9/13/2014
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
CHAPTER - Four
Grounding and Lightning arrestors
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
4.1. Introduction
 Electrical connection of neutral point of a supply system or the non
current carrying part of electrical equipments to the general mass
of earth in such a manner that all times on immediate discharge of
electrical energy
 Why is the practice of earthing electrical system?
 To provide a sufficiently low impedance
 To retain system voltages with in reasonable limits under fault
condition ( Lightning\, switching surges, or contact with higher
voltages system)
 The potential of an installation is measured with respect to the
general mass of earth or commonly called Earth
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Earthing Conductors:
 A protecting conductor connecting the main earthing terminal of
an installation to an earth electrode or other means of earthing.
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Bonding:
 Is the practice of intentionally
electrically connecting all exposed
metallic parts not designed to carry
electricity in a room or building to
protect from electric shock.
Equipotential Bonding
 Electrical Connection maintaining
various exposed conductive parts
and extraneous conductive parts as
substantially the same bonding
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Bonding Conductor:
 A protective conductor providing equipotential bonding
 There are different earthing arrangements:

IT system

TT system

TN system

TN-C-S
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
 The first letter defines the situation of the neutral point in
relation to earth
T: Solidily earthed to neutral
I : unearthed or high impendence earth to neutral
 The Second letter defines the connection method of electrical
installation exposed conductive path
T: The exposed conductive parts are interconnected and
solidly earthed, regardless of whether the neutral part is
earthed or not
N : The exposed conductive parts are directly connected to
neutral conductor
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
 The third and Fourth letter indicates the arrangement of
earthed supply conductor system
C: Combined
N : Separate
IT system earthing
 The neutral is earthed or unearthed via high impendence
 An impendence b/n 1 kohm and 2 kohm are frequently used
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
TT Earthing system
 The neutral is directly earthed,
 The exposed conductive parts of the loads are
interconnected , either together or in groups or individually
earthed
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
TN Earthing system
 The neutral point is directly earthed
 The exposed conductive parts of the loads are connected to
the neutral conductor
 There are two possible
 There are two possible system depending on whether the
neutral conductor (N) and protective conductor (PE) are on
the same or not
TN – C Earthing system
 The Neutral and protective conductors form a single
conductor called PEN
 It is advised to connect PEN to ground
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
TN – s Earthing system
 The Neutral and protective conductors are separate
 It is advised to regularly to connect the protective conductor
to earth
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
TN – C –S Earthing system
 With this arrangement the main distributor neutral is
also used to return earth fault current arising in
consumer installation safety to the source. To achieve
this the distribution will provide consumer earthing
terminal which is linked to the coming neutral
conductor
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Grounding Design consideration
System Earthing
 The system earthing, that is to say the connection between
the transformer neutral points and earth, is of high
importance to the behavior of a power system during an
unsymmetrical fault
Equipment Earthing
 Earthing all metal work of electrical equipments earthing
other than which are normally live or current carrying.
Soil resistivity
 The resistance to earth of an electrode of given dimension
dependent on the electrical resistivity of the soil in which it is
installed
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Element of Electrode
Electrode
Driven to earth wider earth contact area usually 1 to 2 m
length and 16 mm diameter will give relatively low
resistance
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Plate Electrode
 They offer a better grounding system than electrodes
are generally used in permanent communication and
computer sites
Iron or steel type: Not less than 6.3mm thickness
( B/s Iron rusts)
Copper type: Not less than 3.15mm thickness
 Plate electrodes shall be
the size of at least 60cm X
60cm
 Plate electrodes shall be
buried at a depth of not
less than 1.5m
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Soil resistivity testing
 The resistance of an electrode highly influenced by a
resistivity of the soil
 Soil resistivity is the measure of a resistance b/n opposite
sides of a cube of soil with a side dimension of 1 meter
Where : ρ resistivity in Ω –m
R
l
l length in m
A
A Area in mm2
 Soil resistivity values widely depending up on the type of
terrain
Eg: silty on a river bank
Dry sand or granite
1.5 Ω –m
10,000 Ω –m
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Factors affecting resistivity
 Type of earth ( Clay, loam, sand stone, granite)
 Stratification : layer of different soil
 Moisture content ( M  ρ  )
 Temperature
 Chemical composition and concentration of dissolved
salt
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Type of soil or water
Typical
resistivity
Usual limit
(Ω –m)
( Ω –m)
Sea water
2
0.1 to 10
Clay
40
8 to 70
Ground well and spring water
50
10 to 150
Clay and sand mixture
100
40 to 300
Sand
2000
200 to 3000
Solid granite
25,000
10,000 to 50, 000
Ice
100,000
10,000 to 100,000
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Soil resistivity testing Methods
Wenner Method
 It requires the longest cable lay out , largest electrode
spreads and for a large spacing one person per
electrode is necessary to complete the measurement
I
d
d
V
I
  2dR
  2d
V
d
Where d spacing
b/n in m
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Schumbefer array
 The outer electrode are moved four or Five times of
each move of the inner electrodes
 Lowe voltage reading are obtained during this method
I

V
a
d
2a
d 2 R

x
where, x  2a
d
a
d 2 R
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Driven rod Method
 The driven rod method called three pin or Fall – 0f –
potential method
 It is normally suitable for use in circumstances such as
transmission line structure earths or areas of difficult
terrain
I
V
0.62d
b
d
ECEG 4401 | Electrical Installation | By Amare A. 2013/14

2bR
8b
ln( )
D
Where: b The driven
electrode length
R=V/I
D diameter of
driven rod 20
10
9/13/2014
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against electric shock
Direct contact
 Refers to a person coming in
contact with a conductor which is
live in normal circumstance
Indirect contact
 Refers to a person coming in
contact
with
an
exposed
conductive parts of which in not
normally alive bus has become
alive accidentally
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against electric shock
Direct contact
 By means of barrier enclosure ( Key,
 By mean of obstacles (
 By replacing out of arm’s reach
 By using Residual current device ( RCD) – fast sensitive
tripping device
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against electric shock
Indirect contact
 1st Level protection:
Earthing all exposed conductive – parts electrical equipments
[ Equipotential bonding ]
 2nd Level protection:
Automatic disconnection of the supply
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against electric shock
Indirect contact
 1st Level protection:
Earthing all exposed conductive – parts electrical equipments
[ Equipotential bonding ]
 2nd Level protection:
Automatic disconnection of the supply
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against lightning
 Lightning can be linked to distrubtive electrical discharge due
to the dielectric breakdown of the air between the clouds or
between clouds and the ground.
 Breakdown, which is visible in the form of lightning flash, it
accompanied by a sound wave, thunder
Types of lightning
within cloud
cloud – to – cloud
cloud - to - ground
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against lightning
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
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Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Protection against lightning
ECEG 4401 | Electrical Installation | By Amare A. 2013/14
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical and Computer Engineering
Thank you!
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