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1. INTRODUCTION

In the construction of all modern electrical installation, one ground is usually run

through the building to keep the impedance as low as possible low Impedance in

the ground is needed to make sure that the fuse blows when something gets short

circuited to ground wire, the protective earth connection should be able to carry

a heavy current to protect the user from live -to- chassis faults by ensuring that

the fuse or circuit breaker will operate .so the requirement is that the protective

earth conductors can carry a 25A fault current for at least 1 minute.

Unfortunately all buildings have big electrical equipment such as air conditioning

units, refrigerator, washer/driers and other high current devices connected to the

building ground.

A ground system means that there are grounding rods which are in

direct contact with the ground, and the metal conductors which connect

them to grounded parts of electrical installation.

A grounding system must accomplish the following:

Providing a low-impedance path to ground for personal and equipment

protection.

Withstanding and dissipating repeated fault and surge-circuits.

Providing rugged mechanical properties for easy driving with minimum

effort and rod damage.

2. DEFINITIONS:

Earthing: Connecting to the earth or ground.

Neutral Earthing: Connecting to earth, the neutral point, i.e. the star

point of generator transformer, neutral point of grounding transformer.

Reactance Earthing: Connecting the neutral point to earth through a

reactance.

Resistance Earthing: Connecting the neutral point to earth through a

resistance.

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Solid earth (Effective Earthing):

A Power system is solidly earthed when a generator, power transformer,

or earthing transformer neutral is connected directly to the station ground

or to the earth, because of the reactance of the earthed generator

transformer in series with the neutral circuit, solid earthing cannot be

considered a zero- impedance circuit.

Ungrounded system: The system whose neutral points are not earthed,

the system is also called isolated neutral system.

Distribution system: A system operating at a nominal voltage not

exceeding 33kV.

Earth connection A connection to the general mass of earth by means of

an earthing electrode or earthing electrodes electrically connected at a

given location.

Earth resistance: In relation to an earth connection, this means the

resistance to the general mass of earth measured in ohms.

Earthing conductor: A conductor connecting any portion of the earthing

system to works required to be earthed, or to any other portion of the

earthing system.

Earthing electrode: A metal rod, tube, pipe, plate or other conductor

buried in or driven into the ground and used for making a connection to

the general mass of earth.

Earthing values: Values measured with an earth resistance tester.

Earthing system: All conductors, piping, electrodes, clamps and other

connections whereby conductors or other works are earthed.

Exposed conductive parts: Includes electrical equipment that can be

touched by the standard test finger and is not live, but can become live if

basic insulation fails.

Step voltage: The voltage drop caused by a current flowing through the

body between both feet, in contact with the ground one meter apart.

Touch voltage: The voltage drop caused by a current flowing through the

body between both hands and both feet. In this instance, the hands are in

contact with an earthed conductive part within 2.4 meters of the ground.

The feet are within a radius of 1.0m at ground level.

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3. ELECTRODE RESISTANCE TO EARTH:

Resistance to earth is the resistance between the metal of the electrode and

the general mass of earth.

To improve the connection to earth and to reduce the resistance to earth,

two or more ground rods are suggested, the distance between the two rods

must be the depth of the first rod plus the depth of the second rod.

Connection to earth having acceptably low values of impedance is needed

to discharge resulting from nearly strokes, and drain of static voltage

accumulations.

3.1. The factors influencing the earth resistance of an electrode of combination of electrodes:

The composition of the soil in the immediate neighborhood.

The temperature of the soil.

The moisture content of the soil.

The size, shape number and spacing of electrodes.

The depth of electrodes.

The first three influence the resistivity of the soil near the electrode, while the

remaining two factors depend on the type of electrode system used.

3.1.1 Factors affecting on resistance:

Effect of rod size on resistance:

a) Effect of depth

The depth of electrode is an important factor affects on

the resistance. The soil resistivity usually decreases as

the depth increases. Doubling the rod length reduces

resistance by an additional 40 percent.

b) Effect of diameter

Increasing me diameter of the rode, however, does not

materially reduce its resistance. Doubling the diameter for instance reduces

the resistance by less than 10 percent.

c) Effect of soil resistance:

The resistivity of the earth is a prime factor establishing the resistance of a

grounding electrode. The resistivity of soil varies with the depth from the

surface, with moisture and chemical content, and with soil temperature.

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3.1.2 Factors affecting soil resistivity:

1. Type of soil

To study earthing, we have to know the different types of soil because there

is a relation between the composition of soil and the resistivity.

Soil Receptivity (ohm.cm)

Surface, soil, loam 100- 5000

Clay 200-10000

Sand and gravel 5000-100000

Surface limestone 10000-1000000

Limestone 500-400000

Shales 5000-10000

Sandstone 2000-200000

Granite, basalts 100000

Decomposed gneisses 5000-50000

Slates 1000-10000

2. Moisture content

Usually, the percentage of humidity or moisture content is in the range of

10-15% and soil treatment should be made to reach this value.

Moisture content % by weight

Resistivity ohm. cm.

Top soil Sandy loan

0 1.000 * 104

1.000 * 104

2.5 250.000 150.000

5 165.000 43.000

10 35.000 18.500

15 19.000 10.500

20 12.000 6.300

30 6.400 4.200

3. Salt Concentration

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As concentration of the salt increases, the soil resistivity decreases and

earth resistance decreases.

Added salt % Resistivity, ohm.cm

0 10700

0.1 1800

1 460

5 190

10 130

20 100

4. Temperature

The resistivity of the soil is also influenced by temperature

3.2 Driven rods as earthing electrodes: From practical point of view, the most suitable form of earthing electrodes

is the driven rod. Practically no excavation is required and with a suitable

design of extensible rods, electrodes of 30-40 ft. can be installed. The

practical advantages of driven rods over other forms of electrode may be

summarized as follows:

Lower cost.

Temperature Resistivity ohm. cm.

C F

20 68 7.200

10 50 9.900

0 32 (water) 13.8000

0 32 (ice) 30.000

5 23 79.000

-15 14 330.000

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Driven rods can be driven to considerable depth down till moisture level

resulting in big reductions in resistance.

Seasonal variations are very much less with deep rods than with buried

electrode.

On a given area and to a given depth a small number of rods will

approach closely to the resistance of an infinite number of rods on the

same area, this would be the minimum resistance which could be obtained

without extending the area.

The connection between the earth rod and the earthed path can be quite

simple and easily inspected.

4. EFFECT OF CURRENT ON HUMAN BEINGS:

In case of faults in unearthed equipment if a human being touched this

equipment the faulty current will be discharged through his body. this will

lead to a great influence depending on the amount of current passing in the

chest area and the time of passing of this current. The effect of current starting

from 10mA and if this current reached 200mA, then the body will be

completely permanent.

5. MEASUREMENT OF THE ELECTRODE EARTH RESISTANCE:

Fall of potential method

• The most reliable method of

measuring the resistance to earth of

a driven electrode is the ‘fall of

potential’ method. Figure below

shows an arrangement of three

electrodes.

• Let E be the electrode whose

resistance to earth is required to be

measured and let P and C be the

auxiliary rods driven into the earth.

• A known value of current I is circulated between C and E, and the

voltage drop V between E and P is measured.

• The resistance of the electrode E to the earth is V/I.

• The optimum location for the

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EARTHING SCHEMES

T-T

SCHEME

T-N

SCHEME I-T

SCHEME

T-N-S

SCHEME

T-N-C

SCHEME

T-N-C-S

SCHEME

• potential electrode P is 0.62 of the distance from E to C when

• The distance D is at least 30 times the depth of the electrode E.

6. EARTHING SCHEMES

6.1 TT scheme (earthed neutral):

Its principle :

The neutral point of the transformer is connected directly to earth.

Exposed conductive parts of equipment

are connected by protective conductors

to the earth electrode of the installation

which is generally independent with

respect to the earth electrode of the

transformer

Its operation:

The current of an insulation fault is limited by earth connection

impedance.

Protection is provided by the residual current devices ( RCD ) .

Where: RA: resistance of the earth connection of the application frames.

RB: resistance of the neutral earth Connection, Rd: fault resistance.

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Id: fault current flowing in the earth connection resistance RA of the

application frame.

6.2 TN SCHEME: 6.2.1 TNC SCHEME (4 wires) :

Its principle:

The transformer neutral is earthed; the electrical load frames are connected

to neutral.

The same conductor acts as a neutral

and a protective conductor.

This scheme is not permitted for

conductors of les s than 10mm2

& for

portable equipment.

Where: PEN: potential earth and

neutral.

SCPD: short –circuit protection

device.

6.2.2 TNS scheme ( 5 wires ) :

Its principle:

The transformer neutral is earthed; the

electrical load frames are connected to

neutral.

The neutral and the protective

conductor are separate, on underground

cable systems where lead- sheathed

cables exist; the protective conductor is

generally the lead sheath.

This scheme (5 wires) is permitted for

circuits of cross –sectional area of less

than 10mm2

for copper and 16mm2

for

aluminum on mobile equipment.

Where: PE: protective earth.

6.2.3 Tncs scheme :

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When the neutral and the protective conductor are separated downstream

of part of the installation in the

TN-C system.

Note that:

The TN-S cannot be placed

upstream of the TN-C.

TN scheme operation:

An insulation fault on a phase

becomes a short-circuit and the

faulty part is disconnected by a

Short-Circuit Protection

Device (SCPD).

6.3 IT scheme ( unearthed neutral ) :

Its principle :

The transformer neutral is not earthed, but is theoretically unearthed. In

actual fact, it is naturally earthed by the stray capacities of the network

cables by a high impedance of around 1,500Ω (impedance earthed neutral). The electrical load frames are earthed.

Its operation:

• If an insulation fault occurs, a low current develops as a result of the

network’s stray capacities (see fig. a). The contact voltage developed in

the frame earth connection (no more than a few volts) is not dangerous.

• if a second fault occurs on another phase before the first fault has been

eliminated (see fig.b and c ), the frames of the loads in question are

brought to the potential developed by the fault current in the protective

conductor (PE) connecting them. The SCPDs (for the frames

interconnected by the PE) or the RCDs (for the frames with separate earth

connections) provide the necessary protection.

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7. CHOICE CRITERIA:

1st criterion:

No earthing scheme is universal.

To choose the earthing scheme, analyze every case separately.

The best solution often involves several different earthing schemes for

different parts of the installation.

2nd criterion :

It must satisfy the following fundamental criteria:

Protection against electric shock.

Protection against fire of electric origin.

Power supply continuity.

Protection against over voltage.

Protection against electromagnetic disturbances.

3rd criterion :

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Comparison of earthing schemes:

• THE TT SCHEME: It is recommended for installations that have only

limited surveillance or installations subject to extensions or modifications,

that's because it is the simplest scheme to implement private or public

distribution.

•• THE IT SCHEME : It is recommended if the power supply continuity is

imperative (it offers the best guarantee concerning the availability of

power), surgery rooms, intensive care rooms& ups system.

So it requires:

Organization of withstand to over voltage & leakage current.

To promptly eliminate any first fault.

To supervise extensions to the installation.

THE TNS SCHEME: IT recommended for installations that have a high

level of surveillance or installations not subject to extensions or

modifications.

This scheme is generally implemented without medium-sensitivity residual

current devices; the insulation fault currents are high & result in:

Transient disturbance.

High risk of damage.

Even fire.

If medium-sensitivity residual current devices are installed, the protection

against fire is improved & greater flexibility both in design and use.

• THE TNC AND TNCS SCHEMES:

They are not recommended for use, given the risk of fire & electromagnetic

disturbances due to:

Voltage drop along the PEN conductors.

High insulation fault current.

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4th criterion :

In terms of over voltage & electromagnetic disturbances (IT, TTANDTNS)

schemes are equally satisfactory.

5th criterion :

For an economic comparison all costs must be taken into account:

• Including:

Design.

Maintenance.

Modification or extension.

Product losses.

8. SUMMARY:

9. THE TYPES OF EARTHING ELECTRODES:

1-hemisphere earthing electrode.

2-rod earthing electrode.

9.1 For hemisphere earthing electrode:

π

ρ

2=

∞R

9.2 For rod earthing electrode:

)1)8

((ln2

−=

d

L

LR

π

ρ

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Where:

R: Resistance of the electrode in ohms

ρ: Soil resistivity in ohm meter

L: Length of the rod in meters

d: Diameter of the rod in meters

Earthing system calculations in our hotel:

As our hotel locates on the red sea especially in Sharm el sheikh and

according to the previous factors that affecting the soil resistivity and as the

salt concentration is very high in our soil

So from the above tables we find that ρ = 100 ohm.cm.

As we will use rod earthing electrode, so we will calculate the resistance of

this rod according to the following relation:

)1)8

((ln2

−=

d

L

LR

π

ρ

Where

ρ = 100 ohm.cm = 1 ohm.m

L=3m

d= 19mm = 0.019 m

After we substitute with the above values in the relation we can get (R)

R=0.325 ohm

As we find the value of resistance of one rod is very small which is very

good value so, we will use one rod with the above dimensions to earthing

our building and no need for more rods