MODULE EL 105:

UNIT: 105.7 CARRY OUT EARTHING SYSTEM

Learning and instructional objectives:-
 Define earth/earthing
 State the purpose or aims of earthing
 Outline forms of earthing system
 State the basic terms relating to earthing
 Describe various types of earth electrodes
 Describe methods of earthing

EARTHING SYSTEMS/ PERFOM EARTHING SYSTEM

An Overview: Earthing
In electrical engineering, ground (Used in North American and USA) or earth
(used in European) is the reference point in an electrical circuit from which
voltages are measured, a common return path for electric current, or a direct
physical connection to the Earth.

Definition: What is earthing or grounding?

Earthing/grounding may be defined in any of the following:-
1. Grounding is the act of connecting the exposed conductive parts of an
installation to the general mass of earth.
2. Earthing is a connection system between the metallic parts of an electrical
wiring system and the general mass of the earth.
1. In electricity supply systems, an earthing or grounding is circuitry which
connects parts of the electric circuit with the ground
2. A general term used to describe the connection of metallic parts of an
Electrical Installation or an appliance to earth or ground is called Earthing or
Earthed.
3. Earthing is a connection of the exposed conductive parts of an installation
and circuit to the main earthing terminal of that installation or circuit.
 4. Earthing means connecting any non-current carrying conductor part of an
electrical system with general mass of earth in such a manner that there is
an immediate discharge of electrical energy to the earth in the event of
electrical potential developed at that part of the system.

PURPOSE OF EARTHING/OBJECTIVES OF THE EARTHING

1. To save human life (prevent or minimize the risk of shock) due to leakage
current or earth faults by providing an alternative path for the fault current
to flow so that it will not endanger the user . In doing so , this will prevent a
potential difference happening between earth and earthed parts, as well as
letting the flow of fault current to operate the protective systems (earth
leakage protective devices)
2. To keep voltage as constant in the healthy phase (If fault occurs on any one
phase).
3. To Protect Electric system and buildings form lighting.
4. To serve as a return conductor in electric traction system and
communication.
5. To avoid the risk of fire in electrical installation systems.

CLASSIFICATION OF EARTHING

The earthing is broadly divided as

1. System earthing
earthing (Connection between part of plant in an operating system like LV
neutral of a power transformer winding) and earth or Earthing of the
neutral of alternators and transformer
2. Equipment earthing
Equipment earthing (safety grounding) connecting bodies of equipment
(like electric motor body, transformer tank, switchgear box, operating rods
of air break switches, LV breaker body, HV breaker body, feeder breaker
 bodies etc) to earth or Equipment earthing is undertaken to protect human
beings/consumers. If a live source comes into contact with the equipment
body, electrical energy will flow to the earth, without flowing through the
human being/ consumer’s body. This is because of the fact that the human
body has a greater resistance compared
with the resistance to earth. Also refer to a connecting the outer casing or
supporting structures of all live electric equipment to general mass.
PARTS THAT ARE REQUIRED TO BE EARTHED
i. All metallic structures in the wiring system (non current carrying) such as metallic
covers, conduits, ducts, the armour of catenary wires, etc.;
ii. A secondary winding point in a transformer; and
iii. Frame of metal roof truss.

PARTS THAT ARE NOT REQUIRED TO BE EARTHED

i. Short, isolated metallic parts for mechanical protection of cables which have non
metallic sheaths other than conduits which are connected at entry points between
the building and conduits which protect discharge lamp cables;
ii. Cable clips for installing cables;
iii. Metallic covers for lamps;
iv. Small metallic parts such as screws and name plates which are isolated by
means of insulation;
v. Metallic lamp parts for filament lamps in water proof floors.

TYPES OF EAR THING METHOD AND ARRANGEMENT/ TYPES OF EARTHING

ARRANGEMENT
In the regulations for electrical installations, the types of earthing systems are
identified as follows, depending on the relationship of the source (supply authority
network) and of the exposed conductive parts of the installation, to earth.
We must know the suitable earthing types to ensure our system is fully protected
and safety. The five basic type earthing method and system are:-
(1) Unearthed System:
 IT System.
(2) Earthed System:
 TT
 TN (TN-S, TN-C, TN-C-S).
Or
(1). TN – S
(2). TN –C – S
(3). TT
(4). TN –C
(5). IT
Or
(1) TT - Supply authority earth and the installation earth is independent
(2) TN - Earthing of the installation is done to that supplied by the supply
authority
(3) IT - Supply authority has effectively an isolated neutral and the installation
has an independent earth
The first letter defines the neutral point in relation to earth:
1. T = directly earthed neutral (from the French word Terre)
2. I = unearthed or high impedance-earthed neutral (e.g. 2,000 Ω)The second
letter defines the exposed conductive parts of the electrical installation in
relation to earth:
The second letter defines the neutral point in relation to earth:
1. T = directly earthed exposed conductive parts
2. N = exposed conductive parts directly connected to the neutral conductor
UNEARTHED SYSTEM
1. IT system unearthed (high impedance earthed neutral)
First Letter I = the neutral is unearthed at Transformer or Generator side.
Second Letter T = Frame parts of the loads are interconnected and earthed at Load
Side. This system is similar with TT system but totally different in earthing supply.
The distributor systems not have any connection to earth or it has only a high
impedance connection. It means the usual protection is not effective for this
system.
This type is not for consumer power supply.It special for power distributor such as
substation or generator area.
System characteristics
1. It is compulsory to install an over voltage limiter between the MV/LV
transformer neutral point and earth.
2. If the neutral is not accessible, the overvoltage limiter is installed between a
phase and earth.
3. It runs off external over voltages, transmitted by the transformer, to the
earth and protects the low voltage network from a voltage increase due to
flashover between the transformer’s medium voltage and low voltage
windings.
Advantages
1. System providing the best service continuity during use.
2. When an insulation fault occurs, the short-circuit current is very low.
3. Higher operational safety only a capacitive current flows, which is caused by
the system leakage capacitance if an earth fault occurs.
4. Better accident prevention the fault current is limited by the body
impedance, earthing resistance and the high impedance of the earth fault
loop.
Disadvantages
1. Requires presence of maintenance personnel to monitor and locate the first
fault during use.
2. Requires a good level of network insulation (High leakage current must be
supplied by insulating transformers).
3. Overvoltage limiters must be installed.
4. Requires all the installation’s exposed conductive parts to be Same Voltage
level. If this is not possible RCDs must be installed.
5. Locating faults is difficult in widespread networks.
6. When an insulation fault with reference to the earth occurs, the voltage of
the two healthy phases in relation to the earth take on the value of the
 phase-to-phase voltage So when Select Size of equipments it is need to
higher insulation level of the Equipments.
7. The risk of high internal over voltages making it advisable to reinforce the
equipment insulation.
8. The compulsory insulation monitoring, with visual and audible indication of
the first fault if tripping is not triggered until the second fault occurs.
9. Protection against direct and indirect contact is not guaranteed.
10. 10. Short-circuit and earth fault currents may cause fires and destroy parts
of the plant.
EARTHED SYSTEM
(1) TT SYSTEM DIRECTLY EARTHED NEUTRAL
 First letter T = the neutral is directly earthed.
 Second letter T = the exposed conductive parts of the loads are
interconnected and earthed.
 The transformer neutral is earthed.
 The frames of the electrical loads are also connected to an earth
connection.
System characteristics

1. High earth fault loop impedance

2. Low earth fault current
3. Utility company need not to provide earth for consumer

Advantages

1. Save earth wires

 2. The big advantage of the TT earthing system is the fact that it is clear of high
and low frequency noises that come through the neutral wire from various
electrical equipment connected to it.
3. TT has always been preferable for special applications like
telecommunication sites that benefit from the interference-free earthing
4. Does not have the risk of a broken neutral.
5. The simplest system to design, implement, monitor and use.
6. Easily find location of faults.
7. Upon occurrence of an insulation fault, the short-circuit current is small.
8. Reduces the risk of over voltages occurring.
9. Authorizes the use of equipment with a normal phase to earth insulating
level.

Disadvantages

1. High demand of E/F relays.

2. Individual earth system needs higher investment.
3. Higher touch voltage.
4. Induce Potential gradient.
5. Switching upon occurrence of the first insulation fault.
6. Use of an RCD on each outgoing feeder to obtain total selectivity.
7. Special measures must be taken for the loads or parts of the installation
causing high leakage currents during normal operation in order to avoid
spurious tripping (feed the loads by insulating transformers or use high
threshold RCDs, compatible with the exposed conductive part earth
resistance).
8. Very high fault currents leading to maximum damage and disturbance in
telecommunication networks.
9. The risk for personnel is high while the fault lasts; the touch voltages which
develop being high.
 10. Requires the use of differential protection devices so that the fault
clearance time is not long. These systems are costly.
(2) TN SYSTEM: NEUTRAL – CONNECTED
ECTED EXPOSED CONDUCTIVE PART
 First letter T = the neutral is directly earthed at Transformer
Transformer.
 Second letter N = the exposed
exposed conductive part or the frame of electrical
loads are connected to the neutral conductor.
conductor

There are two types of TN system, depending on whether the neutral

conductor and earth conductor are combined or not
(2a) TN – C EARTHING METHOD/SYSTEM
 In TNC system ( The third letter C = combined neutral and earth conductor),
the neutral and earth conductors are combined in single condu
conductor and
earthed at source end
 A TN – C system is one where the neutral (N) and protective earth functions
are combined in a single (i.e. a PEN or protective earth and Neutral
conductor) throughout the system and this combined neutral and earth.

System characteristics
1. Low earth fault loop impedance
2. High earth fault current
3. More than one earth fault loops
Advantages
1. No earth wire required; allow of multi – point earth
2. Better earthing continuity.
continuity
 3. Neutral never have float voltage
4. TNC system may be less costly upon installation ( elimination of one switch
gear pole and one conductor)
Disadvantages
1. If not multi point earthed, and the neutral earth broken, the exposed
metallic part may have float voltage
2. High earth fault level.
3. Intervene the operation of earth fault protective device.
4. Current operated type device is not appropriated, voltage operated
detected type could be employed
5. The fire risk is higher
(2b) TN – S -EARTHING METHOD/SYSTEM
 In TN - S system (The third letter S = separate Neutral and Earth Conductor),
the neutral of the source of energy is connected with earth at one point
only, general near to the source. The neutral and Earth conductors are
separately distributed to load.
System characteristics
1. Low earth fault loop impedance
2. High earth fault current
Advantages
1. Use of over current protective device to ensure protection against indirect
contact
2. Earth fault protection device operates faster
3. Allow mult point earth, better earthing continuity ; minimize the use of
earth fault relay because of low earth fault loop impedance
Disadvantages
1. If not multi point earthed, and the neutral earth broken, the exposed
metallic part may have float voltage
2. High earth fault level.
3. Intervene the operation of earth fault protective device.
4. Current operated type device is not appropriated, voltage operated
detected type could be employed
5. The fire risk is higher

(2c) TN – C – S(PROTECTIVE MULTIPLE EARTHING -PME) EARTHING

METHOD/SYSTEM

 The Neutral and Earth wires are combined within the supply authority cable.
 This combined earth and neutral system called the ‘protective and neutral
conductor’ (PEN) or the ‘combined neutral and earth’ conductor (CNE).
 Typical this will be a concentric cable, with the live as the central core, and a
ring of wire around this for the combined neutral and earth
 The neutral and earth wires are separated, with earth terminal usually being
on the side of cutout. Inside the cutout, the Neutral and earth are linked
 Throughout the supply network or supply distribution, the combined
earth/neutral conductor is connected to ground in multiple places, either
buried underground or at the pole for overhead supplies
 This multiple earthing is why a TN – C- S method is often called PME (
protective multiple earthing)
 The power supply distributor will provide a consumer’s earthing terminal
which is linked to the incoming neutral conductor.
Advantages

1. Cost for core cable is cheaper than a 3 core

2. As the outer sheath is usually plastic, there are no problems with corrosion
Disadvantages
1. When the combined earth/neutral conductor is broken, this will results in
a voltage appearing on the exposed metalwork in the consumer`s
property which can be a shock risk
2. In case of fault, the current flowing in the consumer`s earthing
conductors can be much greater than that for a TNS system
3. It is also possible to get unsual circulating earth currents between
properties particularly where some properties have metal water pipes
and others have plastic.

BASIC TERMS RELATED TO EARTHING SYSTEMS

i. Circuit protective conductor(CPC)/Earth continuity conductor(ECC)

A conductor connecting exposed conductive parts of equipments to the
main earthing terminal or is conductor including any clamp, wire, cables,
conduit pipes, cable`s metallic covering, earth lead or earth terminal of
an installation which is connected to the earth electrode. In other words,
the wire between earthing lead and electrical device or appliance is
called earth continuity conductor. Cpc includes:- Metal conduit
 a. Metal ducting and trunking
b. MIMS cable sheathed
c. Lead covered and wire armoring of cable
d. Neutral conductor of PME
ii. Earth lead/ Earth wire or earth conductor
The earthing lead is the final conductor by which the connection to the
earth electrode or other means of earthing is made. Conductor which
connects the main earth terminal to the earth electrode or a protective
conductor connecting a main earthing terminal of an installation to an
earth electrode or other means of earthing.
iii. Earthed object
Anybody connected to the earth point or earth electrode
iv. Main earthing terminal
The terminal, usually placed at the consumer`s intake position for
connection of consumers cpc`s , bonding conductor and earth lead
v. Direct contact
A contact of a person with the live part
vi. Indirect contact
Refer to a contact of person in with exposed conductive parts made live
by faults
vii. Exposed conductive parts
Conductive parts of equipments which can be touched and which is not
live part but which may become live under fault condition
viii. Extraneous metal work/ conductive part
Exposed metal work which needs to be earthed and which in not parts
of the electric equipments for examples metal sink, baths and cisterns.
They are connected to cpc by conductor known as bonding
ix. Earth leakage current
 Electric current in on unwanted conductive parts under normal
operating conditions. In any electrical installation, some current will flow
through the protective ground conductor to ground.
x. Earth fault current
Abnormal passage of current to general mass of earth

EARTH ELECTRODE
An earth electrode is part of the system that directly in contact with the earth. Or
is a conductor which in contact with general mass of earth.
Type of earth electrode
The following are the common type of earth electrodes used to make contact with
the general mass of the earth:-

i. Rode electrode
ii. Plate electrode
iii. Strip electrode
iv. Earth mats
v. Pipe electrode
Method of installing earth electrode
There are two types of grounding systems;
1. Simple grounding
Simple consist of a single electrode driven into the ground. The use of single
ground electrode is the most common form of grounding and can found in
almost all residential, hotel and min industry and factory.
2. Complex Grounding
Complex grounding system consist of multiple ground earth electrodes
connected, mesh or grid networks. These systems are typically installed at
EARTHING CIRCUIT/COMPONENTS
The earthing circuit and system is made up of three basic parts

a. Circuit protective conductor

 b. Earth conductor
c. Earth electrode
QUALITIES OF GOOD EARTHING
 Must be of low electrical resistance
 Must be of good corrosion resistance
 Must be able to dissipate high fault current repeatedly
power generating substations, central offices, industry and others.

POINTS TO CONSIDER IN I.E.E REGULATIONS

1. Earth lead or wire must be protected when necessary against mechanical

damage and against corrosion
2. Size of Earth Continuity Conductor. The cross sectional area of the Earth
Continuity Conductor should not be less than the half of the cross sectional
area of the thickest wire used in the electrical wiring installation.
3. Earth lead csa must not be less than 6mm2
4. The main earththing terminal, to be of assize and types suitable for the
connection of a number of conductors to it
5. Earth lead and earth point must be marked (Safety Electrical connection, Do
Not Remove)
6. An earthing electrode should not be situated (installed) close to the
building whose installation system is being earthed at least more than
1.5m away.
7. The earth resistance should be low enough to cause the flow of
current sufficient to operate the protective relays or blow fuses. It’s
value is not constant as it varies with weather because it depends on
moisture (but should not be less than 1 Ohm).
8. The earth wire and earth electrode will be the same material.
9. The earthing electrode should always be placed in a vertical position
inside the earth or pit so that it may be in contact with all the different
earth layers.