UNIVERSITY OF NAIROBI

DEPARTMENT OF ELECTRICAL AND INFORMATION

ENGINEERING

PROJECT INDEX: PRJ 136

PME SYSTEMS

BY

NAME: BISKY .I. ASYAGO

REG. NO: F17/2451/2009

SUPERVISOR: PROF. M. K. MANG'OLI

EXAMINER: DR.C.WEKESA

Project report submitted in partial fulfillment of the requirement for the award of the Degree of

Bachelor of Science in Electrical and Information Engineering of the University of Nairobi

Date of submission: 28TH APRIL, 2014

DEDICATION

To my Mother, Cyprose Oluya, true example of unconditional love and boundless support, to

whom I owe everything.

To my Brothers and Sister, Byron, Bell, Brandsar ,Baker and Brenda, my partners in the Quest.

ACKNOWLEDGEMENT

I would like to thank my supervisor Prof. M. K. Mang'oli for his guidance, useful hints and

directions proved to be the strong foundation upon which the project was built.

I wish to sincerely express my heart-felt gratitude to my Lecturers in the Department of

Electrical and Electronic Engineering for the enlightening experience in the field of Engineering.

More thanks to Eng. Osewe for the useful practical knowledge he provided to me with regards to

my project.

I am also grateful to my family for providing me with the moral support through this project,

interest and encouragement

To the Almighty God, whom all the glory and honor belongs.

ABSTRACT

Power systems grounding is probably the most misunderstood element of any Power systems

design. This project studies the characteristics of different Power Systems Grounding techniques

as currently applied and misapplied within industry today. In practice today for utility

distribution network Protective Multiple Earthing (PME) which is a variant of the neutral

earthing is the most widely adopted earthing system arrangement. In this project a comparative

study of PME and Isolated systems is carried out investigating their fault behavior, safety and

requirements. An experiment to demonstrate that there is continuous power dissipation to ground

even under normal system operating conditions in the PME system is also performed and based

on the findings of the study, a recommendation for the Isolated system where and when possible

is made.

Keywords : PME analysis, quantification of power losses

TABLE OF CONTENTS

LIST OF FIGURES

Figure 1.0: Direct and Indirect contact …………………………………………………………..4

Figure 2.1: Interface of earth and electrode………………………………………………………7

Figure 2.2: Diagram illustrating the physical sense of earth resistivity ρ………………….........13

Figure 2.3: IT Grounding System………………………………………………………………..15

Figure 2.4: TT Grounding System………………………………………………………….........16

Figure 2.5: TN-C Grounding System……………………………………………………….........17

Figure 2.6: TN-S Grounding System…………………………………………………………….17

Figure 2.7: TN-C-S (PME) Grounding System………………………………………………….18

Figure 3.1: Fault-loop in PME systems for single-phase loads………………………………….20

Figure 3.2: Fault-loop for a ground fault on the low-voltage PME distribution system………...22

Figure 3.3: Equivalent fault-loop for a short circuit phase-to-PEN in PME…………………….24

Figure 3.4: Interruption of the PEN conductor in PME………………………………………….25

Figure 3.5: Neutral-to-ground potentials…………………………………………………….......26

Figure 3.6: Distributed capacitances in IT grounding…………………………………………...29

Figure 3.7: First fault in IT grounding System…………………………………………………..30

Figure 3.8: Second Fault in IT grounding systems………………………………………………31

Figure 3.9: Position of the IMD……………………………………………….…………………32

Figure 3.10: ECPs Earthed Individually or in Groups…………………………………………...34

Figure 3.11: ECPs Earthed Collectively to a Single Grounding System………………………..35

Figure 3.12: Fall of potential method…………………………………………………………....38

Figure 3.13: Dead Earth Method………………………………………………………………...40

Figure 3.14: Basic clamp-on ground testing methodology………………………………………41

Figure 3.15: Set up for the measurement of the earth electrode resistance……………………...42

Figure 3.16: Set up for the estimation of power losses…………………………………………..42

LIST OF TABLES

Table 2.1: Typical Formulae for Calculating Earth resistance………………………………...8

Table 2.2: Minimum sizes for Steel earth electrodes…………………………………………..8

Table 2.3: Minimum sizes for copper earth electrodes………………………………………...9

Table 2.4: Minimum standard cross-sectional areas for PEs…………………………………..9

Table 2.5: The cross-sectional area of MEBs………………………………………………....10

Table 2.7: Minimum cross-sectional Areas of Earthing Conductors………………………....11

Table 3.1: Maximum disconnecting time for AC final circuits not exceeding 32 A………...29

Table 3.2: Maximum disconnection time in the IT earthing system (second fault)………….32

Table 3.3: capacitive values for HF filters built into various devices………………………...39

Table 4.1: Experimental result A (Switch OFF)……………………………………………...43

Table 4.2: Experimental result A (Switch ON)……………………………………………….43

Table 4.3: Experimental result B (Switch OFF)………………………………………………43

Table 4.4: Experimental result B (Switch ON)………………………………………………..43

Table 4.5: Preference in different types of networks………………………………………….48

Table 4.6: Performance of the two systems……………………………………………………49

Table 4.7: Preference in Different load conditions…………………………………………....50

LIST OF SYMBOLS/ NOMENCLATURE

Symbol Description

VS Step Voltage

VT Touch Voltage

L Rod length

r Rod radius

ρ Soil resistivity

IG Earth fault current

IPH Phase conductor current

ZE Earthing Impedance

PME Protective Multiple earthing

ZPH Phase conductor Impedance

RCD Residual current device

ECP Exposed conductive parts

ZC Faulty circuit loop impedance

EXCP Extraneous conductive parts

EMC Electromagnetic compatibility

EMI Electromagnetic interference

ZS Earth fault current loop impedance

Ia Current setting of the protective device

PEN Protective earthed neutral conductor

TN A neutral grounded electrical supply system

ZPEN Protective and Neutral conductor Impedance

GPR / VG /EPR Ground Potential Rise (Earthing Voltage)

VX The potential at a distance x from the middle of the earth electrode

푽풊∗ The relative value of that individual potential to the earthing voltage

PE Protective earthed conductor /equipment grounding conductor

IT Ungrounded electrical supply system with or without a distributed neutral

TN-C A neutral grounded electrical supply system where the neutral serves as the

protective conductor, i.e. PEN conductor

TN-C-S A neutral grounded electrical supply system where, in part of the installation, a

PEN conductor is used and in other parts a separate PE conductor is used

TN-S A neutral grounded electrical supply system with separate neutral and protective

earthed (PE) conductors

TT A neutral grounded electrical supply system where the source neutral and the

electrical equipment are grounded

1.0 INTRODUCTION

In many cases, misunderstood concepts and perceptions of the purpose and type of Power

Systems grounding to be selected dates back to the 1940's and earlier. Since that time much

research, coupled with experience, has taken place that is now available to industry. In any

medium or low voltage three-phase system there are three single-phase voltages which are

measured between each phase and a common point called the "neutral point". In balanced

operating conditions these three voltages are phase shifted by 120° and have the value:

V√3

V being the phase-to-phase voltage measured between phases. From a physical point of view,

the neutral is the common point of three star-connected windings. It may or may not be

accessible, may or may not be distributed and may or may not be earthed, which is why we refer

to the earthing system.

Grounding or earthing is normally defined as a connection of various exposed conductive parts

of equipment together and to a common terminal which is in turn connected by the earthing

conductor to an earth electrode. There are two misconceptions in this statement. First, grounding

is not only limited to equipment but also involves the electrical power system, the two being

related and may refer to the same physical installation in some cases. Second, the term

grounding, which is used interchangeably with earthing, is not the same thing. Grounding should

be called earthing, only if it involves the physical earth and in case of a mul-functioning of some

part of the system, some of the current returns back to the source through the earth [1].

Therefore, the standard definition of grounding according to [2] is the conducting connection

whether intentional or accidental between an electrical circuit or conductive equipment part and

a common terminal which is in turn connected by a conductor to an earth electrode or to some

conducting body of relatively large extent that serves in place of the earth.

As anticipated above, grounding is broadly classified as equipment grounding and system

grounding. Equipment grounding, referred also as protective grounding is mainly for the

prevention from dangerously high shock that may exist when there is a fault current between an

energized electrical conductor and the structure that either encloses it or is nearby whereas

system grounding is an intentional electrical interconnection between the electrical system

conductors and ground, forming part of the operating system. Objectives behind the system

grounding are to fix the potential at any part of the network with respect to earth and to provide

sufficient fault current so that protection equipment can operate [3].

System grounding can be of four different types, which are ungrounded systems, resistance

grounding, reactance grounding and solid grounding. The neutral may be connected to earth

either directly or via a resistor or reactor. In the first case, the neutral is solidly (or directly)

earthed and, in the second case, the neutral is impedance-earthed. When there is no intentional

connection between the neutral point and earth, the neutral is said to be isolated or unearthed [4].

In low-voltage distribution networks, which distribute the electric power to the widest class of

end users, the main concern for design of earthing systems is safety of consumers who use the

electric appliances and their protection against electric shocks [1] which is the

pathophysiological effect of an electric current through the human body. The fundamental rule of

protection against electric shock is provided by the document IEC 61140 which covers both

electrical installations and electrical equipment; Hazardous-live-parts shall not be accessible and

accessible conductive parts shall not be hazardous. There are two ways of getting a shock from

an electrical installation: first by touching a live part of the installation i.e. direct contact, and

secondly by touching a conductive part that is not supposed to be live, but has been energized

due to failure of the basic i.e. indirect contact. It is obvious that the protection under normal

conditions will be quite easy, as the live parts of the electrical installation will be basically

protected by:

Insulation- This protection consists of an insulation which complies with the relevant

standards,

Barriers or enclosures- This measure is in widespread use, since many components and

materials are installed in cabinets, assemblies, control panels and distribution boards,

Obstacles- This protection is reserved only to locations to which skilled or instructed

persons only have access.

All the preceding protective measures are preventive, but experience has shown that for various

reasons they cannot be regarded as being infallible. Among these reasons may be cited: Lack of

proper maintenance, Imprudence, carelessness, Normal (or abnormal) wear and tear of

insulation; for instance flexure and abrasion of connecting leads, Accidental contact, Immersion

in water, etc. A situation in which insulation is no longer effective In order to protect users in

such circumstances, highly sensitive fast tripping devices, based on the detection of residual

currents to earth are used to disconnect the power supply automatically, and with sufficient

rapidity to prevent injury to, or death by electrocution, of a normally healthy human being

In most cases, and certainly for the general public, the chosen protective measures will include a

protection against direct and against indirect contact or “protection under normal and under fault

conditions. In the protective measures under fault conditions, two possibilities exist: passive

measures such as supplementary insulation, and active measures, which will break the current in

the faulty circuit. Automatic disconnection before the voltage can do harm to a person touching

the accessible conductive part under fault. For the ECPs to be properly protected it is necessary

to have a protective earth.

In general, when an insulation becomes defective, there will be a rapid evolution to a full fault

and therefore the person touching the ECP as in fig. 1 will be subjected to the same fault current

as the person touching the active part. But just connecting the ECPs to earth is not enough. The

fault current in fig.1 creates a potential rise between ECP and the earth and this voltage can be

dangerous if it exceeds a certain value for a certain time.

Fig 1.0

From the above discussion it is evident that earthing is important in electrical power systems to

minimize voltage and thermal stresses on equipment, provide personnel safety, reduce

communications system interference, and assist in rapid detection and elimination of ground

faults.

1.1 Aim of The Project

The aim of this project is to analyze the Protective Multiple Earthing(PME) scheme which is the

adopted system earthing arrangement for most modern low voltage (LV) power systems

including The Kenya Power and Lighting Company (KPLC) with the objective of examining and

quantifying the associated power losses and providing a suitable alternative system.

1.2 Scope of The Project

Chapter 1 gives a brief introduction of the project and measures of protection against direct and

indirect contact.

In Chapter 2, the different system earthing arrangements IT, TT and TN are discussed. Also the

role of earthing systems is introduced

Chapter 3 gives a comparative study of PME and IT systems and also the design methodology

that was adopted in determining and quantifying the power losses in PME system is presented

In Chapter 4, the experimental results and findings of the comparative study in chapter 3 are

presented and a detailed analysis is carried out.

Finally, in Chapter 5, concluding remarks and recommendation of future work to be done are

given.

2.0 LITERATURE REVIEW

2.1 Fundamental Components of Earthing Systems

Earth electrodes, protective conductors, and bonding conductors, are the fundamental

components of the earthing arrangements. A failure in any of these elements can compromise

the electrical safety of the installation as well as its functionality. Earthing arrangements, in fact,

may be used for both functional reasons and safety purposes [6].

2.2 Earth Electrodes

Earth electrodes provides the connection to earth and must provide a reliable link to ground,

primarily for safety purposes, but also for the proper functioning of equipment. Electrodes must

be able to carry ground-fault currents and dissipate them to ground, without causing hazards

caused by thermal effects and/or electric shock. The effectiveness of the earthing system depends

upon its ground resistance RG, which varies with the resistivity of the local soil. Once the

characteristics of the soil and the minimum acceptable value for safety of RG are known, one or

more ground electrodes, even of different nature, must be employed [6].

Identifying the resistance to ground is mostly dependant on soil resistivity of the area to be

grounded. In as much as the earth is a relatively poor conductor of electricity compared to

normal conductors like copper wire its abundance and availability makes it an indispensable

component of a properly functioning electrical system.

2.2.1 Nature of an earth electrode Resistance to current through an earth electrode actually has three components:

Resistance of the electrode itself and connections to it.

Contact resistance between the electrode and the soil adjacent to it.

Resistance of the surrounding earth.

2.2.1.1 Electrode Resistance

Rods, pipes, masses of metal, structures, and other devices are commonly used for earth

connections. These are usually of sufficient size or cross section that their resistance is a

negligible part of the total resistance.

2.2.1.2 Electrode-earth contact resistance

If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is

negligible. Rust on an iron electrode has little or no effect but if an iron pipe has rusted through,

the part below the break is not effective as a part of the earth electrode.

Figure 2.1 Interface of earth and electrode

2.2.1.4 Resistance of surrounding earth

An electrode driven into earth of uniform resistivity radiates current in all directions as in Fig

2.1. The earth shell nearest the electrode naturally has the smallest surface area and so offers the

greatest resistance. The next earth shell is somewhat larger in area and offers less resistance.

Finally, a distance from the electrode will be reached where inclusion of additional earth shells

does not add significantly to the resistance of the earth surrounding the electrode [9].

Approximate values of the earth resistance at 50/60 Hz of typical made electrodes may be

calculated by using the formulas reported in Table 2.1. [6].

Table2.1

Type of Electrode Earth Resistance

Rod ρ L⁄

Buried Horizontal wire 2ρ L⁄

Grid ρ 4r⁄

2.3 Corrosion Phenomena

Earth electrodes must have a minimum size in order to have adequate mechanical strength and

withstand corrosion. Corrosion is an electrochemical process that involves two dissimilar metals

electrically connected when embedded in electrolytes, such as earth, the two metals, respectively,

assume the role of cathode and anode of a galvanic cell. When the current leaves the anode, to

reclose to the cathode through the electrolyte, corrosion at the expense of the anode occurs. The

phenomenon is more pronounced when the ratio of the cathode’s surface to the anode’s surface

is large. In this respect, common minimum sizes for earth electrodes, as per IEC 60364–5-54,2

are reported in Tables 2.2 and 2.3.[6]

Table 2.2

Steel Surface Electrode type Diameter(mm) X-Area(mm2) Thickness(mm) Hot dip galvanized or Stainless

Strip - 90 3

Round rod for deep earth electrode

16 - -

Round wire for Surface earth Electrode

10 - -

Pipe 25 - 2

Table 2.3

Copper Electrode type Diameter(mm) X-Area(mm2) Thickness(mm) Bare strip - 50 2 Bare rope 1.8(individual strands) 25 - Bare Round wire for surface earth electrode

- 25 -

Bare Pipe 20 - 2 Tin coated rope 1.8 for individual

strands 25 -

Zinc coated Strip - 50 2

2.4 Protective conductor

Protective conductors (PEs) provide safety against indirect contact by linking ECPs to the main

earthing terminal, thereby creating a clear path for the fault currents. Cross-sectional areas of

protective conductors must be adequately large, so that fault currents can promptly activate the

protective device and automatically disconnect the supply.

Additionally, protective conductors must be able to withstand the flow of the ground-fault

current without reaching dangerous temperatures to the surrounding environment or shorten the

life of, or damage, their insulation. Minimum standard cross-sectional areas deemed adequate

for PEs are shown in Table 2.4,when the protective conductor is of the same material as the line

conductor. If a protective conductor is common to more than one circuit, it must be selected in

correspondence with the phase conductor’s largest cross-sectional area.

Table 2.4

Minimum Cross-sectional Area of Line Conductor, S (mm2)

Cross-sectional Area of the corresponding PE Conductor

S ≤ 16 S 16< S ≤ 35 16 S > 35 S/2 Protective conductors may not necessarily consist of actual conductors, but metallic layers of

cables (e.g., metallic sheaths, armors, concentric conductors, etc.) and metallic conduits can

serve the same protective purpose as long as their equivalent cross-sectional area complies with

Table 2.2 or 2.3. This conductor, in fact, is generally at zero potential and, therefore, does not

require any dielectric insulation. [6].

2.5 Equipotential Bonding Conductors

Equipotential bonding conductors cancel dangerous potential differences between metal parts,

for example, ECPs and EXCPs, when ground faults occur. They must be reliable, have negligible

resistance and good mechanical strength. As already discussed, main equipotential bonding

conductors are employed for connections of EXCPs to the main earthing bus and for

supplementary bonding in special locations (e.g., locations containing a bath or shower).

The cross-sectional area of MEBs can be selected as per IEC 60364–5-54, which indicates their

minimum standard sizes as provided in table 2.5.

Supplementary bonding conductors must comply with the same requirements as the protective

conductors as to their minimum size.

Table 2.5 Copper(mm2) Aluminum(mm2) Steel(mm2) 6 16 50 If the supplementary bonding conductor connects an ECP to an EXCP, its conductance must be

at least half that of the PE serving the ECP. Also, in this case, a current divider takes place and

the EXCP carries part of the fault current.

2.6 Earthing Conductors and Main Earthing Terminal

Earthing conductors link the earth electrode(s) to the main earthing bus. The cross-sectional area

of these conductors can be selected or calculated as seen for the protective conductors. If the

earthing conductors are embedded in the soil, they must endure mechanical stress and corrosion

without compromising their electrical continuity. To this end, their minimum standard cross

sectional areas can be selected from Table 2.7 as per IEC 60364–5-54. The main earthing

terminal links together protective conductors, equipotential bonding conductors, and earthing

conductors.

Table 2.7 Minimum cross-sectional Areas of Earthing Conductors

Mechanically Protected No Mechanical Protection Copper Iron Copper Iron

Protected against corrosion

2.5 10 16 16

Not protected against corrosion

25 25 25 25

2.7 The PEN Conductor

PEN conductors provide both functions of neutral and protective conductor. The accidental

interruption of the PEN, when the line conductors are live, energizes the equipment, even if

healthy; therefore, its electrical and mechanical continuity must be guaranteed. To this purpose,

the PEN should only be used in a fixed installation and have a cross-sectional area not less than

10 mm2 in copper or 16 mm2 in aluminum. These conditions limit the probability of its

accidental interruption. In some point of the installation, the PEN conductor may split and

originate distinct wires as neutral and PE [14].

2.8 Electrical properties of the earthing system

The electrical properties of earthing depend essentially on two parameters:

earthing resistance

configuration of the earth electrode.

Earthing resistance determines the relation between earth voltage VE and the earth current value.

The configuration of the earth electrode determines the potential distribution on the earth surface,

which occurs as a result of current flow in the earth. The potential distribution on the earth

surface is an important consideration in assessing the degree of protection against electric shock

because it determines the touch and step potentials[8].

2.8.1 Earthing resistance

The earthing resistance has two components:

dissipation resistance RD, which is the resistance of the earth between the earth electrode

and the reference earth

resistance RL of the metal parts of the earth electrode and of the earthing conductor.

The resistance RL is usually much smaller than the dissipation resistance RD. Thus, usually the

earthing resistance is estimated to be equal to the dissipation resistance RD.

In AC circuits one must consider essentially the impedance of an earthing ZE, which is the

impedance between the earthing system and the reference earth at a given operating frequency.

The reactance of the earthing system is the reactance of the earthing conductor and of metal parts

of the earth electrode. At low frequencies - the supply frequency and associated harmonics -

reactance is usually negligible in comparison to earthing resistance, but must be taken into

account for high frequencies such as lightning transients. Thus, for low frequencies, it is assumed

that the earthing impedance ZE is equal the dissipation resistance RD, which is in turn assumed to

be approximately equal to the earthing resistance, R [8].

The earthing resistance R of an earth electrode depends on the earth resistivity ρ as well as the

electrode geometry. In order to achieve low values of R the current density flowing from the

electrode metal to earth should be low, i.e. the volume of earth through which the current flows

is as large as possible. Once the current flows from metal to earth it spreads out, reducing current

density. Low current density extends electrode life.

The calculation of earthing resistance is usually performed under the assumptions that the ground

is boundless and of uniform structure with a given value of resistivity. Earth resistivity ρ

(specific earth resistance) is the resistance, measured between two opposite faces, of a one-metre

cube of earth The earth resistivity is expressed in Ω-m. Where no information is available about

the value of ρ it is usually assumed ρ = 100 Ω-m.

Figure 2.2 - Diagram illustrating the physical sense of

earth resistivity ρ

The earth resistance depends significantly on how deep the electrode is sunk in the ground. This

is because the moisture content is higher and more stable for deeper ground layers than for

shallow layers. Layers near the surface are influenced more by seasonal and short-term weather

variations and are subject to freezing. One can distinguish several types of earth electrodes

including:

simple surface earth electrodes in the form of horizontally placed strip or wire, either as a

single ended strip or a ring

meshed electrodes, constructed as a grid placed horizontally at shallow depth

cable with exposed metal sheath or armor which behaves similarly to a strip-type earth

electrode

foundation earth electrodes formed from conductive structural parts embedded in

concrete foundation providing a large area contact with the earth.

rod electrodes which can consist of a pipe, rod, etc. and are driven or buried to a depth

greater than 1 m and usually from 3 m to 30 m or more.

2.8.2 Earthing voltage and earth surface potential distribution Earthing voltage, as well as distribution of the earth surface potential during the current flow in

the earthing system, are important parameters for protection against electric shock. The potential

of any point located at distance x from the middle of earth electrode, in which earth current IE

flows, can be formulated with the following equation:

푉 = Eq. 2.1

and its relative value,

푉∗ = Eq. 2.2

Where VG is the earthing voltage, which is equal to the earthing potential (assuming that the

potential of the reference earth is equal zero). The earthing potential can be described as follows:

푉 = 퐼 푅 = Eq. 2.3

The potential difference between two points on the earth surface: one at distance x and other at

distance x+ aS, where aS is assumed to be equal to 1 metre, illustrates the step potential ∆VS, i.e.

earth surface potential existing between two feet, when a person stands at that position on the

earth surface:

푉 = − Eq. 2.4

and its relative value,

푉∗ = Eq. 2.5

where x ≥r.

A similar relationship can be described for any other distances x and a. Particularly for x = r and

a = aT = 1m the formula (2.4) enables the calculation of the touch voltage, i.e. the voltage

between a palm and a foot of a person who is just touching the earth electrode or metal parts

connected to it [8]:

푉 = − Eq. 2.6

and its relative value

푉∗ = Eq. 2.7

2.9 Types of System Earthing Arrangements(SEA)

The international standards IEC 60364 distinguishes three families of system earthing

arrangement (SEA) using the two letter codes namely; TT, IT and TN. The first letter indicates

the connection between the power supply equipment (generator/ Transformer) and earth. Where

[1]:

"T"- Denotes direct connection to earth ("T" derived from the French word "Terre" meaning

earth)

"I"- Denotes isolation from earth except perhaps a connection to earth via a high impedance path

The second letter indicates the connection between earth and the electrical installation being

supplied. where:

"T"- Again denotes presence of a direct connection to earth

"N"- Connection to earth is via the neutral of the supply

2.9.1 IT Grounding System

In " IT" (Isolation Terre) earthing arrangement the electrical distribution system is isolated from

earth and therefore the supply does not provide an earthing connection for the consumers. Thus

the consumers have to have local dedicated earth rods. It finds applications in laboratory rooms,

medical facilities, construction sites, repair workshops, mobile electrical installations, and other

environments that are supplied via engine-generators where there is an increased risk of

insulation faults, often use an IT earthing arrangement supplied from isolation transformers [3].

An illustration of a IT network is shown in figure 2.3

Fig 2.3

2.9.2 TT Grounding System

In "TT" (Terre-Terre, or earth-earth) earthing arrangement the earth connection of the consumer

is provided by a local connection to earth independent of the supply side connection to earth. The

big advantage of the TT earthing system is that it is clear of high and low frequency noises that

come through the neutral wire from connected equipment also in locations where power is

distributed overhead and TT is used, installation earth conductors are not at risk should any

overhead distribution conductor be fractured by, say, a fallen tree or branch [3]. Both the supply

and consumer have direct connections to earth as shown in figure 2.4

Fig 2.4

2.9.3 TN Grounding System

In " TN" earthing arrangement also known as neutral earthing, the star point of the supply side is

directly connected to earth and the consumer earth terminal connected to earth via the neutral of

the supply side. This scheme has three different variations namely; TN-C, TN-S and TN-C-S.

2.9.3.1 TN-C

"TN-C " earthing scheme has a combined protective earth and neutral conductor (PEN). That is

one conductor functions as the protective earth as well as the neutral of the power system. TN-C

networks save the cost of an additional conductor needed for separate N and PE connections as is

shown below.

Fig 2.5

2.9.3.2 TN-S

In" TN-S" earthing scheme the protective earth conductor and the neutral conductor of the

system are two separate conductors and are connected together only near the power supply

equipment. In TN-S systems, the consumer has a low-noise connection to earth, which does not

suffer from the voltage that appears on the N conductor as a result of the return currents and the

impedance of that conductor. This is of particular importance with some types of

telecommunication and measurement equipment[3] [6]. This arrangement is shown below.

Fig 2.6

2.9.3.3 TN-C-S/Protective Multiple Earthing

"TN-C-S" earthing scheme Is a hybrid of the TN-C and TN-S earthing schemes where part of

the system uses the combined protective earth and neutral, usually between the substation and

the entry point to the installation

beyond which it splits up into a separate protective earth and a neutral conductor. The combined

PEN is usually earthed at different convenient places along its run to reduce the risk of broken

neutrals and by doing so ensures the reliability of the earth neutral path. It is this multiple

earthing that gives this scheme its name i.e. protective Multiple earthing scheme(PME). This

scheme has the advantages of both the TN-C and TN-S schemes. The protective earth and neutral

conductor should never be joined within the installation [6]. An Illustration of the PME scheme

is shown in figure 2.7

Fig 2.7 Concerning the safety of persons , the three systems are equivalent if all requirements are fully

met.

2.10 Importance of Earthing

The purpose of system grounding in an electrical system is therefore:

Provides a low resistance path to ground for any surges or lightning strikes that may

occur on the electrical system; protection of buildings and installations against lightning.

Provides a low resistance path to ground for fault currents, and thus trips the over- current

protective device (circuit breaker or fuse) quickly when a fault or short circuit occurs; and

most importantly,

Provides a stable reference point for the voltages in each phase of a circuit. A solid

ground point prevents the phase-to-ground and the phase-to-phase voltages from

fluctuating.

Electromagnetic compatibility (EMC) i.e. limitation of electromagnetic disturbances

3.0 METHODOLOGY/DESIGN

3.1 Protective Multiple Earthing (PME/TN-C-S Grounding System)

In PME systems the utility supply neutral conductor is solidly grounded at the source and at each

fourth span along its distribution. At the terminal pole, the PEN is separated into the neutral and

protective conductors. In these conditions, the ground-fault currents, arising at the user’s

installation, will basically return to the supply through the distributor’s neutral conductor, the

PEN. Here the fault-loop does not comprise the actual earth; however, the utility must earth the

center of its transformer’s secondary star point. The purpose of the system ground is to allow the

operating voltage to-earth to remain stable and to limit overvoltages in fault conditions.

It is assumed that in fault conditions, the earth will be partially involved as a return path to the

source because of the connection of the EXCPs to the main grounding bus. As already

substantiated, this bond is essential to guarantee equipotentiality between simultaneously

accessible metal parts. Moreover, in the absence of ground faults, the above earthing

arrangement may cause part of the neutral current to return to the source through the earth.

RN

phase

PEN N

PE

REXCP

IPh

IG1

IG2 Fig 3.1

In this systems, ground-fault currents arising at the user’s location have large magnitude, as their

return path, the PEN, is parallel to earth. Therefore, the total impedance of the fault-loop |ZLoop|

may be low enough to allow users to use overcurrent devices for automatic disconnection. This

renders the presence of RCDs in PME systems not a mandatory safety requirement. The bonding

connection is of utmost importance, at the service entrance of the dwelling unit, between the

PEN and the PE. It is, therefore essential to periodically inspect and maintain such connection.

Upon loss of this bond, the building would become a TT system, wherein in the absence of

RCDs, which are not strictly necessary in PME, users are exposed to electric shock hazards.

3.1.1 Fault-Loop Impedance in PME Systems

In PME the fault-loop impedance includes the impedance Ze of the utility low-voltage

distribution system, which is usually unknown to the customer. Ze, which increases with the

distance of the fault’s location from the supply source, may also change in time without the user

knowing it because of modifications in the utility distribution system.

If the total fault-loop impedance ZLoop = Ze + Zuser is excessive, the ground fault current might be

so low and there would be no automatic disconnection and thus the installation of RCDs in

dwelling houses, even in PME systems, although redundant in the case of low value of ZLoop,

can, indeed, guarantee safety when ZLoop is too high.

3.1.2 Energization of the PEN Conductor in PME Systems

PME systems imply a considerable responsibility of the local utility, since, together with the

electric energy, the distributor provides the users with an earth connection, which must ensure

public safety. In fault conditions, the utility PEN, although multiple grounded, may assume a

voltage, with respect to the earth, as is substantiated later on. Such neutral-to-ground voltage can

be transferred as a shock potential to the users’ ECPs and EXCPs. If utilities cannot “certify” the

neutral potential as harmless to persons, a TT system should be employed, instead.

3.1.3 Ground Fault on the Low-Voltage Utility Distribution System

The PEN conductor may become live due to a ground fault occurring along its distribution

system, for example, as a result of the fall to earth of overhead cables or of a contact of the line

with an EXCP not connected to a protective conductor. An illustration of this condition is as

shown in figure 3.2 where RN represents the ground resistance of the utility’s earth electrode

system: the neutral conductor is earthed not only at intervals along its run (e.g., at the distribution

poles) but also at the customers’ dwelling units by means of their ground electrodes (e.g., cold

water pipe).

RE is the minimum earth resistance of EXCPs not connected to an equipotential system, through

which a fault may occur. The earth current, by circulating through RN and RE energizes the PEN

conductor and, therefore, the user’s ECPs. The contact resistance with earth also limits this

ground current and in some cases can be very high (e.g., line in contact with snow or sand). The

distributor’s overcurrent devices, therefore, may not be able to clear the fault within the

maximum permissible times, exposing persons to the risk of electric shock. To identify safe

values for the PEN potential and the maximum earth resistance RN:

V = V ×R

R + R + Z + Z ≅ V ×R

R + R ≤ 50V

Eq. 3.1

Where both the phase conductor impedance Zph and the internal impedance of the source Zi

ignored because they are generally negligible with respect to RN and RE. As a safety criterion,

we can assume as “safe” the PEN conductor if its potential VN does not exceed the safety limit of

VPh

RN VN

Zi ZPh

RE

IG

Fig. 3.2

50 V. From Eq. (3.1) we derive the condition RN must comply with to keep the PEN potential

rise below 50 V:

RR ≤

50V − 50

Eq. 3.2

3.1.4 Ground Fault on the Medium-Voltage Utility Distribution System

The enclosure of the utility transformer, being an ECP, needs to be earthed and therefore may be

connected to the system ground RN. Low- and medium-voltage systems, then, share the same

earth terminal where the PEN conductor originates. At the occurrence of a ground fault at the

transformer primary, the fault current by circulating through the earth and reclosing toward the

upstream source of the supply network energizes the system ground. RN reaches the potential VN

and so does the PEN conductor.

Consequently, persons in contact with low-voltage ECPs, remotely supplied by the “live” PEN,

are exposed to the whole earth potential VN during the utility fault-clearing time. In PME,

utilities, in order to ensure that the neutral potential due to primary faults is not dangerous, must

accordingly lower the neutral resistance RN. When it is impossible to decrease RN, the distributor

must alternatively separate the service PEN conductor from the transformer enclosure’s earth by

creating two distinct grounds.

3.1.5 Faults Phase-to-PEN in Low-Voltage PME Networks

Another cause of Energization of the PEN conductor may be the accidental contact with the

phase conductor in low-voltage distribution networks The resulting short circuit causes a

circulation of current back to the source through the PEN conductor. We assume to neglect the

fault current derived by the EXCPs at the user’s location, connected for equipotential reasons to

the PEN. Such current is greatly limited by the EXCPs’ resistance-to-ground, which is much

larger than the impedance of the PEN conductor. The equivalent fault-loop circuit is as shown

below

Taking the cross-sectional area of the PEN as half of the phase conductor (common situation),

the user’s ECPs will reach the following prospective touch voltage:

푉 = 푉 ×푍

푍 + 푍 = 푉 ×2푍

2푍 + 푍 =23 × 푉

3.1.6 Interruption of the PEN Conductor in PME

The accidental interruption of the PEN conductor causes all the ECPs supplied downstream of

the interruption to be energized at the line-to-line potential, even if healthy. PME have a much

large geographical extension and therefore the risk of the interruption of the PEN conductor and

of the Energization of the ECPs of more than one customer is higher. Hence, the PEN conductor

is usually placed below the Phase conductors to minimize the probability of its break .

zi ZPH

ZPEN

VPH

VST

Fig 3.3

The loss of the PEN conductor also triggers overvoltages. From Fig 3.4, where two users are

supplied by two different phases and the same PEN. The absence of the PEN as a return path

causes a voltage divider between the two users’ single-phase loads, which are now supplied by

the line-to-line voltage. This may cause the supply to each load to exceed the nominal value,

with great risk of overheating of the equipment and therefore of initiating fire.

3.1.7 Stray Currents

Unavoidable stray currents continuously circulate through the actual earth. As they depend on

the supplied loads, they are likely to escalate over time. Stray currents may produce interferences

among electrical systems by transferring potential rises to healthy systems, thereby exposing

persons to touch voltages

3.1.8 Stray Voltages

The neutral conductor, although earthed at the substation, may, in fact, be energized above

ground, where it enters the user’s premises. The reason being that supply cables employed to

power up the customers, composed of phase and PEN conductors, have finite impedance, which

may cause voltage drops along their runs

User A User B

L1

L2

PEN

L3

PE PE N N

Fig 3.4

The voltage drop along the PEN increases with its length and the contributions from the

customers; therefore, VNG at User B is greater than at User A. The neutral-to-ground voltage,

which may reach several volts, also exists between the neutral and the protective conductors.

In the case of a defect in the insulation between these two conductors, a low-resistance object

might bridge the gap between them, with the possible result of setting on fire any flammable

material eventually present.

3.1.9 Automatic disconnection for PME systems Principle

The automatic disconnection is achieved by overcurrent protective devices or RCD’s. Use of

RCDs on TN-C-S systems means that the protective conductor and the neutral conductor must be

separated upstream of the RCD. Any insulation fault to earth results in a phase to neutral short-

circuit resulting in high fault currents which allow for use of overcurrent protection but can give

rise to touch voltages exceeding 50% of the phase to neutral voltage at the fault position during

the short disconnection time. In order to ensure adequate protection, the earth-fault current:

퐼 = 표푟 0.8 × must be higher or equal to 퐼

Where; 푍 = earth fault current loop impedance

푍 = the faulty-circuit loop impedance

User A User B

L1

L2

PEN

L3

PE PE N N

Fig 3.5

VNG

IN

퐼 = current required to operate the protective device in the time specified

3.1.10 Specified maximum disconnection time

The IEC 60364-4-41 specifies the maximum operating time of protective devices used for the

protection against indirect contact:

For all final circuits with a rated current not exceeding 32 A, the maximum disconnecting

time will not exceed the values indicated in Table 3.1

For all other circuits, the maximum disconnecting time is fixed to 5s. This limit enables

discrimination between protective devices installed on distribution circuits

Table 3.1

Phase Voltage (VPH) Disconnecting Time (Ts)

50 < 푉 ≤ 120 0.8

120 < 푉 ≤ 240 0.4

240 < 푉 ≤ 400 0.2

푉 < 400 0.1

3.1.11 Protection by means of circuit-breaker

If the protection is to be provided by a circuit breaker, it is sufficient to verify that the fault

current will always exceed the current-setting level of the instantaneous or short-time delay

tripping unit (Im). The instantaneous trip unit of a circuit-breaker will eliminate a short-circuit to

earth in less than 0.1 second. In consequence, automatic disconnection within the maximum

allowable time will always be assured, since all types of trip unit, magnetic or electronic,

instantaneous or slightly retarded, are suitable: Ia = Im. The maximum tolerance authorized by the

relevant standard, however, must always be taken into consideration. It is sufficient therefore

that the fault current determined by calculation be greater than the instantaneous trip-setting

current, or than the very short-time tripping threshold level, to be sure of tripping within the

permitted time limit.

3.1.12 Protection by means of fuses

Ia can be determined from the fuse performance curve. In any case, protection cannot be achieved

if the loop impedance Zs or Zc exceeds a certain value. The value of current which assures the

correct operation of a fuse can be ascertained from a current/time performance graph for the fuse

concerned. The fault current as determined above must largely exceed that to ensure positive

necessary to ensure positive operation of the fuse. The condition to observe therefore is that:

퐼 <푉푍

3.1.13 Advantages of the PME Earthing System

The earthing system always provides a return path for faults in the LV grid. The

grounding conductors at the transformer and at all customers are interconnected. This

ensures a distributed grounding and reduces the risk of a customer not having a safe

grounding.

Lower earthing resistance of the PEN conductor.

Has the advantage that in case of an insulation fault, the fault voltages (touch voltages)

are generally smaller than in TT earthing systems. This is due to the voltage drop in the

phase conductor and the lower impedance of the PEN conductor compared with the

consumer earthing in TT systems.

No overvoltage stress on equipment insulation.

PME earthing system could work with simple over current protection.

High reliability of disconnection of a fault by over current devices (i.e. fault current is

large enough to activate the over current protection devices).

3.1.14 Disadvantages of the PME Earthing System Faults in the electrical network at a higher voltage level (MV) may migrate into the LV

grid grounding causing touch voltages at LV customers.

A fault in the LV network may cause touch voltages at other LV customers.

Potential rise of exposed conductive parts with the neutral conductor in the event of a

break of the neutral network conductor as well as for LV network phase to neutral and

phase to ground faults and MV to LV faults.

The utility is not only responsible for a proper grounding but also for the safety of

customers during disturbances in the power grid

Protection to be fitted in case of network modification (increase of fault loop impedance).

3.2 IT Grounding System

In IT (Isolation Terre) systems as mentioned in chapter 2, the power source, is not solidly

connected to earth, and therefore, is defined as ungrounded. Enclosures of ECPs, though, must

be grounded, individually, in groups or collectively.

The insulation of secondary sides of supply transformers from the earth may be obtained

through a high- resistance grounding resistor, typically in high-/medium-voltage systems. It is

not advisable, although not forbidden by technical standards, to distribute the neutral conductor

in order to facilitate its insulation from ground. However electrical systems cannot be completely

isolated from ground, even in the absence of any intentional connection to the earth, the system

is still coupled to ground through the distributed capacitances. By this way, system fixes the

neutral point and the voltages are not floating.

At the system frequency, cables and earth can be seen as armatures of a capacitor, whose

dielectric is the surrounding air. In addition, the resistance offered by the cable insulation to

ground, which has the magnitude of a few mega ohms per kilometer, constitutes another leaking

path in parallel to the system net capacitance. The leakage resistance is, indeed, very high and,

therefore, may be considered as an open circuit in parallel to the system capacitance

Figure 3.6.

3.2.1 Overvoltages Due to Faults in IT Systems

The main issue in IT systems is the possibility of overvoltages induced by ground faults.

3.2.1.1 First Fault in the IT earthing system

The occurrence of a ground fault causes the system capacitance to become unbalanced. The earth

electrode resistance RG, in fact, is in parallel to the capacitance of the faulty phase. Upon the first

fault the system evolves from a balanced three-phase capacitive load, with no neutral wire, to an

unbalanced capacitive-resistive load. Because of this unbalance, a potential difference VNG = VN-

VG, also referred to as neutral potential rise, appears between the point of neutral N at the source

and the ground G at the faulty ECP. The presence of VNG changes the voltage between the line

conductors and the ground(i.e.,V1G,V2G,andV3G) with respect to the systems voltages (i.e., V1N,

V2N, and V3N). The two sets of vector quantities identical in normal conditions will now differ.

The current circulating after the first fault, is mainly a small capacitive and will always go

undetected by overcurrent devices. In the case of indirect contact with a faulty ECP, persons are

exposed to the risk of electrocution. Cf increases with the cables’ extension and so does the

ground-fault current flowing through the person.

Figure 3.7

To avoid this hazard, the ECPs must be earthed, as previously anticipated. The presence of the

intentional ground lowers the prospective touch voltage to the potential drop across 푅 , with

evident benefit for the safety against indirect contact. If V = R I ≤ 50V, where I is the first-

fault current to ground, the automatic disconnection of supply is not necessary, as the ground

fault does not cause any hazard to persons and may persist within the ECP. However it is

essential to know that there is a fault and need to track and eliminate it promptly, before a second

fault occurs. To meet this need the fault information is provided by an Insulation Monitoring

Device (IMD) monitoring all live conductors, including the neutral. When the neutral is not

distributed (three-phase three-wire distribution), the following condition must be satisfied:

푍 ≤0.866푉

퐼

When the neutral is distributed (three-phase four-wire distribution and single phase distribution),

the following condition must be satisfied:

푍 ≤0.5푉퐼

3.2.1.2 Second fault in the IT earthing system

During an unresolved first fault to ground, a second fault involving a different phase might take

place. In this case, the phase-to-phase voltage drives the fault current, and an actual short circuit

occurs.

Figure 3.8

In this situation, at least one of the protective devices safeguarding the circuits will trip and

disconnect the supply. On the other hand, the second fault exposes persons to risk of

electrocution in the time frame the overcurrent devices take to trip. With reference to Fig. 3.10,

prospective touch voltage is calculated as; VST1 on ECP 1

퐼 =√3푉

푅 + 푅

푉 =√3푉

푅 + 푅 × 푅

The fault-loop has a driving potential which is not the phase voltage (240 V), but the line to line

voltage (415 V). It is clear that if RG2 were low when compared to RG1, persons in contact with

ECP 1 would be exposed to nearly the whole line-to-line voltage. To prevent these hazards, and

as already anticipated, the first fault should be resolved in the shortest possible time by the

maintenance team.

3.2.2 Insulation Monitoring Device

The IMD supervises the insulation reactance and/or resistance between the power lines and the

earth.

Figure 3.9

The IMD continuously monitors the impedance to ground (i.e., resistance and capacitive

reactance) by injecting both a d.c. and an a.c. current through the neutral point of the system.1 If

such impedance decreases below a predetermined value, due to a first fault to ground, an

audible/visual alarm will be initiated. Such alarm will alert the maintenance crew and will stay

on for the entire duration of the fault. Once the faulty circuit has been located and fixed,

operators will manually switch it off.

3.2.3 Protection Against Direct and Indirect Contact by Using RCDs in IT Systems

The additional protection, normally provided by RCDs in PME, is not effective in IT systems.

The fault current, in fact, cannot activate the RCD, because it flows entirely back through its

toroid via the system distributed impedance. The RCD does not sense any unbalance and,

therefore, cannot intervene as the nature of the fault-loop, in fact, prevents their proper operation

and renders their installation ineffective.

3.2.4 Protection Against Indirect Contact in the Event of a Second Fault to Ground

After the occurrence of the first fault to earth, the IT system is no longer ungrounded, because of

the accidental connection of the faulty phase to earth. In the event of a second fault involving a

different phase, the IT system “evolves” into TT or TN according to the earthing arrangement of

ECPs (i.e., individually or collectively).

3.2.4.1 ECPs Earthed Individually or in Groups If ECPs are earthed individually, or in groups, in the event of a second fault, the system

becomes TT and we are in the case in Fig. 3.10. Protection against indirect contact is achieved if

the following condition, applied to the generic ith ECP, is fulfilled:

푅 퐼 ≤ 50푉

The optimum protection against indirect contact in TT systems is carried out by RCDs. The fault

current circulating through the earth, due to the second fault may in fact be too low to operate

promptly the overcurrent devices. RCDs can clear the fault within the safe time required,

generally in correspondence of a ground current of at least five times their residual operating

currents.

The additional costs due to the necessity of RCDs, effective under second fault conditions in the

previous arrangement, and due to the installation of individual ground electrodes, usually induce

designers to collectively earth the ECPs to a single ground electrode.

3.2.4.2 ECPs Earthed Collectively to a Single Grounding System

The ECPs may be earthed collectively, that is, connected to one single grounding system. If the

neutral is not distributed, at the occurrence of a second fault, involving a different live conductor,

the system becomes TN.

In this case, the voltage between the line conductors, equal to 1.732 times the voltage between

line conductor and neutral, drives the fault current. The second fault may randomly occur in a

different circuit, which, for example, supplies an ECP remotely located with respect to the

location of the first fault. Thus, the fault impedance may be due to the contributions of line

conductors and protective conductors of different cross-sectional areas. This possibility renders

extremely challenging the prediction of the total fault-loop impedance in IT systems “evolved”

into a TN.

ECP 1 ECP 2

L1

L2

L3

IG Figure 3.10

To take into account the second fault in a different circuit, the protection against indirect contact

of persons touching one faulty enclosure is effective if the following condition is fulfilled:

√3푉2푍 ≥ 퐼

If the neutral conductor is distributed to loads, a first or second fault may involve this conductor.

In this case, the voltage Vph between the faulty line and neutral conductor will drive the fault

current. Condition for safe automatic disconnection of supply is:

푉2푍′ ≥ 퐼

Table 3.2 Maximum disconnection time in the IT earthing system (second fault).

Phase Voltage (V) VL=50V, Breaking time (sec) VL=25V, Breaking time (sec)

No Neutral With Neutral No Neutral With Neutral

120 0.8 5 0.4 1.0

240 0.4 0.8 0.2 0.5

400 0.2 0.4 0.06 0.2

ECP 1 ECP 2

L1

L2

L3

IG

Figure 3.11

3.2.5 Advantages of IT Earthing Arrangement

Continuity of supply. The main advantage of using the IT earthing system for network

operation is without doubt the continuity of supply it offers, as there is no need to trip on

the first fault Another of this system’s strong points is guaranteed safety against the fire.

Increased availability .This corresponds to an average electrical power availability that

is 91 times better in IT than in TN or TT. Consequently, preference is frequently given to

the IT earthing system for use in: hospitals, airport take-off runways, vessels, plants with

continuous manufacturing processes, laboratories, cold storage units, electrical power

plants.

Increased safety against the fire hazard. Electricity is often the cause of fire.

Standards, set the threshold for this risk at 500mA on an insulation. This value can be

considerably exceeded, particularly with stray currents that flow through building

structures when faults occur in the TN system. IT is the only earthing system that

monitors insulation of the neutral conductor, It is because the current of the first fault is

particularly low that the IT earthing system has been chosen for use in certain

establishments at risk from fire and explosion.

Less downtime on control and monitoring circuits. Even if safety of persons with

respect to the electrical hazard is guaranteed by the various earthing systems, or by use of

Safety by Extra Low Voltage (SELV), safety of persons with respect to mechanical

hazards may not be guaranteed in certain cases. More care must therefore be taken when

wiring such circuits in the TT and TN than in the IT system, as the latter warns the

operator of the incident (first insulation fault), thereby guarding against electrical and

mechanical hazards.

3.2.6 Disadvantages of IT Earthing Arrangement

The restrictions for using the IT system are linked to loads and networks.

Limits linked to loads With a high earth capacitive coupling (presence of filters). A

number of devices fitted with capacitive filters offer the same disadvantage, due to their

number, as very long networks when the IT system is used. These capacitive leakages

have a particularity, with respect to distributed capacity mainly due to network cables, i.e.

they can be unbalanced. Office computer equipment: micro-computers, monitors and

printers, concentrated on the same single-phase feeder, is an example of this. Typical

values are provided below.

Table 3.3 capacitive values for HF filters built into various devices.

Device Network/ Earth capacitance

Micro computer 20nF-40nF

UPS 40nF

Variable speed controllers 70nF

Fluorescent tubes 20nF

Limits due to the physical characteristics of networks. When an insulation fault

occurs, they can also cause flow of residual currents likely to generate nuisance tripping

“by sympathy” of the RCDs placed on very long or highly capacitive feeders thus use of

the IT system is thus advised against for very long networks, containing long feeders, for

example for electrical power distribution in a number of buildings at a distance from one

another.

Case of networks with replacement power supply. The fact that a network can be

supplied by several sources makes it necessary to detect the first fault and to trip on the

second fault, irrespective of the voltage source in operation.

3.3 Estimation of Power losses in PME

Because the neutral conductor is connected to earth, it forms a deliberate or intentional short

circuit and because the earth is a conductive mass though poor as it may be some current flows

through the this link to ground. This current is referred to as the stray current. To demonstrate

and quantify this phenomenon an experiment was performed.

For the quantification of the power losses the knowledge of the earth resistance was vital and so

a measurement of the earth resistance had to be done.

3.3.1 Conventional Test Method for Earth Resistance There are three basic test methods as noted below;

Fall-of-potential method, or three-terminal test.

Dead Earth method (two-point test).

Clamp-on test method

3.3.1.1Fall of Potential (wenner)Method

This three-terminal test is the method that can be carried out with three or four terminal earth

testers. Although four terminals are necessary for resistivity measurements, the use of either

three of four terminals is largely optional for testing the resistance of an installed electrode

Figure 3.13 Fall of potential method

With a three-terminal instrument, connect X to the earth electrode. With a four-terminal tester,

P1 and C1 terminals on the instrument are ‘jumpered’ and connected to the earth electrode under

test. The driven reference rod C should be placed as far from the earth electrode as practical; this

distance may be limited by the length of extension wire available, or the geography of the

surroundings. Leads should be separated and not run close and parallel to each other, to eliminate

mutual inductance. Potential reference rod P is then driven in at a number of points roughly on a

straight line between the earth electrode and C.

3.3.1.1.1 Simplified analysis for fall-of-potential test

This reading with P at 50% of the distance from the earth electrode to C is noted as R1.

Reference probe P is then moved to a location 40% of the distance to C. The reading at this point

is noted as R2. A third reading, R3, is made with P at a 60% distance. The average of R1, R2 and

R3 is calculated as RA. The maximum deviation from the average is then determined by finding

the greatest difference between individual readings and the average. If 1.2 times this percentage

is less than the desired test accuracy, RA can be used as the test result. If the result is not within

the required accuracy, probe C has to be placed farther away and the tests repeated.

3.3.1.2 Dead earth method

When using a four-terminal instrument, P1 and C1 terminals connect to the earth electrode under

test; P2 and C2 terminals connect to an all-metallic metal system (e.g. a water pipe system). The

water system could be any suitable metal structure or similar. With a three-terminal instrument,

connect X to the earth electrode, P and C to the pipe system, see Figure 3.3. If the water system

is extensive (covering a large area), its resistance should be low and is usually much less than

one ohm. You can then take the instrument reading as being the resistance of the electrode under

test.

Figure 3.14

It is, however important to note the limitations to this test method as follows:

The dead earth metal (e.g. water pipe) system must be extensive enough to have a

negligible resistance.

The dead earth metal (e.g. water pipe) system must be metallic throughout

The earth electrode under test must be far enough away from the dead earth (e.g. water

pipe) system to be outside its sphere of influence.

3.3.1.3 The clamp-on ground tester

Unfortunately, the Fall of Potential method also comes with several drawbacks:

It is extremely time consuming and labor intensive.

Individual ground electrodes must be disconnected from the system to be measured.

There are situations where disconnection is not possible.

The clamp-on ground tester performs a ‘stake less’ test which is a ground resistance test

performed without disconnecting the ground. Based on Ohm’s Law (R=V/I), the stake less test

induces a known voltage in a loop circuit that includes ground, measures resultant current flow

and calculates the loop resistance of the circuit. Four pole earth testers also perform a stake less

test; however they use two clamps, a voltage clamp (V clamp) and a current clamp (I clamp), and

keep the clamps separate to prevent interaction between the two. All elements of the loop are

measured in series. The method benefits from low return path resistance.

Fig 3.15 Basic clamp-on ground testing methodology

All elements of the loop are measured in series. The method benefits from low return path

resistance, i.e. where the ground resistance is large compared to the effective return loop

resistance. For a six rod system the resistance-to-ground measure at R6 is:

Rloop = Vloop/Iloop = R6 + (1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5))

3.3.2 Proposed Method In this project a new method for the estimation of the earth electrode resistance was formulated.

The method employs the use of a simple electronic circuit as shown below to measure the earth

resistance.

Figure 3.16 Set up for the measurement of the earth electrode resistance

With switch off:

푉 =푉20 × 10 = 0.5푉

With switch on:

The voltage across the resistor is measured as 푉 and the current flowing to earth calculated as;

퐼 =푉푅

Then change in voltage can be worked as;

∆푉 = 푉 − 푉

This change in voltage is due to the earth resistance 푅 , to calculate the electrode resistance the

equations representing the earth fault current as shown below

∆푉푅 =

푉푅

From which

푅 =∆푉 × 푅푉

푉 , 50Hz

푅

푅 푅 푅 = 10Ω,14

푊푎푡푡푠

푉

푉

With 푅 known the dissipated power can be estimated as follows;

푃 = 푖 푅

4.0 EXPERIMENTAL RESULTS

A. The following results were obtained from the set up to measure the earth electrode resistance

Table 4.1 Switch OFF

푉 (V) 푉 (V) 푉 (V) 푉 = 푉 (V) 퐼

2.7 1.40 0 1.36 0.136

2.6 1.35 0 1.29 0.129

2.5 1.30 0 1.24 0.124

2.4 1.25 0 1.20 0.120

2.3 1.20 0 1.15 0.115

2.2 1.10 0 1.10 0.110

2.1 1.05 0 1.05 0.105

2.0 1.00 0 0.99 0.099

Table 4.2 Switch ON

푉 (V) 푉 (V) 푉 (V) 푉 (V) ∆푉(V) 퐼 (A) 푅 (Ω)

2.7 1.36 1.22 0.146 1.254 0.0146 85.89

2.6 1.31 1.19 0.140 1.210 0.0140 86.43

2.5 1.26 1.14 0.136 1.164 0.0136 85.59

2.4 1.21 1.09 0.131 1.119 0.0131 85.42

2.3 1.15 1.04 0.125 1.075 0.0125 86.00

2.2 1.10 0.99 0.120 0.980 0.0120 81.67

2.1 1.05 0.95 0.115 0.885 0.0115 76.96

4.1 Analysis The average earth resistance was obtained as

푅 =85.89 + 86.43 + 85.59 + 85.42 + 86.00 + 81.67 + 76.96

7 = 83.99 ≈ 84Ω

The relative value of earth current to the current that flows in the circuit when isolated from earth

was calculated as;

퐼∗ = at different source voltages and tabulated as shown below

Table 4.3

푉 퐼∗ 퐼∗ 푎푠 %

2.7 0.1081 10.8

2.6 0.1085 10.9

2.5 0.1097 11.0

2.4 0.1092 10.9

2.3 0.1087 10.9

2.2 0.1091 10.9

2.1 0.1095 11.0

Giving the average earth current as a percentage of the isolated circuit current as

퐼∗(%) =10.8 + 10.9 + 11.0 + 10.9 + 10.9 + 10.9 + 11.0

7 = 10.9%

The instantaneous ac electrical power that is dissipated to earth due the system grounding can be

estimated as;

푝 = 푖 푅

푝 = (퐼 sin휔푡) 푅 = (0.11퐼 sin휔푡) 푅

The curve of the earth current as a function of the source voltage was plotted and showed a

linear relationship as shown in figure 4.1 from which table 4.4 was deduced.

Figure 4.1

Table 4.4

x f(x) f'(x) f''(x) 0 0 0 0

40 0.2168 0.0054 0 80 0.4336 0.0054 0

120 0.6503 0.0054 0 160 0.8671 0.0054 0 200 1.0839 0.0054 0 240 1.3006 0.0054 0

Table 4.5 Comparison of the Performance of the two systems

Characteristics TN-C-S IT 1 IT 2 Comments

ELECTRICAL

Fault Current -- + -- Only the IT system offers virtually negligible first-fault

currents

Fault voltage - + - In the IT system, the touch voltage is very low for the first

fault, but is considerable for the second

Touch voltage - + - In the TT system, the touch voltage is very low if system is

equipotential, otherwise it is high

PROTECTION

Indirect contact + + + All SEAs are equal if properly implemented

Persons with emergency sets - + - Systems where protection is ensured by RCDs are not sensitive

to a change in the internal impedance of the source

Protection against fire (with an

RCD) + + + All SEAs in which RCDs can be used are equivalent.

OVERVOLTAGES

Continuous overvoltage + + +

Transient overvoltage - + - Systems with high fault currents may cause transient

overvoltages

Overvoltage if transformer

breakdown

(primary/secondary)

+ + +

ELECTROMAGNETIC

COMPATIILITY

Immunity to nearby lightening

strikes + + +

Continuous emissions of an

EM field - + + Connection of the PEN to the metal structures of the building

is conducive to the continuous generation of electromagnetic

fields

Transient non-equipotentiality

of the PE - + - The PE is no longer equipotential if there is a high fault current

CONTINUITY OF SERVICE

Interruption for first fault - + + Only the IT system avoids tripping for the first insulation fault

Voltage dip during insulation

fault - + - Both TN-C-S and IT (2nd fault) systems generate high fault

currents which may cause phase voltage dips

INSTALLATION

Special devices + - - The IT system requires the use of IMDs

Number of earth electrodes + -/+ -/+ The IT system offers a choice between one or two earth

electrodes

Number of cables + - - Only the TN-C system offers, in certain cases, a reduction in

the number of cables

MAINTENANCE

Cost of repairs -- - -- The cost of repairs depends on the damage caused by

the amplitude of the fault currents

Installation damage - ++ - Systems causing high fault currents require a check on

the installation after clearing the fault

5.0 CONCLUSION

In terms of the protection of persons, the three system earthing arrangements (SEA) are

equivalent if all installation and operating rules are correctly followed. Consequently, selection

does not depend on safety criteria. It is by combining all requirements in terms of regulations,

continuity of service, operating conditions and the types of network and loads that it is possible

to determine the best system.

From the findings of the experiment it is true that there is electrical energy dissipated to ground

and is dependent on the magnitude of the current drawn by the load as well as the earth

resistance. Also evident is that the isolated systems portray superior qualities to the PME systems

except for the likelihood of high overvoltages in the occurrence of a second fault or

simultaneous faults. Given that the requirements to install the two system earthing arrangements

is the almost the same, except for high basic insulation levels in IT, and that both systems

guarantee automatic disconnection of the supply, the IT system earthing arrangement provides a

better solution.

5.2 REFERENCES

[1] Schneider Electric-Electrical Installation Guide 2008-LV Distribution

[2] IEC 60364-1:2005(E), Electrical installation of buildings, Part 1: General principles.

[3] IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems

,IEEE Std. 142, USA, 2007

[4] Schneider Electric Industrial electrical network design guide T & D 6 883 427/AE

[5] Dr. Massimo A.G. Mitolo-Electrical safety of Low voltage systems pp.143-145

[6] V.K. Mehta & Rohit Mehta Principles of Power System Chapter 26 pg 587-599

[7] Prof Henryk Markiewicz & Dr Antoni Klajin Power Quality Application Guide Earthing &

EMC

[8] Megger Getting Down to Earth A practical guide to earth resistance testing pg 7-33

[9] Schneider Electric - Electrical installation guide 2008-Protection against electric shocks

[10] Bernard Lacroix and Roland Calvas - Cahier Technique Merlin Gerin n° 173 Earthing

systems worldwide and evolutions

[11] Cahier technique no. 178 The IT earthing system (unearthed neutral) in LV

[12] Dr. C.R. Bayliss CEng FIET and B.J Hardy ACGI CEng FIET Transmission and

Distribution Electrical Engineering Third Edition, Elsevier Ltd, 2007- Chapter 8