TN-S system earthing

A TN-S system, shown in fig, has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source, and the consumer’s earthing terminal is typically connected to the metallic sheath or armored of the distributor’s service cable into the premises.

Type of earthing systems

BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT.

T          = Earth (from the French word Terre)

N         = Neutral

S          = Separate

C         = Combined

I           = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth. All exposed-conductive-parts of an installation are connected to an earth electrode.)

When designing an electrical installation, one of the first things to determine is the type of earthing system. The distributor will be able to provide this information. The system will either be TN-S, TN-C-S (PME) or TT for a low voltage supply given in accordance with the Electricity Safety, Quality and Continuity Regulations 2002. This is because TN-C requires an exemption from the Electricity Safety, Quality and Continuity Regulations, and an IT system is not permitted for a low voltage public supply in the UK because the source is not directly earthed. Therefore TN-C and IT systems are both very uncommon in the UK.

Grounding Codes and Standards

 Ideally a ground should be of zero ohms resistance. The NEC has stated that "A single electrode consisting of a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode..." Once you have added the supplemental ground you have met the requirement for the NEC.

This does not mean that the value of the ground now has to be 25 ohms or less. The ground resistance values objectives vary from industry to industry.

Telecommunications industry has often used 5 ohms or less as their value for grounding and bonding. The goal in ground resistance values is to achieve the lowest ground resistance value possible that makes sense economically and physically

The need to ground

There are important reasons why a grounding system should be installed.

1- The most important reason is to protect people

2- Protection of structures and equipment from unintentional contact with live conductors


3- Ensure maximum safety from electrical system faults and lightning.

The meaning of earthing

It is a fundamental fact that electricity ALWAYS flows to the point of lowest potential. The task is to ensure that electricity, including faults, lightning and electronic noise, and flows to this point with maximum safety to people, while maintaining the reliability of equipment. Therefore we must ensure the safe, controlled flow of electricity with minimum voltage drop to earth in all cases.

The different between earth and ground 

  Ground:

The NEC, National Electrical Code defines a ground as: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

Earth:

The conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero. (In some countries the term "ground" is used instead of "earth.")