-C-S system): a distribution system in which one live part of a power source is earthed, 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, PEM or PEL conductors, and in other part of the electrical installation by protective earthing conductors (PE) [defined in the IEC 60364-1].

BS7671 provides the following definition: a system in which neutral and protective functions are combined in a single conductor in part of the system.

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

The letter codes used for designations of the types of system earthing have the following meanings.

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 electrical distribution network (if any) may be provided.

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:

  • 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-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 conductors (PE). In the head part of the electrical 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 conductor and the neutral, mid or earthed line conductor.

TNCS Earthing System Explained

In the TN-C-S type of system earthing (Fig. 1 and 2), one part of the power supply that is live is earthed, usually the transformer neutral. The exposed-conductive-parts of the electrical installation of a building have an electrical connection to the earthed part of the power supply which is live. In order to provide this connection, PEN conductors are generally used in the LV electrical distribution network, while PE protective conductors are used in the electrical installation of a building.

In the TN-C-S system, as in the TN-C system, PEN conductors are used in the distribution network, and protective conductors are used in the electrical installation of a building, just as in the TN-S system.

With the TN-C-S type of system earthing, the PEN conductor is always separated into a protective conductor and a neutral conductor at some point in the electrical installation of a building. This separation can be effected at the origin of the electrical installation of a building, at the input terminal or at the protective busbar of the switchgear (Fig. 1). This should be done in the electrical installations of residential and public buildings, commercial establishments and medical facilities.

TNCS Earthing System Diagrams

Diagrams of the TN-C-S earthing systems are shown below.

TN-C-S system 3-phase 4-wire. PEN conductor is separated at the origin of the electrical installation
Figure 1. TN-C-S system 3-phase 4-wire. The PEN conductor is separated at the origin of the electrical installation.

The PEN conductor can also be separated at the input terminal or at the protective busbar of another switchgear, which is connected to the main switchgear via a distribution electric circuit that has the PEN conductor as part of its conductors (Fig. 2).

TN-C-S system 3-phase 4-wire. PEN conductor is separated for part of the electrical installation of building
Figure 2. TN-C-S system 3-phase 4-wire. The PEN conductor is separated for part of the electrical installation of a building.

Diagrams 1 and 2 show:

In the first case (see Diagram 1), two protective and neutral conductors are used in the whole electrical installation of a building. In the second case (see Diagram 2), the PEN conductor is used at the head part of the electrical installation of a building, while the protective conductor and the neutral conductor are used after the separation point.

Exposed-conductive-parts of class I electrical equipment are connected respectively to protective conductors throughout the electrical installation of a building (see Figure 1), or they are connected to PEN conductors in the head part of the electrical installation of a building and to protective conductors in the rest of the electrical installation (see Figure 2).

With the TN-C-S type of system earthing, it is theoretically possible to separate the PEN conductor into a protective conductor and a neutral conductor at any point in the electrical distribution network. However, it is more reliable to separate the PEN conductor within the electrical installation of a building, e.g. on the input terminals of the switchgear or on the protective busbar of the switchgear.

If the transformer substation is built into the building, it is advisable to make the electrical installation of a building with the TN-S type of system earthing, since the power distribution system will not have a distribution line.

Advantages of the TN-C-S Earthing System

The TN-C-S type of system earthing is widespread in the electrical installations of residential buildings, which is due to a number of advantages:

  1. To implement the TN-C-S system, it is possible to use existing low-voltage distribution networks without reconstructing them.
  2. The TN-C-S system can be seen as a logical development of the TN-C system. Therefore, the electrical installations of a building, corresponding to the TN-C-S type of system earthing, can be regarded as one of the options for “modernization” of low-voltage electrical installations, widespread throughout the territories of different countries of the world. It is relatively easy for designers, electricians and maintenance personnel to understand the logical transformation of the TN-C system into the TN-C-S system, and the basic requirements that must be followed when performing protective conductors in electrical installations of buildings with this type of system earthing.
  3. With TN types of system earthing, the earth-fault current flowing in a faulty electrical circuit from the phase conductor to the exposed-conductive-part and the protective conductor may be equal to that of a single-phase short-circuit. It is therefore possible to use overcurrent protection devices such as circuit-breakers and fuses as part of an electrical protective measure such as an automatic disconnection. However, in some cases, it is not possible to provide the rated tripping time with overcurrent protection devices. In these cases, an RCD must be used to automatically disconnect the power supply.
  4. In electrical circuits of electrical installations of buildings corresponding to the TN-C-S type of system earthing, which are protected by residual current devices (RCDs), it is quite easy to detect errors made in the connection of protective and neutral conductors. RCDs will disconnect the circuits they protect for no reason, signaling the following mistakes made during installation of the wiring conductors:
  • connecting neutral conductors to exposed-conductive-parts of class I electrical equipment
  • connection of protective conductors to terminals of electrical equipment intended for connection of neutral conductors
  • the electrical connection of protective conductors and neutral conductors

A higher level of electrical safety can be achieved by using the TN-C-S type of system earthing in the electrical installations in buildings than by using the TN-C type of system earthing. A higher level of electrical safety is primarily achieved by using individual protective conductors in electrical installations of buildings, through which leakage currents flow under normal conditions. Their values are much lower than the load currents that normally flow through PEN conductors.

Minor currents have a smaller adverse effect on the electrical contacts in protective conductor circuits. A protective conductor is therefore much less likely to lose electrical continuity than a PEN conductor.

If it is necessary to increase the level of electrical safety, the electrical installation of a building must be of the TN-S type of system earthing. This will require the construction of a new low-voltage distribution line or the reconstruction of an existing one.

Currently, the TN-C-S system is widely used throughout the world, including in the UK, USA, Canada and Australia. To implement the TN-C-S system, existing and new low-voltage distribution networks are used, overhead lines and underground cables of which have 3 phase conductors and a PEN conductor. On the basis of these networks it is also possible to realise TN-C and TT systems.

How to Make a TN-C-S Type of System Earthing?

Electrical installation of a single-family detached home.

Make the type of system earthing TN-C-S for the electrical installation of a single-family detached home is simple enough. Separation of the PEN conductor should be made at the input terminals of the main switchgear (see Figure 1 of the article). Further, two conductors should be used in the entire electrical installation of the building: a protective conductor and a neutral conductor, which should have no intentional or accidental electrical connection with each other beyond the point of separation of the PEN conductor.

The electrical installation of a single-family detached home is usually connected to an AC electrical distribution network.

Electrical installation of newly constructed multi-family residential buildings.

In the electrical installations of newly constructed multi-family residential buildings, the TN-C-S type of system earthing can be realised in only one way, providing for the separation of the PEN conductor of the distribution line at the origin of the electrical installation of a building, namely at the input terminals of the main switchgear (see Fig. 3).

Electrical installation of a multi-family residential building corresponding to the tn-c-s-type of system earthing
Figure 3. The electrical installation of a multi-family residential building corresponding to the TN-C-S-type of system earthing. The PEN conductor is separated in the main switchgear.

Existing electrical installations of multifamily residential buildings.

In existing electrical installations of multi-family residential buildings, the TN-C-S type of system earthing could be done differently. For example, the PEN conductors of the electrical risers could be separated into protective and neutral conductors in the floor distribution boards, which are installed on the floors of the residential building and connected to the electrical risers (see Figure 4).

The electrical installation of a multi-family residential building corresponding to the TN-C-S type of system earthing. PEN conductor separated in floor distribution boards.
Figure 4. The electrical installation of a multi-family residential building corresponding to the TN-C-S type of system earthing. The PEN conductor is separated in floor distribution boards.

Diagrams 3 and 4 show:

1 – earthing at the source
2 – earthing in the electrical installation of a building
3 – exposed-conductive-part
TS – transformer substation
UC – underground cable
OL – overhead line

Note: Figures 3 and 4 show the electrical installations of the apartments as single-phase class I appliances.

In the first option, the electrical riser (see Figure 3), which is part of the electrical distribution circuit and is designed to transmit electricity from the main distribution board to the floor distribution boards, must have 5 conductors – 3 phase conductors, a neutral conductor and a protective conductor. In the second option (see Figure 4), the electrical riser is made of 3 phase conductors and the PEN conductor.

The first option of constructing electrical circuits of protective conductors in electrical installations of residential buildings corresponding to the TN-C-S type of system earthing, which is required by IEC 60364-1 and IEC TS 62257-5-2015, is preferable in terms of providing protection against electric shocks than the second option. The first option of implementing the TN-C-S type of system earthing should be followed when reconstructing the existing electrical installations of residential buildings.

Other Examples of the TN-C-S System

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.
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 the PEL conductor into the earthed line conductor and the protective conductor for part of the installation.
TN-C-S system single-phase, 2-wire where the PEL conductor is separated into the protective conductor and the earthed phase conductor at the origin of the electrical installation
Figure 8. TN-C-S system single-phase, 2-wire where the PEL conductor is separated into the protective conductor and the earthed phase conductor at the origin of the electrical installation.
TN-C-S system 3-phase 3-wire with pel conductor separation at the origin of the electrical installation
Figure 9. TN-C-S system 3-phase 3-wire with pel conductor separation at the origin of the electrical installation.

References

  1. IEC 60364-1
  2. IEC TS 62257-5-2015
  3. BS7671