9/13/2014 - Electrical and Computer Engineering 1 CHAPTER -Four Grounding and Lightning arrestors...

9/13/2014

1

CHAPTER CHAPTER -- FourFour

Grounding and Lightning arrestors

Addis Ababa UniversityAddis Ababa Institute of TechnologySchool 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

The potential of an installation is measured with respect to the

general mass of earth or commonly called Earth

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)

ECEG 4401 | Electrical Installation | By Amare A. 2013/14

9/13/2014

2


Earthing Conductors:

A protecting conductor connecting the main earthing terminal of

an installation to an earth electrode or other means of earthing.



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.

Electrical Connection maintaining

various exposed conductive parts

and extraneous conductive parts as

substantially the same bonding

Equipotential Bonding


9/13/2014

3


Bonding Conductor:

A protective conductor providing equipotential bonding

There are different earthing arrangements:

IT system

TT system

TN system

TN-C-S



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


9/13/2014

4


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



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


9/13/2014

5


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



TN – s Earthing system

The Neutral and protective conductors are separate

It is advised to regularly to connect the protective conductor

to earth


9/13/2014

6


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



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

installedECEG 4401 | Electrical Installation | By Amare A. 2013/14

9/13/2014

7


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



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


9/13/2014

8


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

l length in m

A Area in mm2A

lR

Soil resistivity values widely depending up on the type of

terrain

Eg: silty on a river bank 1.5 Ω –m

Dry sand or granite 10,000 Ω –m



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


9/13/2014

9


Type of soil or water Typical

resistivity

( Ω –m)

Usual limit

(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




18

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

dR

I

Vd

2

2

I

V

d d d

Where d spacing b/n in m


9/13/2014

10



19

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

axwhere

x

Rd

a

Rd

2,

22

2

I

V

d a a d




20

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

)8

ln(

2

D

b

bR

I

V

0.62d

db

Where: b The driven electrode length

R=V/ID diameter of

driven rodECEG 4401 | Electrical Installation | By Amare A. 2013/14

9/13/2014

11



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





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


9/13/2014

12




Indirect contact

1st Level protection:

Earthing all exposed conductive – parts electrical equipments

[ Equipotential bonding ]

2nd Level protection:

Automatic disconnection of the supply





Indirect contact

1st Level protection:

Earthing all exposed conductive – parts electrical equipments

[ Equipotential bonding ]

2nd Level protection:

Automatic disconnection of the supply


9/13/2014

13



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






9/13/2014

14






Thank you!ECEG 4401 | Electrical Installation | By Amare A. 2013/14

Date post:	13-Mar-2018
Category:	Documents
Upload:	ledan
View:	261 times
Download:	6 times

9/13/2014 - Electrical and Computer Engineering 1 CHAPTER -Four Grounding and Lightning arrestors...

Documents