Earthing

Earthing : Earthing means connecting the neutral point of a supply system or the non – current carrying metal parts used in electrical distribution system to the general mass of earth by wire of negligibale resistance in such a manner that at all times an electrical discharge of electrical energy takes place without danger. This brings the body of the equipment to zero potential and thus will avoid the shock to the operator.

Purpose of earthing: 1. To avoid electrical shock to human body.2. To avoid risk of fire due to earth leakage current through unwanted path.3. To ensure that no current carrying conductor rise to potential with respect to earth than its

designed insulation.

Important term used in Earthing

Earth electrode: A conductor or group of conductores in intimate contact with and providing an electrical connection to earth.

Earth electrode resistance: The resistance of an earth electrode to earth.

Touch voltage : The potential differance between a grounded metallic structure and the point on the earth's surface seperated by a distance equal to the normal maximum horizontal reach approximately one meter.

Step Voltage : The potential differance between two points on the earth's surface seperated by a distance one pace, that will be assumed to be one meter in the direction of maximum potential gredient.

Earth Grid: A system of grounding electrodes consisting of inter- connected connectors burried in the earth to provide a common ground for electrical divices and mettalic structure.

Earth mat: A grounding system formed by grid of horizontally buried conductors and which serves to dissipate the earth fault current to earth and also as an equipotential bonding conductor system.

Ground symbol s

Signalground

Chassisground

Earthground

Types of Earthing

1) Neutral Earthing 2) Body Earthing

Neutral Earthing Solidly Earthing / Effectively Earthing Non effectivaly Earthing

a) Reactor Earthing b) Resistor earthing

Isolated Earthing

Earthing design considerations A correctly designed and installed earthing system will safeguard both lives and equipment.A good earth connection should have,

Low electrical resistance to earth Good corrosion resistance Ability to carry the required current repeatedly A reliable life span of the earthing system.

The crucial factors that determine the resistance to earth of an electrode are:

Soil resistivity Electrode dimensions Area available Earth electrode materials

Soil resistivity

Physical composition Different soil compositions give different average resistivities

Effect of soil type on resistivitySoil type Typical resistivity

Ohm-m

Marshy ground 2 - 2.7

Loam and clay 4 - 150

Chalk 60 - 400

Sand 90 - 8,000

PeatRock 200 upwards

Sandy gravel 300 - 500

Rock 1,000 upwards

Moisture

Increased moisture content of the ground can rapidly decrease its resistivity.It is especially important to consider moisture content in areas of high seasonal variation in rainfall.Wherever possible the earth electrode should be installed deep enough to reach the "water table" or "permanent moisture level".

Effect of moisture on resistivity

Moisture content % by weight

ResistivityOhm-mTop soil

ResistivityOhm-mSandy loam

0 1,000 x 10^4 1,000 x 10^4

2.5 2,500 1,500

5 1,650 430

10 530 185

15 310 105

20 120 63

30 64 42

Chemical composition

Certain minerals and salts can affect soil resistivity. Their levels can vary with time due to rainfall or flowing water.Note that although the addition of salts can lower soil resistivity, they are not recommended due to corrosion and leaching.

Effect of salt on resistivity forsandy loam, 15.2% moisture

Added salt(% by weight of moisture)

ResistivityOhm-m

0.0 107.0

0.1 18.0

1.0 4.6

5.0 1.9

10.0 1.3

20.0 1.0

Temperature

When the ground becomes frozen, its resistivity rises dramatically. An earth that may be effective during temperate weather may become ineffective in winter.

Please note that, if your soil temperature decreases from +20°C to -5°C, the resistivity increases more than ten times.

Effect of temperature on resistivity forsandy loam, 15.2% moisture

Temperature(°C)

Temperature(°F)

ResistivityOhm-m

20 68 14

10 50 99

0 32 (water) 138

0 32 (ice) 300

-5 23 790

-15 14 3,300

Different methods of earthing

1. Earhing through a water mains.2. Wire or strip earthing.3. Rod earthing.4. Pipe earthing.5. Plate earthing.

Classification of system based on types of system earthing :

1. TN System2. TT System3. IT System

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):

T : direct connection of a point with earth;

I : no point is connected with earth (isolation), except perhaps via a high impedance.

The second letter indicates the connection between earth and the electrical device being supplied:

T : direct connection with earth, independent of any other earth connection in the supply system;

N : connection to earth via the supply network.

S : Seperate

C : Combined

1. TN network

In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer.

The conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral (N). Three variants of TN systems are distinguished:

TN−S : PE and N are separate conductors that are connected together only near the power source. Separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.

TN−C : A combined PEN conductor fulfills the functions of both a PE and an N conductor.Combined PE and N conductor all the way from the transformer to the consuming device.

TN−C−S : Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN).Combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.

It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S.

2. TT network

In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.

3. IT network

In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance.