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).

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).

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.

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).

TT System

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

IT System

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

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