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Grounding Systems and Their 3-Letter Classification
Grounding Systems and Their 3-Letter Classification

Protective Ground Connection

Low voltage systems supplying to consumer premises are predominantly solidly grounded. Protective ground connection to consumer premises (or extending the supply system ground to consumer premises) is however done in different ways. The common system categories are defined below using a 3-letter classification (based on IEE Standards).

Note that in these descriptions, ‘system includes’ both the supply and the consumer installation, and ‘live parts’ include the neutral conductor.

First letter

T – The live parts in the system have one or more direct connections to ground.

I – The live parts in the system have no connection to ground or are connected only through a high impedance.

Second letter

T – All exposed metal parts / enclosures of electrical equipment are connected to the ground conductor which is then connected to a local ground electrode.

N – All exposed metal parts / enclosures of electrical equipment are connected to the ground conductor which is then connected to the ground provided by the supply system.

Remaining letter(s)

C – Combined neutral and protective ground functions (same conductor).

S – Separate neutral and protective ground functions (separate conductors).

Common types of systems

TN system

A system having one or more points of the source directly grounded with the exposed metal parts being connected to that point by protective conductors. It is further subdivided into the following types depending on the neutral-ground connection configuration.


TN-C system

A system in which the same conductor functions as the neutral and protective conductor throughout the supply and consumer installation (Figure 1).

Schematic of a TN-C system
Figure 1 – Schematic of a TN-C system


TN-S system

A system in which separate conductors are provided for neutral and protective ground functions throughout the system. In this type of system, the utility provides a separate ground conductor back to the substation.

This is most commonly done by having a grounding clamp connected to the sheath of the supply cable which provides a connection to the ground conductor of the supply side and the grounding terminal of the consumer installation (Figure 2).

Schematic of a TN-S system
Figure 2 – Schematic of a TN-S system

TN-C-S system

A system in which the neutral and protective functions are done by a single conductor in a part of the system. In this system, in supply side neutral and ground are combined, but they are separated in the installation.

This is also called as protective multiple earthing (PME for short). The grounding terminal of the consumer installation is connected to the supplier’s neutral.

Any breakage of the common neutral cum ground wire, called sometimes as PEN (protective earth and neutral) conductor, can result in the enclosures of electrical equipment inside the premises assuming line voltage when there is insulation failure.

It is therefore essential to maintain the connection integrity of this common neutral-cum-ground conductor (Figure 3).

Schematic of a TN-C-S system
Figure 3 – Schematic of a TN-C-S system

TT System

No ground provided by supplier; installation requires own ground rod (common with overhead supply lines) (Figure 4).

Schematic of a TT system
Figure 4 – Schematic of a TT system

IT System

Supply is, for example, portable generator with no ground connection, installation supplies own ground rod (Figure 5).

Schematic of an IT system
Figure 5 – Schematic of an IT system

Resource: Practical grounding, bonding, shielding and surge protection – G. Vijayaraghavan
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About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

5 Comments


  1. Lothar
    Nov 04, 2015

    this is useless hahahhaa


  2. precious tumwesigye
    Jul 26, 2014

    thanks


  3. Nadir
    Feb 06, 2014

    Good site

  4. […] of Earthing Strip/Conductor (R)== 1.7 Ω 4. Calculate Min. Cross Section area of Earthing ConductorCross Section Area of Earthing Conductor As per IS 3043(A) =(If x√t) / KWhere:t = Fault current […]


  5. eroncelli
    Apr 12, 2013

    Just a simple clarification to this subject:
    N – All exposed metal parts / enclosures of electrical equipment are connected to the ground conductor “is then connected to the ground provided by the supply system”

    The sentence is incorrect: the real meaning is that metal parts/enclosures are connected to the “same” earth as neutral.
    That is the typical case of installation of a MV/LV transformer by the consumer (where a common grounding for neutral and metal parts is a preferrable solution)

    Thanks for the attention
    Eugenio Roncelli

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