Module No E02 Earthing and Lightning Protection

8/10/2019 Module No E02 Earthing and Lightning Protection

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Capabil ity & Improvement Dept.2004For Internal Use Only

PETRO NA S

G A S

TRAINING MODULE

ELECTRICAL

TITLE : EARTHING AND LIGHTNING PROTECTIONMODULE NO : E02


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Earthing and Lightning Protection System-Module No. E 02 Page 2 of 42

EARTHING & LIGHTNING PROTECTION SYSTEM

OBJECTIVES

Upon completion of this module, the technician would be able to demonstrate knowledge

and understanding on the following:

1. Electrical Fundamentals on earthing and lightning protection system

2 General principles on earthing and lightning protection system installation

3 Importance of earthing and lightning protection system in an electrical installation

4 Different types of earthing arrangement

5 Basic materials used in earthing system and lightning protection system installation

6 Requirements in earthing system installation

7 Maintenance and inspection for earthing and lightning protection system

8 Periodic maintenance and inspection on the installation

9 Different tests and testing instruments for earthing and lightning protection system

10 Earth resistance measurement

11 How to improve earthing system

12 Different means of protection against lightning surges

13 Principle of protection of Surge Arrester

14. Earthing and lightning protection system installed in the plant.


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TABLE OF CONTENTS

Section Topic Page

1.0.0DEFINITIONS OF TERMS ..........................................................................................6

2.0.0EARTHING FUNDAMENTALS ...................................................................................8

2.1.0GENERAL INFORMATION ............................................................................................................8

2.2.0OBJECTIVES OF EARTHING ........................................................................................................8

2.3.0SAFE WORK ENVIRONMENT THROUGH EARTHING..................................................................8

3.0.0EARTHING SYSTEM INSTALLATION........................................................................9

3.1.0SYSTEM EARTHING (GROUNDING ) ...........................................................................................9

3.2.0EQUIPMENT EARTHING (GROUNDING).....................................................................................9

3.2.1CONNECTION OF ELECTRICAL EQUIPMENT TO EARTHING SYSTEM .......................................................... ..... 11

4.0.0IMPORTANCE OF EARTHING IN AN INSTALLATION ..............................................12

4.1.0PROTECTION BENEFITS FROM EARTHING/GROUNDING ........................................................12

5.0.0BASIC MATERIALS USED FOR EARTHING SYSTEM...............................................13

5.1.0EARTH ELECTRODES................................................................................................................13

5.2.0EARTHING CONDUCTORS.........................................................................................................14

5.2.1SIZE OF EARTHING CONDUCTOR ............................................................. .......................................................... 14

5.3.0 MAIN EARTHING TERMINALS OR EARTHING BARS ...............................................................14

5.4.0PROTECTIVE CONDUCTORS .....................................................................................................15

6.0.0EARTHING ARRANGEMENT AND PROTECTIVE CONDUCTORS.............................16

6.1.0EARTHING ARRANGEMENTS ....................................................................................................17

6.1.1TN-CSYSTEM........................................................... ............................................................... ........................... 17

6.1.3TN-C-SSYSTEM ....................................................... ............................................................... ........................... 18

6.1.4TTSYSTEM ............................................................... ............................................................... ........................... 19

6.1.5 ITSYSTEM ............................................................... ............................................................... ........................... 19


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7.0.0GENERAL REQUIREMENTS FOR EARTHING INSTALLATION ................................21

7.2.0INSTALLATION OF EARTHING SYSTEM PATH..........................................................................21

7.2.1SOLID GROUNDING ............................................................ ................................................................ ................ 21

7.2.2LOW RESISTANCE GROUNDING (NEUTRAL GROUNDING RESISTORS OR REACTORS) .................................... 22

8.0.0MEASUREMENTS,TESTS AND TEST INSTRUMENTS..............................................25

8.1.0IMPORTANCE OF CONDUCTING TEST ON EARTHING/GROUNDING SYSTEMS ........................25

8.2.0TEST METHOD FOR EARTHING SYSTEM COMPLIANCE TESTING...........................................25

8.2.1FALL OF POTENTIAL METHOD ................................................................ .......................................................... 25

8.2.2 DIRECT MEASUREMENT BY THE USE OF CLAMP-ON-METER ......................................................... ................ 26

8.3.0TESTING ON EARTHING INSTALLATION ..................................................................................26

8.3.1EARTH LOOP IMPEDANCE TESTING ....................................................... .......................................................... 26

8.3.2 EARTHING ELECTRODE RESISTANCE TESTING .......................................................... ..................................... 28

8.3.3TYPICAL EARTH ELECTRODE TESTING INSTRUMENTS.......................................................... .......................... 29

8.3.4MEASUREMENT OF EARTH RESISTANCE-PROCEDURE .......................................................... .......................... 30

9.0.0EARTHING SYSTEM IMPROVEMENT .....................................................................31

10.0.0MAINTENANCE AND INSPECTION OF EARTHING SYSTEM INSTALLATION ........32

10.1.0POSSIBLE CAUSES OF ACCIDENTAL EARTHING ....................................................................32

10.2.0CONTINUITY OF THE EARTHING CIRCUIT .............................................................................32

10.3.0PERIODIC CHECKS ON THE EARTHING SYSTEM...................................................................32

10.3.1 INSPECTION OF EARTHING SYSTEM INSTALLATION............................................................ .......................... 33

10.3.2TYPICAL CHECKLIST FOR INSPECTION OF EARTHING SYSTEM ................................................................ ..... 33

11.0.0LIGHTNING PROTECTION SYSTEM .....................................................................34

11.1.0EFFECTS OF LIGHTNING STROKES ........................................................................................34

11.2.0HARMFUL EFFECTS OF VOLTAGE SURGES............................................................................34

11.3.0 MEANS OF PROTECTION AGAINST LIGHTNING SURGES.......................................................35

11.4.0TYPICAL DEVICES/MATERIALS FOR LIGHTNING PROTECTION ...........................................35


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12.0.0LIGHTNING PROTECTION BY SURGE ARRESTER ...............................................36

12.1.0TYPES OF SURGE ARRESTERS ................................................................................................36

12.2.0PRINCIPLE OF SURGE ARRESTER PROTECTION ....................................................................36

12.2.1GAPPED ARRESTERS (GAPPED SILICON CARBIDE ARRESTERS) ............................................................... ..... 36

12.2.2GAP LESS ARRESTERS (METAL OXIDE VARISTORS SURGE ARRESTERS) ...................................................... 37

12.3.0TYPICAL INSTALLATION AND TYPE OF SURGE ARRESTER ...................................................38

12.4.0TESTS ON LIGHTNING ARRESTERS. .......................................................................................39

13.0.0MAINTENANCE AND INSPECTION OF SURGE ARRESTERS ..................................40

13.1.0PERIODIC INSPECTION ON THE SURGE ARRESTER................................................................40

14.0.0SURGE COUNTERS...............................................................................................41

14.1.0INSTALLATION OF SURGE COUNTER .....................................................................................41

14.2.0MAINTENANCE AND INSPECTION OF SURGE COUNTER ........................................................42


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1.0.0DEFINITIONS OF TERMS

1. Earthing- the act of connecting exposed conductive parts of an installation to the main earthing

terminal of an installation.

2. Earthing conductor- a protective conductor connecting the main earthing terminal of an installation

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

3. Earth electrode- a conductor or group of conductors in intimate contact with, and providing an

electrical connection to, Earth.

4. Earth electrode resistance- the resistance of an earth electrode to Earth.

5. Exposed conductive part- a conductive part or metalwork of electrical equipment which can be

touched and which is not a live part but may become live under fault conditions.

6. Main earthing terminal- the terminal or bar provided for the connection of protective conductors,

including equipotential bonding conductors, and conductors for functional earthing if any, to the

means of earthing

7. Circuit protective conductor- a conductor connecting exposed conductive parts to the main

earthing terminal.

8. Equipotential bonding- an electrical connection between exposed conductive parts and extraneous

conductive parts which puts them at approximately the same potential.

9. Extraneous conductive part- metalwork that is not part of the electrical installation, and which is

liable to introduce a potential, generally earth potential

10. Bonding conductor- a conductor used for equipotential bonding

11. Live part - a phase or neutral conductor, or part intended to be energized in normal use.

12. Protective conductor- a conductor used as means of protection against electric shock by connecting

extraneous conductive parts together, or to exposed

13. Electrical equipment- any item for such purposes as generation, conversion, transmission,

distribution or utilization or electrical energy, such as transformers, apparatus, measuring instruments,

protective devices, wiring materials, accessories and luminaires.

14. System- an electrical system consisting of a single source of electrical energy and an installation.

15. TN system- a system having one or more points of the source of energy directly earthed, the

exposed-conductive parts of the installations being connected to that point by protective conductors.

16. TN-C system- a system in which neutral and protective functions are combined in a single conductor

throughout the system.

17. TN-S system- a system having separate neutral and protective functions throughout the system.

18. TN-C-S system- a system in which neutral and protective functions are combined in a single

conductor in part of the system.


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19. TT system- a system having one point of the source of energy directly earthed, the exposed-

conductive-parts of the installation being connected to earth electrodes independent of the earth

electrodes of the source.

20. IT system- a system having no direct connection between live parts and Earth, the exposed-

conductive parts of the electrical installation being earthed.

21. System grounding (earthing)- intentional connection of neutral point to ground.

22. Equipment grounding (earthing)- refers to the connection of noncurrent carrying parts to earth

(ground).


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2.0.0EARTHING FUNDAMENTALS

2.1.0GENERAL INFORMATION

In Britain, it is termed as earth while in Northern America, it is called ground. Basically,

these two terms means exactly the same thing, only different terms are used in different countries.Earthing or grounding is connecting an electrical system to the earth with a wire. Excess or stray

current travels through this wire to an earthing device (commonly called a ground) deep in the earth.

Grounding prevents unwanted voltage on electrical components.

Sometimes an electrical system will receive a higher voltage than it is designed to handle. These

high voltages may come from a lightning strike, line surge, or contact with a higher voltage line.

Sometimes a defect occurs in a device that allows exposed metal parts to become energized. Earthing will

help protect the person working on a system, the system itself, and others using tools or operating

equipment connected to the system. The extra current produced by the excess voltage travels relatively

safely to the earth.

2.2.0OBJECTIVES OF EARTHING

The objectives of earthing system may be summarized as follows:

1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.

2. To assure correct operation of electrical/electronic devices.

3. To prevent damage to electrical/electronic apparatus.

4. To dissipate lightning strokes.

5. To stabilize voltage during transient conditions and therefore to minimize the probability of flash over

during transients.

6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.

2.3.0SAFE WORK ENVIRONMENT THROUGH EARTHING

Earthing creates a path for currents produced by unintended voltages on exposed parts. These

currents follow the grounding path, rather than passing through the body of someone who touches the

energized equipment.

Leakage current occurs when an electrical current escapes from its intended path. Leakage's are

sometimes low-current faults that can occur in an electrical equipment because of dirt, wear, damage, or

moisture. A good grounding system should be able to carry off this leakage current.

Proper grounding protects against ground faults. Ground faults are usually caused by misuse of a

tool or damage to its insulation. This damage allows a bare conductor to touch metal parts or the tool

housing. When you ground a tool or electrical system, you create a low-resistance path to the earth. This

path has sufficient current-carrying capacity to eliminate voltages that may cause a dangerous shock.


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3.0.0EARTHING SYSTEM INSTALLATION

The two most common earthing system installation that will be described are:

1. System earthing (system grounding), and

2. Equipment earthing (equipment grounding)

3.1.0SYSTEM EARTHING (GROUNDING )

System earthing is the intentional connection of neutral point to earth. If neutral point is earthed,

the phase to ground voltages under each fault condition do not rise to high value. Earth fault protection

becomes easy.

An earthed distribution system has one conductor of the system solidly connected to a common

system ground. The earthed system offers advantages of :

1) greater safety to personnel and equipment

2) reduced exposure to over voltages, and

3) easier location of ground faults

Accidental grounds in a grounded system generally cause the opening of breakers or the blowing

of fuses. However, the best protection against damage due to a ground fault may be obtained with a

ground-sensing relay that operates a tripping coil in the breaker. In the absence of a ground relay, the trip

setting of the breaker should be set as low as possible; however, the trip setting must be high enough to

prevent false tripping that may be caused by motor starting current.

3.2.0EQUIPMENT EARTHING (GROUNDING)

Equipment earthing is connecting to earth the noncurrent carrying metal parts. The noncurrent

carrying metal parts include motor body, switch gear structure, transformer core and tank, sheaths of

cables, body of portable equipment, etc.

L 1

L 2

L 3

N

E

Ex. Grounded distribution system: Grounded Neutral


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The framework of all electrical equipment, large or small, should be connected with low-

resistance metallic conductors to a common point, usually the earth. Such connections eliminate the

shock hazard to operating personnel caused by normal leakage current and normal capacitive effects, and

reduce the hazard to personnel if an insulation fault should occur within the apparatus...

The equipment grounding conductors under normal conditions carry no current. The only time

they carry current is under abnormal conditions when the electrical equipment is faulty and has become a

shock or fire hazard. Under a fault condition, the earthing conductor that is connected to the outer sheet of

the equipment must be able to provide a very low resistance path back to the source of power so that

enough current will flow causing a breaker or fuse to open the circuit and automatically disconnect the

hazard from the system. It is not the purpose of this equipment earthing system to send current through

L 1

L 2

L 3

N

E

Ex. Equipment Earthing System

N

Metallic

Enclosure


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the ground. The onlytime the current is intentionally sent into the earth is during a lightning strike or line

surge due to a nearby lightning strike.

3.2.1CONNECTION OF ELECTRICAL EQUIPMENT TO EARTHING SYSTEM

Apparatus Parts to be earthed Method of connectionPower transformer Transformer tank Connect the earthing bolt on

transformer tank to station

earthing system. Connect

the neutral to earthing

system

Support of bushing

insulators, lightning arrester,

fuse, etc.

Device flange or base plate Connect the earthing bolt on

transformer tank to station

earthing system. In the

absence of earthing bolt or in

case of connection to non

conducting structures,

connect device fastening bolt

to earth.

Earthing switch and surge

arrester

Earth terminal of each pole

on 3 phase surge arrester.

Earth terminal of earthing

switch

When the device is mounted

on a steel structure, connect

each supporting structure of

the apparatus to earthing

main via separate conductor

Cabinets of control relay

panels

Framework of switch gear

and cabinets

Weld the framework of each

separately mounted board

and cabinet to earth

conductor of earthing system

minimum at 2 points

High voltage circuit breakers Operating mechanism, frame Connect the earthing bolt on

the frame and operating

mechanism of the CB to

earthing system

Isolator Isolator base (frame)

operating mechanism base

plate

Weld the isolator base frame,

connect it to the bolt on the

operating mechanism bed

plate and station earthing

Shunt reactor Neutral, tank Connect neutral point of 3phase shunt reactor to the

station earth

Steel doors and wire guards

in chambers or cubicles door

or guard steel mount

Door or guard steel mount Weld the mount of each door

and guard and connect to

earth system


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4.0.0IMPORTANCE OF EARTHING IN AN INSTALLATION

Earthing of an installation offers the following advantages:

1. It eliminates the shock hazard to operating personnel caused by normal leakage current and normal

capacitive effects.

2. It reduces the hazard to personnel if an insulation fault should occur within the apparatus

3. It protects equipment from damage,

4. It allows fault or lightning energy to dissipate easily,

5. It is essential to the effective operation of circuits used in process control systems and in

communication

6. It allows operation of fault protection equipment,

7. It eliminates the buildup of static electricity

8. It provides a reference for the easier location of earth faults.

4.1.0PROTECTION BENEFITS FROM EARTHING/GROUNDING

Earthing or grounding protect against;

Equipment faults to ground

Distribution faults to ground

Flash overs and lightning strikes on a system

Direct lightning strike

Radiation hazard

Static build up


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5.0.0BASIC MATERIALS USED FOR EARTHING SYSTEM

Basic materials used for earthing system installation are:

1. Earth electrode

2. Earthing conductor

3. Main earthing terminals or earthing bars

4. Protective conductor

5.1.0EARTH ELECTRODES

Earth electrode is a metal plate or metal pipe or metal conductors electrically connected to earth.

Copper. Aluminum, mild steel and galvanized iron are the materials generally used for earthing

electrodes.

The following types of earth electrodes are recognized by the regulations and may be used:

1. Earth rods (5/8 in. Dia.) or pipes (3/4 in. dia.),

2. Earth tapes or wires,

3. Earth plates (1/4 in. Thick x 2 sq.ft.),

4. Underground structural metalwork embedded in foundations,

5. Welded metal reinforcement of concrete (except pre-stressed concrete) embedded in the

earth,

6. Metallic sheaths or other coverings of cables, providing;

A) they are not liable to deterioration through excessive corrosion, and

B) the cable sheath is in effective contact with earth

The design used, and the material from which earth electrodes are made, shall be able to withstand

damage due to corrosion. The earth electrode should be buried at a depth such

that the earth electrode resistance is not increased by the soil drying or freezing.

Connections to earth electrodes shall

not be made with aluminum, or copper clad

aluminum conductors. Connections to earth

electrodes shall be soundly made, so that they

are electrically and mechanically satisfactory;

they shall have a label permanently attached

in a visible position, durably marked in

legible type with the words, Safety electrical

connection do not remove. The connection

must also be suitably protected against

corrosion.

EARTH RODS

EARTH PLATES


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5.2.0EARTHING CONDUCTORS

Earthing conductor is a protective conductor connecting the main earthing terminal of an

installation to an earth electrode or to other means of earthing. Minimum cross-sectional area of a buried

earthing conductor is 25 mm2copper.

The connection of an earthing conductor to an earth electrode or other means of earthing shall be

soundly made and be electrically and mechanically satisfactory, and labeled in accordance with the

Regulations. It shall be suitably protected against corrosion.

5.2.1SIZE OF EARTHING CONDUCTOR

Plant earthing ring conductors shall have a cross-sectional area of 70 mm2.The cross-sectional

areaof branch conductors connecting equipment and structures to the plant earth ring shall be:

1. To metallic enclosures of HV electrical equipment 70 mm2

2. To metallic enclosures of LV electrical equipment having a supply cable with a conductor cross

section of 35 mm2and more 70 mm2

3. To metallic enclosures of LV electrical equipment having a supply cable cross-sectional area less

than 35 mm2 25 mm2

4. To control panels, etc.., 25 mm2

5. To non-electrical equipment exposed to lightning, e.g. Tanks, columns and tall structures

70 mm2

6. To other non-electrical equipment 25 mm2

5.3.0 MAIN EARTHING TERMINALS OR EARTHING BARS

A main earthing terminal or bar shall be provided for every installation, to connect the following

to the earthing conductor:

1. The circuit protective conductors,

2. The main equipotential bonding conductors,

3. Any functional earthing conductors, and

4. The lightning protection system bonding conductor.

The consumers main earthing terminal shall be

connected to the earthed point of the source of electrical

EARTHING BAR


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energy for a TN-S system, by the supply undertaking to the neutral of the source of electrical energy for a

TN-C-S system, and connected by an earthing conductor to an earth electrode for a TT or IT system.

The main earthing terminal shall be accessible, to enable the earthing conductor to be

disconnected for test purposes. The joint shall be mechanically strong, reliably maintain electrical

continuity, and be capable of disconnection only by means of a tool.

5.4.0PROTECTIVE CONDUCTORS

It is a conductor used for some measures of protection against electric shock and intended for

connecting together any of the following parts:

1. Exposed-conductive parts

2. Extraneous-conductive-parts

3. The main earthing terminal

4. Earth electrode (s)

5. The earthed point of the source, or an artificial neutral

A protective conductor (minimum cross-sectional area shall not be less than 2.5 mm2 copper),

may consist of one or more of the following:

1) a single core cable

2) a conductor in a cable3) an insulated or bare conductor in a common enclosure with insulated live conductors

4) a fixed bare or insulated conductor

5) a metal covering, for example, the sheath, screen or armouring of a cable

6) a metal conduit or other enclosure or electrically continuous support system for conductors

7) an extraneous-conductive-part like suitable structural metalwork


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6.0.0EARTHING ARRANGEMENT AND PROTECTIVE CONDUCTORS

M

M

M

BP

C

T

1 4

1 14

2

3

E

Legend: 1,2,3,4 = protective conductors

1 = circuit protective conductor

2 = main equipotential bonding conductor

3 = earthing conductor

4 = supplementary equipotential bonding conductors (where required)

M

B = main earthing terminal

= exposed-conductive-part

C = extraneous-conductive-part

P = main metallic water pipe

T = earth electrode (TT and IT systems)

E = other means of earthing (TN systems)


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6.1.0EARTHING ARRANGEMENTS

Before commencing an installation the type of earthing arrangement shall be determined: i.e. the

type of system which the installation and supply will comprise. Depending upon the requirements of the

installation, joint or separate earthing arrangement may be used both for protection or functional purposes.

The main earthing terminal shall be connected with Earth by one of the methods described and

identified as follows (TN-C, TN-S, TN-C-S, TT, IT system), depending upon the relationship of the

source, and of exposed-conductive-parts of the installation, to Earth.

6.1.1TN-CSYSTEM

Neutral and protective functions combined in a single conductor throughout system.

For a TN-C system, means shall be provided for the connection of PEN conductor to the main earthing

terminal.

6.1.2 TN-S system

source

of earth

source of energyL1

L2

L3

combined

protective and

neutral conductor

additional

source earth

consumers'

installations

equipment

in

installation

exposed

conductive

parts

PEN

source

of earth

source of energyL1

L2

L3

consumers'installations

equipment

in

installation

exposed

conductive

parts

N

protective

conductor (PE)


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TN-S system.

Separate neutral and protective conductors throughout the system.All exposed-conductive parts of

an installation are connected to this protective conductor via the main earthing terminal of the installation.

The protective conductor (PE) is the metallic covering of the cable supplying the installations or separate

conductor.

For a TN-S system, means shall be provided for the main earthing terminal of the installation to be

connected to the earthed point of the source of energy. Part of the connections may be formed by the

suppliers lines and equipment.

6.1.3TN-C-SSYSTEM

Neutral and protective functions combined in a single conductor in a part of the system.

The usual form of a TN-C-S is as shown, where the supply is TN-C and the arrangement in the

installation is TN-S. The supply system PEN conductor is earthed at several points and an earth electrode

may be necessary at or near a consumers installation.

All exposed-conductive-parts of an installation are connected to the PEN conductor via the main

earthing terminal and the neutral terminal, these terminals being linked together.

For a TN-C-S system means shall be provided for the main earthing terminal of the installation to

be connected by the supplier to the neutral of the source of energy.

source

of earth

source of energyL1

L2

L3combined

protective and

neutral conductor

additional

source earth

consumers'

installations

equipment

in

installation

exposed

conductive

parts

PEN


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6.1.4TTSYSTEM

All exposed-conductive parts of an installation are connected to an earth electrode which is

electrically independent of the source earth.

For a TT system, the main earthing terminal shall be connected via an earthing conductor to an

earth electrode.

6.1.5 ITSYSTEM

source

of earth

source of energyL1

L2

L3

consumers'

installations

equipment

in

installation

exposed

conductive

parts

N

installation installation

earth earth

electrode electrode

source

of earth

source of energyL1

L2

L3

consumers'

installations

equipment

in

installation

exposed

conductive

parts

installation installation

earth earth

electrode electrode

earthing

impedance


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IT system.

All exposed-conductive-parts of an installation are connected to an earth electrode.

The source is either connected to Earth through a deliberately introduced earthing impedance or is

isolated from Earth.

For a IT system, the main earthing terminal shall be connected via an earthing conductor to an

earth electrode.


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7.0.0GENERAL REQUIREMENTS FOR EARTHING INSTALLATION

The earthing of the installation shall be:

1. Continuously effective,

2. Able to carry earth fault currents and earth leakage currents without danger, particularly from

thermal, thermomechanical, and electromechanical stresses,

3. Adequately robust, or have additional suitable mechanical protection,

4. Arranged so that the risk of damage to other metallic parts through electrolysis is avoided.

7.1.0 EARTHING INSTALLATION PRACTICE IN INDUSTRIAL DISTRIBUTION AND

UTILIZATION SYSTEM

Nominal Voltage Grounding Practice and Reason

1,000 V and below (phase to phase) Solid Grounding

- Low earth fault current

- No sustained earth fault

- Easy fault detection

- Easy protection

- Higher safety

1,000 V to 15,000 V (phase to phase) Low Resistance Grounding or Reactance

Grounding

- To limit fault currents

- To prevent damage to machines from over-

voltage

Above 15,000 V (phase to phase) Solid Grounding

- Fault currents limited by fault resistance

- No rotating machines connected at this voltage

7.2.0INSTALLATION OF EARTHING SYSTEM PATH

7.2.1SOLID GROUNDING

The earthing system of power plants and substations is usually formed by several vertical ground

rods connected to each other and to all equipment frames, neutrals and structures that are to be earthed.

Such a system that combines a horizontal grid and a number of vertical ground rods penetrating lower soil

layers has several advantages in comparison to a grid alone. Sufficiently long ground rods stabilize the

performance of such a combined system. Rods are more efficient in dissipating fault currents because the

upper soil layer usually has higher resistivity than the lower layers. Therefore, the touch and step voltages

are reduced significantly compared to that of the grid alone.


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The earthing system usually consists of an earthing grip of copper wires or strips. In order to

avoid the interference with installations for corrosion prevention, the grid shall consists of insulated wires

or strips instead of bare conductors, whilst earthing electrodes should be of suitable material.

The earthing path may include solidly connected metallic conduit, metallic piping, one-piece

copper conductor of appropriate size, the steel structure of a building, the hull of a ship, etc..

The connection to earth may be accomplished by connection to a metallic underground water-

supply system or to driven earth electrodes using unpainted galvanized pipes, rods, or metal plates. The

pipes (3/4 inch in diameter) or rods (5/8 inch in diameter) should be driven to a depth of at least 8 feet.

Each plate electrode should be at least inch thick and have a cross-sectional area of at least 2 sq. ft.

Where over one electrode is used, spacing between them should be no less than 6 feet.

Earthing electrodes are spotted throughout the grid to provide a low enough earthing resistance.

Generally a resistance of the entire grid below 4 (four) ohms will be satisfactory.

Example of typical solid grounding installation

7.2.2LOW RESISTANCE GROUNDING (NEUTRAL GROUNDING RESISTORS OR REACTORS)

Neutral Grounding Resistors are used in industrial power systems for resistance grounding of

wye-connected generators and transformers. A neutral

grounding resistor limits the fault current to a value which is

sufficient enough to operate protective relays, yet prevent

unwanted fault damage. Resistance grounding can limit point-

of-fault damage, eliminate transient overvoltages, reduce the

flash hazard, limit voltage exposure to personnel, and provide

adequate tripping levels for selective ground-fault detection

and coordination.

Chemically Charged Ground Rod


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In order to limit the fault current on transformers with a wye connected secondary, neutral

grounding resistors or reactors are often used on medium voltage systems from 1000 volts to 15,000 volts

phase to phase. These current limiting devices are connected in series with the transformer secondary

neutral. In the event of a phase-to-ground fault, the current will flow through, and be limited by, the

neutral resistor or reactor.

The three electrical ratings required to select a grounding resistor are: Voltage Rating, Current

Rating, and Time Rating.

Voltage Rating:based on the system phase-to-neutral voltage (phase-to-phase voltage divided by

square root of 3.

Current Rating:Low Resistance Grounded Systems, generally the range is from 25 to 600 amps.

High Resistance Grounded Systems, the current is limited to 10 amps or less

R


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the protective device. The circuit would remain connected and energized and the fault current would

continue to flow. This could result in damage to equipment or the installation, and severe or fatal electric

shock to personnel in contact with grounded equipment.

Earth Loop Impedance Testing is used to determine the total alternating current of the circuit that

would be involved under fault conditions. Loop testing is required to ensure that the earth path is adequate

for the maximum possible fault current.

The test is conducted by using a ground loop impedance tester. The tester places a limited fault

current (about 20 amps) on the circuit under test for a limited time (about 20 ms). By measuring the

voltage drop across a reference resistor, the tester indicates the ohmic value of the fault loop. Digital loop

testers are available and are designed for a quick, accurate and reliable testing.

Loop impedance tests should be used to identify circuits with high resistance. The high resistance

may denote poor connections or excessive conductor length.

Example of Ground Loop Impedance Testing

R loop

R testV testV supply

I

Circuit with the loop tester on

Loop impedance R loop = (Vsupply Vtest)/I


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Example of Digital Loop Tester

(Megger LT7)

The Megger LT7 Digital loop tester is a compact portable instrument designed to measure earth

loop impedance and prospective short circuit current.

8.3.2 EARTHING ELECTRODE RESISTANCE TESTING

Earth electrode testing is used to determine the effectiveness and integrity of the earthing system.

It will ensure a safe, low resistance path for the electrical current to flow to earth in the event of a fault.

Periodic testing is necessary because variations in soil resistivity are caused by changes of soil

temperature, soil moisture, conductive salts in the soil, and corrosion of the earth connectors. An earth

electrode rod must have a resistance to ground of 25 ohms or less.

The test set used will ordinarily be a earth resistance test set, designed for the purpose, using the

principle of the fall of potential of AC circulated current from a test spot to the earth connection under

test. This instrument is direct reading with a scale calibrated in ohms of earth resistance.


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8.3.3TYPICAL EARTH ELECTRODE TESTING INSTRUMENTS

A. DET62D Ground Resistance Tester

The DET62D is a fully automatic, 3-terminal

instrument built into a rugged, water resistant case for

outdoor use. The instrument is suitable for the testing of

single ground electrodes such as lightning conductors and

other small grounding systems, or for measuring the

resistance of conductors. Ground resistance can be

measured directly from 100mohm to 2kohm.

B. DET 5/4R Ground Resistance Tester

The DET 5/4R ground tester provides the higher resistance range necessary for the testing of

ground electrodes and measuring ground resistivity. The

fully automatic operation is started by simply choosing a 3

or 4 terminal measurement, all other operations are

automatic. These instruments are suitable for soil resistivity

measurements used to establish the optimum ground

electrode system design and location, and archaeological and

geological investigations. There are 4 ranges covering

measurements from 10mohm to 20kohm.

C. Clamp-On Ground Resistance Tester

The 3711 Clamp-on Ground Resistance Tester is used in multi-

grounded systems without disconnecting the ground under test. This meter

simply clamps around the ground conductor or rod and measures the

resistance to ground. By performing measurements on intact ground systems,

the user also verifies the quality of the grounding connections and bonds.

Resistance and continuity of grounding loops around pads and buildings may

also be measured


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8.3.4MEASUREMENT OF EARTH RESISTANCE-PROCEDURE

Earth Resistance Testing using Earth Resistance Tester

1. Refer to the above diagram for the connections

2. Drive to the ground Current Spike (C) at a distance (D) at least 10 times the length of Electrode (X)

3. Drive to the ground Potential Spike (P) at a distance midway of X and C

4. Connect the earth leads (wires) to the tester and spikes with the proper connectors and clips

5. Press Test button on the tester and record the readings as shown on the window.

6. Repeat measurement with spike P installed at a distance about 10% of D measured from the first

position of P (midway)

7. Take readings on position L and position R

8. Evaluate the three resistance readings. If all three resistance readings are within 5% of one another,

earth resistance is acceptable.

9. Record the average Earth Electrode Resistance.

10.If the resistances are not within 5% of each other, it means the earth resistances areas are overlapping

and the test must be repeated with spikes P and C further apart.

Generally a resistance of the entire grid below 4 (four) ohms will be satisfactory.

XP

3 m 3 m

ELECTRODEUNDER TEST ( X )

POTENTIALSPIKE

CURRENTSPIKE (C)

20-30 m

READING NO.3

READING NO.1

READING NO.2

AVERAGE

Ohms

Ohms

Ohms

Ohms

EARTH TESTER

PSE

XH

C!

k

R P

R C

NOISE

BRUIT

MEASUR E R E

R

MEGGERR

DET62DEARTHTESTER

(P)

L R

D20-30 m20-30 m 20-30 m


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9.0.0EARTHING SYSTEM IMPROVEMENT

To obtain a sufficiently low earth resistance, the following can be done:

1) Area of the mesh is increased,

2) Spacing of buried rods of mesh is reduced,

3) Soil is irrigated,

4) Number of earthing electrodes are increased

5) Application of substances like sodium chloride (common salt), calcium chloride, sodium

carbonate, copper sulfate, charcoal, soft coke.

6) Implementation of the chemically charged ground rods

7) Use of deep-driven ground rods reaching underground water table or lower soil layers with low

resistivity


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10.0.0MAINTENANCE AND INSPECTION OF EARTHING SYSTEM

INSTALLATION

10.1.0POSSIBLE CAUSES OF ACCIDENTAL EARTHING

1. Chafing of a cable insulation against the sharp edge in the opening of a connection box

2. Damage in the insulation of electrical machinery due to the effects of age, heat, and vibration,

permitting the entry of conducting dust.

3. The dust which forms a bridge between the exposed conductor and the frame, makes the wiring

grounded.

4. The moisture which condenses inside the apparatus and eventually enough water may collect to

make an electrical connection between the wiring and the frame, and the equipment is

grounded

10.2.0CONTINUITY OF THE EARTHING CIRCUIT

1) The continuity of the earthing circuit shall be ensured by effective connections through

conductors or structural members

2) Bonding of equipment with bolts or other identified means shall be permitted where paint and

dirt are removed from the joint surfaces or effectively penetrated

3) Moving machine parts, other than accessories or attachments, having metal to metal bearing

surfaces shall be considered as bonded. Sliding parts separated by a nonconductive fluid under

pressure shall not be considered as bonded

3) Portable, pendant, and resilient mounted equipment shall be bonded by separate conductors

4) Lids, doors, cover plates, etc.., shall be connected to the earthing circuit by a earthing

conductor. Where no electrical equipment is attached to the lids, door, cover plates, the usual

metal hinges and the like shall be considered sufficient to provide continuity.

5) Raceways, wire ways, and cable trays shall not be considered as earthing or bonding conductor

6) When a part is removed, the continuity of the grounding circuit for the remaining parts shall

remain intact.

10.3.0PERIODIC CHECKS ON THE EARTHING SYSTEM

Earthing systems should be checked after commissioning and periodically during the course of

maintenance.

The following checks apply for newly installed earthing system:

1. Buried elements of earthing systems are checked for condition at random by unearthing; other

elements are examined at accessible places;


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2. Circuit continuity is checked between earthing devices and earthed elements. Open circuit and

poor contact shall not be admitted.

3. The earthing system resistance shall be checked.

4. Cross-sectional areas of earth conductors should be checked.

10.3.1 INSPECTION OF EARTHING SYSTEM INSTALLATION

1. Disconnect each earthing electrode, measure the earthing resistance of each electrode. Earth

electrode must have a resistance of 25 ohms or less.

2. Measure the earthing resistance of the grid only.

3. Connect all electrodes to the grid and measure earthing resistance. Generally a resistance of the

entire grid below 4 (four) ohms will be satisfactory.

4. Connections and clamps are to be inspected as fas possible. At least a number of spot checks

shall be made of the buried connections.

5. Check the protective measures against mechanical damage of the grid and the earthing

electrodes.

10.3.2TYPICAL CHECKLIST FOR INSPECTION OF EARTHING SYSTEM

Location: _____________________________________________________

Number of Electrodes: __________________________________________

Type of Electrodes: _____________________________________________

Type of Conductors: ____________________________________________CHECK LIST

ActionsCheck

MarkRemarks

1. Earthing resistance of each electrodeNo 1.

2.

3.

4.

5.

2. Earthing resistance of grid only

3. Earthing resistance of system

4. Inspection installation of electrodes

5. Inspect bolted and clamped connections

6. Check protective measures

Date of Inspection: ___________________ Inspection conducted by: ____________________


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11.0.0LIGHTNING PROTECTION SYSTEM

There is no question about the hazards posed by lightning strikes and their associated effects.

Fires, injury or loss of life, damage and destruction of property, and the significant downtime and outage-

related revenue losses due to equipment damage all make lightning a serious threat. Although lightning is

an unavoidable hazard to electrical systems, its effects on the system can be minimized through proper

application of lightning arresters.

11.1.0EFFECTS OF LIGHTNING STROKES

Direct lightning strokes and even induced voltage due to lightning strokes elsewhere in the system

may cause damage or complete destruction to unprotected electrical apparatus. Direct or induced

voltages caused by lightning strokes can cause a high-voltage wave to travel along a power line. Unless

this high surge voltage is reduced before reaching the terminals of an electrical machine, severe damage

will result.

11.2.0HARMFUL EFFECTS OF VOLTAGE SURGES1. Possible failure of insulation of transformers, motors.

2. Possible flash over at very high stressed points, external or internal to equipment

3. Aging of non-restorable insulating materials.


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11.3.0 MEANS OF PROTECTION AGAINST LIGHTNING SURGES

Protective device Where applied Remarks

Lightning Masts (earthed) Above tall buildings Protect buildings against

direct strokes.

Surge Arresters On incoming lines in eachsubstation

Near terminals of

transformers, motors and

generators

Diverts over voltages toearth without causing short-

circuit

Overhead Grounded Wires Above overhead lines

Above the substation area

Provide effective protection

against direct strokes on

line conductors, towers,

substation equipment

Rod gaps Across insulator string,

bushing insulators

Difficult to coordinate

Horn gaps Across insulator string,

bushing insulators

Difficult to coordinate

11.4.0TYPICAL DEVICES/MATERIALS FOR LIGHTNING PROTECTION


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12.0.0LIGHTNING PROTECTION BY SURGE ARRESTER

Surges and spikes from nearby lightning strikes, arc-welders and high voltage cables can destroy

or disrupt unprotected electronic equipment. These destructive forces enter mains power circuits within

buildings by a variety of methods and cause powerful surges.

The primary route is where power-often dirty and spike laden actually enters the building and it

is at this point that surges should be stopped in order to prevent them from propagating further. However,

surges and RFI can also corrupt mains power supplies from within the building.

By providing power protection at the main distribution board and then at each piece of equipment,

mains power borne surges and spikes are eliminated before they can cause damage.

Surge Arrester (Lightning Arrester) is a device designed to protect electrical equipment from high

voltage surges and to limit the duration and amplitude of the discharge current. The arrester holds the

voltage across its terminals below the protective level and therefore the voltage appearing across surge

arrester terminals and on the terminal of the protected equipment does not exceed the protective level.

12.1.0TYPES OF SURGE ARRESTERS

1. Gapped Arresters with Silicon-Carbide resistor discs in series with gap units. This is called

valve type or conventional arrester.

2. Gap less Arresters with zinc-oxide resistor discs in series. These are called Metal-oxide

Arresters

12.2.0PRINCIPLE OF SURGE ARRESTER PROTECTION

12.2.1GAPPED ARRESTERS (GAPPED SILICON CARBIDE ARRESTERS)

Surge arrester is connected between line and earth. It consists of resistor elements in series with

gap elements. The resistor element offer nonlinear resistance such that for normal frequency power

system voltages the resistance is high. For discharge currents the resistance is low. The gap units consists

of air gaps of appropriate length. During normal voltages the surge arrester does not conduct. When a


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voltages, the resistors become conductive so that a surge current can flow to earth and the over voltage is

reduced to the value of the voltage drop across the arrester. The surge currents have values up to 1 kA

with switching over voltages, and of 1...10...20 kA with lightning over voltages.

12.3.0TYPICAL INSTALLATION AND TYPE OF SURGE ARRESTER

ROTATINGMACHINE

BUS BUS

TRANSFORMER

OVERHEAD LINE

DISTRIBUTIONLINE

SURGEDIVERTER

13 22

4

1. First apparatus on incoming line in substation2. For Transformer protection3. For protection of High Voltage Motors and Generators4. For Distribution line protection

LOCATION OF SURGE ARRESTERS


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13.0.0MAINTENANCE AND INSPECTION OF SURGE ARRESTERS

In normal service the surge arresters may be exposed to different stresses that alone or together

may cause increased resistive leakage current and overloading. These stresses are as follows:

1. Normal operating voltage.

2. Temporary over voltages.

3. Switching over voltages.

4. Lightning over voltages.

5. External pollution.

13.1.0PERIODIC INSPECTION ON THE SURGE ARRESTER

Periodic Inspection on the surge arrester are limited to the following:

1. Checking whether an arrester may have blown off, burned or damaged surface. In this event, a

new arrester must be fitted.

2. Checking the degree of pollution of the porcelain housing. In the event of heavy and uneven

contamination, cleaning and then siliconizing is to be recommended. Before cleaning or

otherwise working on lightning arrester, it must be disconnected from the power line.

3. Reading the surge counting device.

4. Earthing terminal connections.


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14.0.0SURGE COUNTERS

Surge counter is a monitoring instrument suitable for installation in the

earth connection of a surge arrester which must be isolated from earth for the

counter to operate correctly. Generally, a surge counter is connected in series.

The riser from the station earth mat is connected to the surge counter and surge

counter is connected to the surge arrester earthing terminal.

14.1.0INSTALLATION OF SURGE COUNTER

Surge counter is normally should be mounted at eye-level on a horizontal surface such as cross-

channel or angle support. A clearance of at least 30 mm must be allowed between the live terminal of the

surge counter and any earthed object. Connections to the surge counter may be made with either cable or

bare conductor with suitable terminations and earth studs, with are nickel plated brass.


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14.2.0MAINTENANCE AND INSPECTION OF SURGE COUNTER

Under normal operating conditions surge counter does not require any maintenance, except for the

following:

1. Periodic cleaning of the glass and the line bushing.

2. Checking of the tightness of connections.

References:

IEE Wiring Regulations

Handbook on the IEE Wiring Regulations

Preventive Maintenance of Electrical Equipment by Charles I. Hubert

Testing, Commissioning, Operation and Maintenance of Electrical Equipment by S. Rao

Module No E02 Earthing and Lightning Protection

Documents

Transcript of Module No E02 Earthing and Lightning Protection