TN-S Earthing System (or TN-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 by protective earthing conductors (PE) [defined in the IEC 60364-1].
BS7671 provides the following definition: a system having separate neutral and protective conductors throughout the system.
The Meaning of the Letters T, N, 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 distribution system:
- S – the specified connection is provided throughout the 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 distribution system by means separate conductors – the protective earthing conductor and the neutral, mid or earthed line conductor;
TN-S Earthing System Explained
In the TN-S type of system earthing (see Fig. 1), one part of the power source that is live is earthed, usually the neutral of the transformer. 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. PE protective conductors are used to provide this connection in the entire electrical distribution system – both in the LV distribution network and in the electrical installation of a building.
With the TN-S type of system earthing, a higher level of electrical safety can be achieved in the electrical installations of buildings than with the TN-C type of system earthing. The higher level of electrical safety is primarily due to the use of protective conductors through which leakage currents flow under normal conditions. Their values are much lower than the load currents which normally flow through PEN conductors.
Minor electrical currents have less negative effect on the electrical contacts in protective conductor circuits. Therefore, the probability of loss of electrical continuity in a protective conductor circuit is significantly lower than in a PEN conductor.
Currently, the TN-S system is hardly used in some countries of the world (e.g. Russia). It will also have limited future penetration due to more expensive electricity distribution networks. To implement a TN-S system in a low-voltage electrical distribution network, overhead lines and underground cables distribution line should be used, having one more conductor than is necessary when implementing TN-C, TN-C-S and TT systems.
However, if the transformer substation is integrated into the building, the electrical distribution system will not have a distribution line. It is therefore sensible to carry out the system in question with the TN-S type of system earthing. The electrical installation of a single-family detached home that is connected to its own nearby transformer substation can also easily be performed with the TN-S type of system earthing.
In the electrical installations of buildings that contain a large amount of technological information processing equipment, international regulations recommend the use of the TN-S type of system earthing. That part of the electrical installation of a building which has its own power supply, used mainly to supply power to the information processing equipment, must comply with the TN-S type of system earthing.
Examples of TNS Earthing System Diagrams
Diagram 1 shows:
- 1- earthing at the power source
- 2 – earthing arrangement of the electrical installation of a building
- 3 – exposed-conductive-parts
- 4 – protective contact of the plug socket
- TS – transformer substation
- UC – underground cable
- OL – overhead line
Advantages of the TNS Earthing System
The TN-S system is the most reliable and safe earthing system, which protects electrical equipment and, most importantly, people from electric shocks by means of RCDs and equipotential bonding systems (EBS).
Another advantage of this system is the absence of high-frequency interference (from appliances such as electric shavers, vacuum cleaners, drills) and other interferences with consumers’ power lines.
The TN-S system does not require monitoring of the earthing circuit, because it is not necessary.
Disadvantage of the TNS Earthing System
I believe that the only disadvantage of this system is the expensive wiring due to the presence of power cables (wires) with a large number of cores.