i

DESIGN AND CONSTRUCTION OF A DIGITAL

UNDERGROUND CABLE FAULT LOCATOR

BY

Itodo Friday Victory

UE/10104/07

FEBRUARY 2012

ii

DESIGN AND CONSTRUCTION OF A DIGITAL UNDERGROUND

CABLE FAULT LOCATOR

BY

ITODO FRIDAY VICTORY

UE/10104/07

A PROJECT REPORT PRESENTED TO

THE DEPARTMENT OF ELECTRICAL AND ELECTRONIC

ENGIINEERING, UNIVERSITY OF AGRICUTURE, MAKURDI,

BENUE STATE, NIGERIA.

IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE

AWARD OF THE DEGREE OF BACHELOR OF ENGINEERING

(B.Engr.) IN ELECTRICAL AND ELECTRONIC ENGINEERING,

iii

FEBRUARY 2012

DECLARATION

I declare that this project work described in this report represents my original work and has not

been submitted to any university or similar institution for any degree.

ITODO, FRIDAY VICTORY --------------------------------------------

UE/10104/07 Signature/Date

iv

APPROVAL PAGE

This is to certify that this project was carried out by ITODO, FRIDAY VICTORY

(UE/10104/07) which met the requirement of the College of Engineering; University of

Agriculture, Makurdi, for the award of Bachelor in Engineering (B.Engr.) Degree.

Approved by:

Project Supervisor:

(Dr J. A. ENOKELA) Signature/Date

Project Coordinator:

(Engr N. S. Tarkaa) Signature/Date

Head of Department:

(Dr Jonathan A. Enokela) Signature/Date

External Examiner

Signature/Date

v

DEDICATION

This project is dedicated to God almighty for granting me the knowledge, support and protection

throughout my whole life to this moment and to my late father Mr. Itodo Friday.

vi

ACKNOWLEDGMENT

Gods support through this career cannot be quantified; I owe everything to Him for His infinite

mercy and immeasurable love over me. If not because of Him, it would not have been possible

for me to successfully complete this program.

The contribution of my mother Mrs. Mary Onoja Itodo is all-directional, it therefore pleases my

heart to give her thanks and may the almighty God bless her for her enamors support.

It gives me a great joy to use this opportunity to thank my able supervisor, Dr. J.A.Enokela, of

Electrical/Electronic Engineering Department, University of Agriculture Makurdi for his careful

supervisory roles played in this work. His advice and guidance to this work can never be

forgotten. I appreciate all staff of Electrical/Electronic Engineering Department for their

individual and collective contributions to my academic up-bringing.

I thank Dr. Onoja.F.Ameh whom has stood by me from my childhood till this defining moment

of my life cum Mr. Kennedy Iwundu and Mr. Simon Onuh for their immense and immeasurable

contribution to my life in all entireties.

I am grateful to my beloved family members: my uncle Mr. J. A.Onoja, for his fatherly advice

and support. Appreciation is extended to my brother Mr.U.F. Itodo and my good friend Miss

Berikisu Musa for their unending contribution to the success of this work

Finally, I would like to appreciate every other person who contributed in one way or the other to

the successful completion of this project work.

The acknowledgement is not exhaustive but for time and space, I say thank you all.

vii

ABSTRACT

Frequent faults in underground cables due to breakdown of the paper or polymer plastic

insulation due to chemical action, reaction or even poor workmanship during installation and

the difficulty in locating the approximate fault area has long been a serious engineering

problem. Most underground faults are located by unearthing the entire length of armoured cable

to enable visual inspection to be carried out. In case where visual inspection is not helpful, the

entire length of such a cable is replaced. This method is not only expensive but also, long outage

of a cable from service results in heavy loss of revenue to the power distribution company,

production loss of industries as well as unpleasant condition to the general public, since the

consumers are left without electricity for the whole period taken to unearth the cable and carry-

out necessary repairs. To salvage these challenges, an efficient instrument capable of locating

the fault in minimum possible time and restoring the supply, that is, the digital underground

cable fault locator is designed and constructed. This research work will help in easy

identification and location of underground cable fault without unearthing the entire length of

cable before repair or replacing the entire cable due to difficulties in detecting or locating the

fault. It will also help to reduce loss of revenue due to damage in trying to locate or detect fault

and long power down time will be reduced as minimum time will be used to restore supply. In

the design and construction of digital underground cable fault locator, the locator circuit is

designed to use the sectionalizing test method to locate the fault distance. Its main limitation is

the fact that measurement and monitoring must be conducted at regular uninterrupted intervals

of 10m following the underground mapping of the trouble spot. The device is program based

and uses microcontroller as the interface between the input section of the device (detector circuit

and the analogue to digital converter which comprises the comparators and the pull-up resistor

configured in the ACTIVE LOW arrangement) and the output of the device which is a seven

segment LED display.

TABLE OF CONTENT

viii

Cover page - - - - - - - - - - i

Title page - - - - - - - - - - ii

Certification- - - - - - - -- - - iii

Approval page - - - - - - - - - - iv

Dedication - - - - - - - - - - v

Acknowledgement - - - - - - -- - - vi

Abstract - - - - - - - -- - - vii

Table of Content - - - - - - - - - viii

List of Figures - - - - - - - - - xi

List of Tables - - - - - - - - - - xii

CHAPTER ONE Introduction - - - - - - - 1

1.1 General Overview - - - - - - - - - 1

1.2 Statement of research problem - - - - - - - 2

1.3 Significance of the Research - - - - - - - 2

1.4 Aims And Objectives Of The Study - - - - - - 3

1.5 Scope of the Research - - - - - - - - 3

1.6 Review 0f Literature - - - - - - - - 3

CHAPTER TWO Background Of the Design

2.1 Anatomy Of Underground Distribution Cables - - - - - 9

2.2 Aging Mechanisms In Underground Cables - - - - - - 11

2.3 Underground Cable Fault Location Methods - - - - - - 12

2.3.1 Thumping Method - - - - - - - - 13

2.3.2 Sectionalizing- - - - - - - - - - 13

2.4 Definition of underground cable Faults - - - - - 13

2.4.1 Earth fault- - - - - - - - - 15

ix

2.4.2 Short circuit fault- - - - - - - - 16

2.4.3 Open circuit fault-- - - - - - - - 16

2.5 The Block Diagram of the Circuit- - - - - - 17

2.5.1 The locator circuit- - - - - - - - 17

2.5.2 Analog To Digital Converter- - - - - - 18

2.5.2.1 Lm393 Comparator - - - - - - - 19

2.5.3 Pic16f84a Microcontroller- - - - - - - 21

2.5.3.1 Pins on PIC16F84A microcontroller have the following meaning- - 22

2.5.3.2 Memory Organization - - - - - - - 24

2.5.3.3 Microcontroller Board - - - - - - 24

2.5.3.4 PIC 16F84A least circuit - - - - - - 25

2.5.3.5 Microcontroller PIC16F84A knows several sources of resets - - 26

2.5.3.6 The Registers- - - - - - - - - 27

2.5.3.7 Programming the Microcontroller.- - - - - - 28

2.5.3.7 Status Register - - - - - - - 29

2.5.3.8 TRISA and TRISB - - - - - - - 29

2.5.3.9 PORTA and PORTB - - - - - - - 30

2.5.3.10 Program Structure for PIC 16F84A - - - - - 32

2.5.3.11 Programming Concepts- - - - - - - 34

x

2.5.3.12 PIC Instruction Set BTFSS - - - - - - 36

2.5.3.13 The XOR-Trick. - - - - - - - 37

2.5.3.14 Power requirements - - - - - - - 38

2.5.4 Seven Segment Display Unit - - - - - - 38

2.6 LM7805 Voltage Regulator. - - - - - - 39

CHAPTER THREE DESIGN AND ANALYSIS

3.1 Detector Circuit. - - - - - - - 42

3.2 Analogue To Digital Converter - - - - - 43

3.3 Inputs - - - - - - - - - 46

3.4 Output - - - - - - - - - 48

3.4.1 Seven Segment Display - - - - - - 49

3.5 Program for the Underground Cable Fault Locator - - - 53

CHAPTER 4 PROGRAM TESTINGAND HARDWARE CONSTRUCTION

4.1 Programming the PIC (Peripheral Interface Controller) microcontroller - 56

4.2 MPLAB - - - - - - - - - - 57

4.3 Writing The Program - - - - - - - - 60

4.4 Simulator - - - - - - - - - 61

4.5 MPLAB Programming - - - - - - - 64

4.6 The Burning Process - - - - - - - 69

4.7 Summary Of Project Construction And Results- - - - - 72

4.7.1 Program Debugging- - - - - - - - - 73

xi

4.7.2 Burning The Program- - - - - - - - 74

4.7.3 Copy Of The Burnt Program- - - - - - - - 77

4.7.4 Breadboarding And Testing- - - - - - - - 81

CHAPTER FIVE CONCLUSIONS AND FUTURE WORK

5.1 Conclusions - - - - - - - - - 82

5.3 Future Work - - - - - - - - - 82

Reference - - - - - - - - - - 3

xii

LIST OF FIGURES

Figure

Page

Fig.2.1 Anatomy of typical single phase underground cable - - - - 9

Fig. 2.2: Single line earth fault - - - - - - - - 15

Fig. 2.3: Double-line to earth fault Earthing - - - - - - 15

Fig 2.4: Three-line to earth fault - - - - - - - - 15

Fig. 2.5: Single-line to earth through a resistance fault. - - - - - 16

Fig. 2.6: Double line short circuit fault- - - - - - - 16

Fig. 2.7: Three phase short circuit fault - - - - - - - 16

Fig.2.8; Shows the block diagram of the digital underground cable fault locator - - 17

Fig. 2.9: The detector circuit - - - - - - - - 17

Fig. 2.10: 3-bit flash analog to digital converter- - - - - - 18

Fig. 2.11: Conversion of analogue signal to digital signal - - - - - 19

Fig. 2.12: Low power low offset voltage dual comparator - - - - - 20

Fig. 2.13; The pin diagram of PIC16F48A microcontroller - - - - - 21

Fig.2.14; The block diagram of PIC16F84A microcontroller - -- - - 23

Fig.2.15; PIC16F84A microcontroller board arrangement - - - - - 24

Fig.2.16; PIC16F84A RC oscillator connection - - - - - - 25

Fig.2.17; PIC16F84A reset circuit. - - - - - - - - 26

Fig.2.18; LED display anatomy. - - - - - - - - 39

Fig.2.19; pin-out of Lm7805 voltage regulator - - - - - - 40

Fig.3.1; Block diagram of digital underground cable fault locator (DUCLF) - - 41

Fig.3.2; Detector circuit - - - - - - - - - 42

Fig.3.3; Analogue to digital converter of the fault locator - - - - - 44

xiii

Fig.3.4; The primary configuration of the project microcontroller - - - - 46

Fig.3.5; Active Low/High arrangements - - - - - - - 47

Fig.3.6; Input Circuitry to Port A of PIC16F48A - - - - - - 48

Fig.3.7; microcontroller port B connections to the seven segment display - - - 49

Fig.3.8; Schematic diagram of digital underground cable fault locator (DUCFL) - - 52

Fig.4.1; MPLab IDE (v8.60) window - - - - - - - 58

Fig.4.2; MPLab IDE project welcome window - - - - - - 58

Fig.4.3; MPLab IDE project wizard window - - - - - - 59

Fig.4.4; MPLab IDE project making window - - - - - - 59

Fig.4.5; Finish window of project Initialization of MPLab IDE - - - - 60

Fig.4.6; MPLab IDE project creation window - - - - - - 60

Fig.4.7; MPLab IDE Debug pane window - - - - - - - 61

Fig.4.8; MPLab IDE special function registers window - - - - - 63

Fig.4.9; MPLab IDE watch window - - - - - - - - 63

Fig.4.11; MPLab IDE build option window - - - - - - - 64

Fig.4.12; MPLab IDE compile setting window - - - - - - 65

Fig.4.13; MPLab IDE header files window - - - - - - - 65

Fig.4.14; MPLab IDE program target window - - - - - - 65

Fig.4.15; MPLab IDE reset tool window - - - - - - - 66

Fig.4.16; MPLab IDE naming project window -- - - - - - 66

Fig.4.17; MPLab IDE project directory window - - - - - - 67

Fig.4.18; MPLab IDE check box window - - - - - - - 67

Fig.4.19; MPLab IDE menu window - - - - - - - 67

xiv

Fig.4.20; MPLab IDE .asm extension window - - - - - - 68

Fig.4.21; MPLab IDE assembly code editor window - - - - - 68

Fig.4.22; MPLab IDE .ikr file window - - - - - - - 68

Fig.4.23; MPLab IDE simulation/debug window - - - - - - 69

Fig.4.24; MPLab IDE project file directory window - - - - - 70

Fig.4.25; Dataman intelligent universal programmer - - - - - 71

Fig.4.26; Dataman programmer software window - - - - - - 72

Fig.4. 27; Screenshot of the project compilation - - - - - - 72

Fig.4. 28; Screen shot of project single step debugging - - - - - 73

Fig.4.29; Screen shot of STATUS register watch window -- - - - - 73

Fig.4.30; Screenshot of TRISA register watch window - - - - - 74

Fig.4.31; Screenshot of created files from project build - - - - - 74

Fig.4.32 The programmer interface with the microcontroller - - - - 75

Fig.4.33; Screenshot of Dataman software buffer radio window - - - - 76

Fig.4.34; Screenshot of loaded HEX file on Dataman buffer - - - - 76

Fig.4.35; Screenshot of Dataman software programming progress bar - - - 77

Fig.4.36; picture of the completed project - - - - - - - 81

xv

LIST OF TABLES

Table Page

Table 2.1; Summary of ageing mechanism in cables - - - - - - 12

Table 2.2; Register file map of PIC16F84A - - - - - - - 28

Table 2.3; The status register of PIC16F84A - - - - - - 29

Table 2.4; graphical illustration of port A - - - - - - - 30

Table.2.5; Graphical illustration of TRISA- - - - - - - 32

Table 2.6; the used instruction set for the Project. - - - - - 35

Table 2.7; Op-Code Field Description - - - - - - - 35

Table 2.8; Flag check in the Status Register - - - - - - 37

Table 2.9; XOR Truth Table - - - - - - - - - 37

Table 3.1 On/Off status in the common cathode abcdefg seven segment display for 0-9 50

Table 3.2; The hexadecimal encodings for displaying the digits 0 to F - - - 51

1

CHAPTER ONE

INTRODUCTION

1.1 GENERAL OVERVIEW

Study of cable failures and development of accurate fault detection and location methods has

been interesting yet challenging research topics in the past and present. Fault detection entails

determination of the presence of a fault, while fault location includes the determination of the

physical location of the fault. Accurate permanent fault detection techniques and relatively

accurate fault location methods have been developed for overhead distribution systems.

However, fault detection and location technology for underground distribution systems is still

in developing stages. From a macroscopic perspective, cable faults refer to the abnormalities

associated with any type of deterioration phenomena manifested in the cable electrical

signals.

The power industry has been developing in a challenging and competitive environment due

to, the ongoing restructuring and deregulation. This structural change has required the electric

utilities to reduce operating costs and optimize usage and maintenance of electrical assets

without sacrifice the quality and reliability of the power delivered to the customers.

Underground distribution systems are valuable assets of electric utilities, which supply power

to the end customers at low voltages. Many of the system components, particularly

underground cables, fail over time, in part due to the deterioration of the insulating materials

used in their structure. Studies reveal that cable failure rates in power systems continue to

worsen as the cable ages [1].

In the past, analogue system was used to detect and locate faults. However, the need for

improvement has made it necessary to shift from analogue to digital system of fault detection

and location. This shift requires developing new tools and methods to detect and locate faults

of underground distribution systems including power cables.

2

In addition to degrading system reliability, cable failures cost substantial amounts of money

for the utilities as replacing or repairing a cable is a very costly process. Quick detection and

location of cable faults within a minimum time would undoubtedly be a great benefit to the

utilities enabling them to avoid catastrophic failures, unscheduled outages, and thus loss of

revenues. This project presents a tool, digital underground cable fault locator (DUCFL) and a

methodology for such a location system known as sectionalisation.

1.2 STATEMENT OF RESEARCH PROBLEM

Frequent faults in underground cables due to the breakdown of the paper or polymer plastic

insulation due to chemical action, reaction or even poor workmanship during installation and

the difficulty in locating the approximate fault area have long been a serious engineering

problem. Most underground faults are located by unearthing the entire length of armoured

cable to enable visual inspection to be carried out. In case where visual inspection is not

helpful, the entire length of cable is replaced. This analogue method is not only expensive,

but also, long outage of a cable from service results in heavy loss of revenue to the power

distribution company, production loss of industries as well as unpleasant conditions to the

general public, since the consumers are left without electricity for the whole period taken to

unearth the cable and carry out necessary repairs. To salvage these challenges, an efficient

instrument capable of locating the fault in minimum possible time and restoring the supply is

needed. This research is aimed at designing and constructing a digital underground cable fault

locator to solve this problem.

1.3 SIGNIFICANCE OF THE RESEARCH

Underground cables constitute the heart of any distribution system such as the power and

communication utilities.

This research work will help these utilities in easy identification and location of underground

cable fault without unearthing the entire length of the cable before repair or replacing the

entire cable due to difficulties in detecting or locating the fault.

The research will also help to reduce the loss of revenue due to damage in trying to locate

or detect faults and long power down time will be reduced as minimum time will be used to

restore supply.

3

Thus, the research will help to enhance the quality and reliability of the power or signal

delivered to the customers and also reduces cost of services to the customers.

This project will also help researchers for future work.

1.4 AIMS AND OBJECTIVES OF THE STUDY

At the end of this study, a digital underground cable fault locator would have been

Designed

Constructed

Able to locate underground cable fault and display the results using digital methods.

1.5 SCOPE OF THE RESEARCH

The research would be based on designing and constructing a digital tool or instrument that is

capable of locating an underground cable fault.

1.6 REVIEW OF LITERATURE

Electrical cable is composed of one or more electric conductors covered by insulation and

sometimes protective sheath, used for transmitting electric power or the impulse of an electric

communications system.

For electric power transmission, three-wire cables sheathed with lead and filled with oil under

pressure are employed for high-voltage circuits; secondary distribution lines usually employ

insulated single-conductors cables. In residential electric wiring, B-X cable is often used.

This type of cable contains two insulated conductors, which are wound with additional layers

of insulation and covered with a helically wound strip of metal for protection [5].

Conductor material and insulation type will be specified. Restricting extensions of existing

systems to a specific conductor material and insulation type in order to match an existing

cable type is permitted only when a need has been established.

Neutral cables, where required, will be installed with 600V insulation unless concentric

neutral cable is used. In duct lines, neutrals will be installed in the same conduit with

associated phase cables.

a. Conductor material. Since underground conductors are continuously supported, soft-drawn

copper or aluminium alloy 5005 provides adequate strength. However, the selection of copper

4

or aluminium will be justified based upon an analysis using life, environmental, and cost

factors. The need for mechanical flexibility requires that conductors be stranded, and the

NEC makes this mandatory for cables larger than No. 8 AWG installed in raceways. The

installation of conductors larger than 500 kcmil is not economical, and such large cables

should be used only under exceptional circumstances. Large ampacities can be served by

parallel or multiple circuits. Three 15 kV, single conductor, non-metallic-jacketed cables

larger than No. 4/0 AWG will require use of ducts larger than the standard four-inch size (i.e.

three single conductor cables making up a three-phase circuit and each having individual

overall diameters greater than 1.25 inches will need to be installed in a duct larger than four

inches). One three conductor cable is more costly than three single conductor cable, and use

of multiple-conductor cable will be restricted to special conditions.

Metallic-armoured cable is such a special condition.

b. Insulation and jacket material. The type of insulation used will be dependent upon the

voltage level and type of service required. Factors affecting selection will be the effects of the

surrounding environment, the importance of the load in regard to operation of the installation,

and whether peak loading is continuous or intermittent.

(1) Medium-voltage cable. Cable will be specified as 133 percent insulation level

(ungrounded) which allows greater margin for voltage surges, insulation deterioration, and

fault clearing time than does the use of the 100 percent insulation level (grounded). When

marking guide specifications, refer to NFPA 70, which currently limits the minimum size to

No. 1 AWG at 133 percent insulation for 15 kV to 28 kV systems and No. 2 AWG at 133

percent insulation for 8 kV to 15 kV systems. Medium-voltage cable above 3 kV will be

shielded.

(a) Non-metallic-jacketed cable. Non-metallic jacketed cable will be used, except where

circumstances warrant other coverings. Insulation will be either cross linked-polyethylene

(XLP) for short life requirements, or ethylene-propylene-rubber (EPR) for long life

requirements, in accordance with NEMA WC-7 and WC-8.

This option allows the use of cables which are available as stock items in small quantities. In

some environments, however, the selection of other jacket materials may be necessary

because properties of some jacket materials may not provide adequate cable protection.

Special shielding or coverings will not be specified, unless the designer has checked that the

5

footage installed for each different cable diameter is large enough for manufacturers to make

the special runs required.

(b) Metallic-armoured cable. Armoured cable is justified only when cable is installed under

water (submarine cables) and sometimes when installed in cable trays or trenches. Armored

cable will have XLP or EPR insulation covered with a thermoplastic core covering and then

provided with interlocked-metal tape armour. A non-metallic jacket is required for

underground installations, where corrosion and moisture protection is required, for

installations in outdoor cable trays, or for submarine cables. Submarine cable may also

require a lead covering. Cable having steel armour will be three-conductor type to avoid the

high hysteresis and eddy current losses which can result when single-conductor cable is used.

(c) Lead-covered cable. Lead-covered cables will not be used, unless extenuating

circumstances prevail such as for submarine cable. The lead covering is both more costly and

more difficult to handle. The use of laminated insulation such as for paper-insulated-lead-

covered (PILC) or for varnished-cambric-lead-covered (VCLC) instead of the solid or

extruded dielectrics such as cross linked-polyethylene (XLP) or ethylene propylene- rubber

(EPR) is not approved. In addition, these cables have lower temperature ratings.

(2) Low-voltage cables. Cables suitable for below grade installations are listed in the NEC.

Insulation will be either XLP (NEMA WC 7) or EPR (NEMA WC 8) and jackets or other

protection will be in accordance with the applicable Underwriters Laboratories (UL)

specification covering that NEC type. Use of metal-clad (MC) cable will be limited as

previously discussed for metallic armoured cable. The use of the less expensive Moisture-

and-Heat-Resistant Thermoplastic (THWN) or Moisture-and-Heat-Resistant Cross-Linked

Synthetic Polymer (XHHW) is not recommended for underground work as their thinner

insulation has been designed for interior usage.

Moisture-and-Heat Resistant Thermoplastic (THW) wiring does have the same thickness of

insulation as Heat-Resistant Rubber (RHH)/Moisture-and-Heat Resistant Rubber

(RHW)/Underground Service-Entrance (USE) wire, but polyvinylchloride insulation is

considered to have only fair electrical and mechanical insulation properties as compared to

the excellent properties exhibited by XLP and EPR insulation. UF cable may have a greater

insulation thickness, but some sizes have a lower ampacity rating than does USE cable.

6

c. Cable ampacity. The current carrying capacities of cable will be in accordance with

ampacities given in the NEC and IEEE/ICEA publications. There are many factors taken into

account in determining these allowable ampacities such as operating temperatures, soil

effects, shielding losses, and conductor configurations, but the variables which cause the

most concern are circuit loading and location in a duct bank. Because of load diversity, peak

demands for cables in a duct bank will not occur concurrently in most cases. This diversity

factor will be taken into account when computations expected heat build-up in a duct bank.

Heat dissipation from a cable is also influenced by the position occupied by the cable in a

duck bank. Cables in duck bank corners dissipate heat more effectively than cables in interior

ducts, because of the greater soil dissipating area and the smaller heat contribution from

neighbouring cables. Calculations of the position effect indicate that, to equalize operating

temperatures, full load ratings of cables appropriate for isolated (one-way) ducts should be

decreased for multiple duct banks. For example, in an eight-way-duct bank the recommended

full-load percentage decrease for each corner duct is 95 percent and for each interior duct is

83 percent giving an average load percentage decrease of 89 percent. This rerating still allows

provision for loads in excess of the normal feeder capacity usually found on military

installations, as the summation of feeder capacities is generally from three to eight times the

overall capacity of a main electric supply station.

In communication systems, cables commonly consist of numerous pairs of paper-insulated

wire, encased in a lead sheath; the individual pairs of wire are intertwined to minimize induce

interference with other circuits in the same cable. To avoid electrical interference from

external circuits, cables used in radio broadcasting are often shielded with a winding of metal

braid, which is grounded. The development of the coaxial cable was an important advance in

the communications field. This type of cable consists of several copper tubes; each tube

contains a wire conductor that extends along its centre. The entire cable is sheathed in lead

and is generally filled with nitrogen under pressure to prevent corrosion. Because the coaxial

cable has a broad frequency range, it is valuable in transmission of carrier-current telephone.

For safety purpose, cables are laid underground; originally channel will be dug into the

ground along the route of a pre-planned network where a four-inch earthenware pipe would

be laid. Depending on the needs of the network, either a wide or thin cable would be pulled

through the pipe by rope; leaving some spare space for future use, spare rope was left in the

ducts for future cables, which today are frequently fibre optics. Fenect and other techniques

(sometimes the old ways are the best) blocked cable ducts can be a problem, especially if the

7

blockage is 60 feet away from an opening. The best way to shift the obstruction is to attack

the duct like blocked drain with a very long pole.

Means for a method of laying electrical cable underground and providing a high conductivity

environment therefore, consisting of a cable flow machine plus means to fill the evacuated

tube in which the cable is laid and buried 600mm below the surface. The diameter of the duct

depends on what size cable that will be installed. Cable faults such as short circuit faults,

open circuit faults, earth faults and high resistance joint and splices are traced by faulting

circuits indicators.

In the design of digital underground cable fault locator, the detector reads up the resistance of

the faulty underground cable which is proportional to its length at fault and feeds this

analogue signal to the digital integrated circuits which comprises the comparator, PIC

microcontroller and the digital display driver. The concept of digital data manipulation has

made a dramatic impact on our society. One has long grown accustomed to the idea of digital

computers, evolving steadily from mainframe and mini computers. More significant,

however, is a continuous trend towards digital solutions in all other areas of electronics.

Instrumentation was one of the first noncompeting domains where the potential benefits of

digital data manipulation over analogue processing were recognized. Early digital electronics

systems were based on magnetically controlled switches (or relays). They were mainly used

in the implementation of very simple logic networks. The age of digital electronic computing

only started in full with the introduction of the vacuum tube [2].

The first truly successful IC logic family, TTL (Transistor-Transistor Logic) was pioneered in

1962 [30]. Other logic families were devised with higher performance in mind. Examples of

these are the current switching circuits that produced the first sub nanosecond digital gates

and culminated in the ECL (Emitter-Coupled Logic) family [36]. TTL had the advantage,

however, offering a higher integration density and was the basis of the first integrated circuit

revolution. MOS digital integrated circuits started to take off in full in the early 1970s.

Remarkably, the first MOS logic gates introduced was of the CMOS variety [37], and this

trend continued till the late 1960s, the complexity of these devices for two more decades.

Interestingly enough, power consumption concerns are rapidly becoming dominant in CMOS

design as well and this time alleviates the problem.

Integration density and performance of integrated circuits have gone through an astounding

revolution in the last couple of decades. In the 1960s, Gordon Moore then with fair child

8

corporation and later cofounder of Intel predicted that the number of transistors that can be

integrated on a single die would grow exponentially with time. This prediction, later called

Moores law, has proven to be amazingly visionary [36].

9

CHAPTER TWO

BACKGROUND OF THE DESIGN

2.1 ANATOMY OF UNDERGROUND DISTRIBUTION CABLES

The core component of any underground system is the cable that supplies power from the

source to the load. The longevity and reliability along with desired safety and aesthetic issues

of underground cables have made underground distribution systems an unprecedented

substitute for overhead distribution lines. Underground cables have been designed for various

applications and voltage levels and extensive improvements in design process have been

achieved. Today pressurized cables are available up to 765 KV and even 1100 KV through

the gradual advancements in materials and manufacturing processes [7]. For primary

distribution systems, cables are typically designed with the following major components,

conductor, conductor shield, insulation, insulation shield, concentric neutral, and jacket.

These components are illustrated in figure 2.1.

The conductor can be either aluminium or copper in solid or stranded form. The selection of a

conductor type depends on ampacity, voltage, physical properties, flexibility, shape, and

economics [2], however it is recommended to use solid or stranded-filled conductors for

reliability [1]. Conductor shields and insulation shields synergistically provide a uniform

cylindrical surface next to the cable insulation to establish the most uniform possible

distributions of electrical stress. Research performed on cable failures has shown that

existence and development of voids or protrusions near the conductor shield-insulation

interface played an important role in the failure process [1]. This region experiences

10

extremely high electrical stresses and these irregularities help boost a non-uniform electrical

field, stressing the cable insulation and eventually causing it to fail. The extruded conductor

shield is a layer of semiconducting material, used to prevent excessive electrical stress in

voids between the conductors.

Insulation can be of a variety of materials such as EPR1, XLPE2, paper, and TRPE3

compounds, whose thickness is a function of cable voltage rating such that the higher the

voltage rating, the thicker the insulation. The extruded insulation shield also consists of a

semi-conductive layer similar to the conductor shield. The function of the insulation shield is

to confine the electric field within the cable, symmetrically distribute electrical stress, reduce

the hazard of shock, limit radio interference, and protect cable induced potential when

connected to overhead lines [8]. The shield may be a metallic tape or a non-metallic tape,

drain wires, or concentric neutral wires. The outer shield is normally connected to ground.

Concentric neutral conductors serve as the metallic component of the insulation shield and as

a conductor for the neutral return current [2]. Due to some mechanical and electrical

considerations, concentric neutral conductors are built from copper even if the central cable

conductor is aluminium.

The cable jacket is the outermost layer of the cable. The purpose of the jacket is to provide

mechanical, thermal, chemical, and environmental protection. It can be made of polyethylene,

polyvinyl chloride, nylon, as well as other plastics. Certain cables use a sheath or armour

instead of a jacket, which provide a much better protection to the cable than a jacket [2].

The first widely accepted concentric neutral cables were unjacketed. The bare concentric

neutral (BCN) cables were directly buried exposing the concentric neutral conductors to the

surrounding soil and consequently provided very effective ground. This design was desired

from a personnel safety point of view in case of a dig-in. Due to the presence of a low

resistance path through neutral conductors, adequate fault current could be conducted to

operate protective devices. The low resistance between the neutral and earth would also

reduce the touch potential at the dig-in site, significantly [1].

Despite the numerous advantages of BCN cables, major durability problems hindered their

wide instalment in underground systems. Soon engineers found that cable moisture and/or

concentric neutral corrosion played a major role in increasing the failure rate of unjacketed

underground cables. Due to the lack of a protective jacket, BNC cables were subject to

corrosion. Once corroded, the only neutral current path was through ground rods which were

11

a totally unsatisfactory condition from the safety and reliability stand point. Therefore,

jacketed concentric cables (JCN) achieved wide acceptance with a special attention to system

grounding.

It is worth mentioning that while U.S utilities installed BCN cables, European and Japanese

utilities installed only jacketed cables and as a result these utilities have experienced much

higher reliability than in the United States. Today, the U.S utilities mainly use jacketed cables

which are also use in Nigeria. [1].

2.2 AGEING MECHANISMS IN UNDERGROUND CABLES

Deterioration of insulation is an inevitable phenomenon in underground cables leading to

insulation failures. The aging is caused by single or synergistic action of several aging factors

that include thermal, electrical, mechanical and environmental [4][5]. Persisting aging factors

eventually cause the cable insulation to fail through a number of mechanisms summarized in

Table 2-1.

Activation of aging mechanisms either change the bulk properties of the insulating materials

referred to as intrinsic aging or cause degradation known as extrinsic aging. The degradation

is the result of the presence of contaminants, defects, voids, and protrusions in the insulation

material and their interaction with different aging mechanisms [5][6]. Under normal

conditions, electrical stresses are the predominant aging factors that may fail cables through

partial discharge and treeing mechanisms aggravated by the presence of water. In organic

extruded dielectric and in particular in cross-linked polyethylene (XLPE) cables, the majority

of cable failures are related to the treeing activity. Treeing refers to any kind of damage in the

insulation medium in which the deterioration path resembles the form of a tree.

This pre-breakdown phenomenon takes place in the form of electrical trees or water tress

under DC, AC and impulse voltages [7]. The primary cause of treeing in dry dielectrics is

partial discharges under high electric stresses and moisture at lower electric stresses. On the

other hand, not all degradation phenomena are associated with electrical stresses. Cable might

fail under abnormal conditions- through thermally aged insulation breakdown [8]. Moisture

increases dielectric losses so localized heat generation is produced and thermally degrades the

paper insulation. The following sections will briefly discuss electrical aging mechanisms i.e.

partial discharges, electrical trees, and water trees as the most commonly sought failure

phenomena.

12

2.3 UNDERGROUND CABLE FAULT LOCATION METHODS

To date, various research studies have been conducted to develop methods for fault

identification and location in underground systems [9]-[11] and some commercial detection

systems are also available for diagnostic testing [12]-[13]. The present methods, although

conceptually different, can be categorized in terms of the mutually exclusive active/passive

terms. The term active describes detection schemes that require an external electric source to

energize the system and generate the diagnosis signals. The opposite holds true for passive

methods in which there is no external injection to the cable system. Active methods are often

destructive which implies that they may further degrade cable insulation that has not already

failed. Thus, the portion of the system involved in the fault must be replaced before restoring

power. Offline methods consist of detection techniques that operate while a section of the

cable is de-energized. Passive methods are preferred over active diagnosis techniques because

it is not destructive.

Existing methods target two main categories of insulation categories. While some of the

methods are used to provide an overall assessment of the insulation, there are other methods

that perform an incremental condition assessment of the underground cable.

13

From a field application point of view, the existing methods can be categorized into the

following classes [10]: i) Thumping method ii) Methods based on sectionalisation. The

following sections discuss these methods with particular attention to the advantages and dis-

advantages of each method.

2.3.1 THUMPING METHOD

When a high voltage is supply to a faulted cable, the resulting higher-current arc makes a

noise loud enough for you to hear above ground. This method has its drawback as it requires

a current on the order of tens of thousand of amps at voltages as high as 25kV to make an

underground noise loud enough for you to hear above ground.

The heating from this high current often causes some degradation of the cable insulation.

Moderate testing may produce no noticeable effects, sustained or frequent testing can cause

the cable insulation to degrade to an unacceptable conduction.

2.3.2 SECTIONALIZING

Sectionalisation method is risk reducing cable reliability, because it depends on physically

cutting and splicing the cable into successively smaller sections which will narrow down the

search for a fault.

For example, on a 9m length, you would cut the cable into two 4.5m sections and measure

both ways with the digital underground cable fault locator

The defective section shows a lower IR than the good section. You would repeat this divide

and conquer procedure until reaching a short enough section of cable to allow repair of the

fault from voltage divider.

2.4 DEFINITION OF UNDERGROUND CABLE FAULTS

Underground cable incipient faults are the primary causes of catastrophic failures in the

distribution systems. These faults develop in the extruded cables from gradual deterioration

of the solid insulation due to the persisting stress factors. The initial incipient activity is

caused by the electrical stresses applied to the voids or protrusions near the conductor shield

insulation interference. This region undergoes an extremely high electrical stresses and such

irregularities serve as stress amplifiers when they produce a non-uniform electrical field.

Once initiated, the gradual damage propagates locally through the insulation in the form of a

14

tree and the incipient process develops. The aging in the insulation can progress due to the

contribution of electrical stresses in the form of partial discharges i.e. electrical trees or from

the presence of moisture in the form of water trees. Electrical trees are swift whereas the

propagation time of the water trees is expressed in years [10]. Water trees fail the cable when

they convert to electrical trees as a result of heat generation or under other stress factors.

Once this happens, the time to failure is normally short because the initiated electrical tree

propagates rapidly through the already weakened dielectric. The only window for detection is

during the conversion process [8].

Electrochemical trees are also likely to develop which are believed to be due to the presence

of chemicals in the region [1]. Regardless of the type of aging mechanism, the term incipient

fault encompasses the insulation treeing process from inception to completion before leading

to a catastrophic failure. From a macroscopic perspective, underground cable faults refer to

the abnormalities associated with any type of deterioration phenomena manifested in the

underground cable electrical signals.

15

2.4.1 EARTH FAULT: This is the most common of all. It occurs when the conductor is in

contact with the lead sheath and thereby transferring charges to the general mass of the earth

and the fault resistance may be low or high. Earth fault normally encountered in real life are:

- Single-line to earth fault.

Fig. 2.2: Single line earth fault

- Double-line to the earth fault

Fig. 2.3: Double-line to earth fault Earthing

- Three-line to earth fault

Fig 2.4: Three-line to earth fault

- Single-line to earth through a resistance

Conductors

Conductors

Conductors

16

Conductors

Resistor

Fig. 2.5: Single-line to earth through a resistance fault.

2.4.2 Short circuit fault: This fault is less common than the earth fault and is usually found

in combination with an earth. This fault occurs as a result of damaged insulation and can

result in overheating of conductors and often causes sparking or arcing at the point where it

occurs [5]. It can be any of the following:

- Double line short circuit fault:

Short circuiting

Fig. 2.6: Double line short circuit fault

- Three phase circuit fault.

Fig. 2.7: Three phase short circuit fault

2.4.3 Open circuit fault: The fault can be a break in a cable or a loose joint connection. A

clear broken cable is seldom met and only occurs when the cable has been unduly stretched

by accident.

Conductors

Conductors

17

2.5 The Block Diagram of the Circuit

Fig.2.8; The block diagram of the digital underground cable fault locator

2.5.1 The detector circuit

Fig.2.9; The detector circuit

The fundamental concept of the detector circuit is voltage divider principle. In its basic

application, a dc voltage is applied to the locator circuit. The value of R2 is precisely known.

An unknown resistance R1 is connected which is determined by the resistance of the faulty

cable and it varies with the length of location of fault. Since resistance is directly proportional

cable About 100R

R2

R1

GND

Ref

U2A3

2

8

1

4

U3A3

2

8

1

4

U4A3

2

8

1

4

R3

R4

R5

R6

GND

GND

GNDGND

A2

A3

A4

Vo

Vs

Detector Circuit

18

to the length of a cable ( ), R1 varies with the point at which the fault is detected on the

cable. By voltage divider rule,

0 =2

1 + 2 2.1

Where, R1 is the resistance of the faulty conductor from the probe terminal to the location of

the fault.

From equation (2.1), R1 (resistance of the measured cable) determines the output signal

(voltage) that will be fed into the digital/analogue converter.

The resistance of a cable is proportional to its length, L and inversely proportional to its cross

sectional area, A.

.

2.2

Or

=

2.3

2.5.2 ANALOG TO DIGITAL CONVERTER

Figure2.10; 3-bit flash analogue to digital converter

19

The principle of operation is based on the comparator principle to determine whether or not

to turn on a particular bit of the binary number output.

The resistor net and comparators provide an input to the combinational logic circuit, so the

conversion time is just the propagation delay through the network - it is not limited by the

clock rate or some convergence sequence. It is the fastest type of ADC available, but requires

a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are

available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned.

Also called the parallel A/D converter, this circuit is the simplest to understand. It is formed

of a series of comparators, each one comparing the input signal to a unique reference voltage.

The comparator outputs connect to the inputs of a PIC 16F84A microcontroller.

As the analogue input voltage exceed the reference voltage at each comparator, the

comparator outputs will sequentially saturate to a high state [36][37].

Time

Analog input

-1

0

1

Time

V(v

olt

)

V(v

ol t)

Fig. 2.11: Conversion of analogue signal to digital signal

2. 5.2.1 LM393 COMPARATOR

The LM393 series are dual independent precision voltage comparators capable of single or

split supply operation. These devices are designed to permit a common mode

rangetoground level with single supply operation. Input offset voltage specifications as

low as 2.0 mV make this device an excellent selection for many applications in consumer,

automotive, and industrial electronics [22].

A dedicated voltage comparator chip such as LM393 is designed to interface with a digital

logic interface (to a TTL or a CMOS). The output is a binary state often used to interface real

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar8.html#c1http://en.wikipedia.org/wiki/Transistor-transistor_logichttp://en.wikipedia.org/wiki/CMOS

20

world signals to digital circuitry. If there is a fixed voltage source from, for example, a DC

adjustable device in the signal path, a comparator is just the equivalent of a cascade of

amplifiers. When the voltages are nearly equal, the output voltage will not fall into one of the

logic levels, thus analogue signals will enter the digital domain with unpredictable results. To

make this range as small as possible, the amplifier cascade is high gain. The circuit consists

of mainly bipolar transistors except perhaps in the beginning stage which will likely be field

effect transistors. For very high frequencies, the input impedance of the stages is low. This

reduces the saturation of the slow, large P-N junction bipolar transistors that would otherwise

lead to long recovery times. Fast small Schottky diodes, like those found in binary logic

designs, improve the performance significantly though the performance still lags that of

circuits with amplifiers using analogue signals. Slew rate has no meaning for these devices.

For applications in flash ADCs the distributed signal across 8 ports matches the voltage and

current gain after each amplifier, and resistors then behave as level-shifters[20][22].

The LM393 accomplishes this with an open collector output. When the inverting input is at a

higher voltage than the non inverting input, the output of the comparator connects to the

negative power supply [20].When the non inverting input is higher than the inverting input,

the output is 'floating' (has a very high impedance to ground) [36][37].

With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5

and can interface with TTL logic:

else 0

Fig. 2.12: Low Power Low offset Voltage Dual Comparator

.

http://en.wikipedia.org/wiki/Bipolar_transistorhttp://en.wikipedia.org/wiki/Field_effect_transistorhttp://en.wikipedia.org/wiki/Field_effect_transistorhttp://en.wikipedia.org/wiki/Field_effect_transistorhttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/Schottky_diodehttp://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Open_collectorhttp://en.wikipedia.org/wiki/Pull-up_resistorhttp://en.wikipedia.org/wiki/Transistor-transistor_logic

21

2.5.3 PIC16F84A MICROCONTROLLER

A microcontroller is a microprocessor which has I/O circuitry and peripherals built-in,

allowing it to interface more or less directly with real-world devices such as lights, switches,

sensors and motors. They simplify the design of logic and control systems, allowing complex

(or simple!) behaviours to be designed into a piece of electronic or electromechanical

equipment. They represent an approach which draws on both electronic design and

programming skills; an intersection of what was once two disciplines, and is now called

embedded design.

Modern microcontrollers make it very easy to get started. They are very forgiving and often

need little external circuitry. Among the most accessible are the PIC microcontrollers.

The range of PICs available is very broad from tiny 6-pin 8-bit devices with just 16 bytes

of data memory which can perform only basic digital I/O, to 100-pin 32-bit devices with 512

kilobytes of memory and many integrated peripherals for communications, data acquisition

and control [24].

A diagram showing the pin-outs of the PIC 16F84A is given in figure 2.11.

Fig. 2.13; The pin diagram of PIC16F48A microcontroller

RA0 to RA4

RA is a bidirectional port. That is, it can be configured as an input or an output. The number

22

following RA is the bit number (0 to 4). So, we have one 5-bit directional port where each

bit can be configured as Input or Output.

RB0 to RB7

RB is a second bidirectional port. It behaves in exactly the same way as RA, except there are

8 - bits involved.

VSS and VDD.

These are the power supply pins. VDD is the positive supply, and VSS is the negative

Supply or 0V. The maximum supply voltage that you can use is 6V, and the minimum is 2V

OSC1/CLK IN and OSC2/CLKOUT: these pins are where we connect an external clock, so

that the microcontroller has some kind of timing.

MCLR

This pin is used to erase the memory locations inside the PIC (i.e. when we want to re-

program it).

In normal use it is connected to the positive supply rail.

INT.

This is an input pin which can be monitored. If the pin goes high, we can cause the program

to restart, stop or execute any other single function we desire.

T0CK1.

This is another clock input, which operates an internal timer. It operates in isolation to the

main clock.

2.5.3.1 PINS ON PIC16F84A MICROCONTROLLER HAVE THE FOLLOWING

MEANING:

Pin no.1 RA2 Second pin on port A. Has no additional function.

Pin no.2 RA3 Third pin on port A. Has no additional function.

Pin no.3 RA4 Fourth pin on port A. TOCK1 which functions as a timer is also found on

this pin.

Pin no.4 MCLR Reset input and Vpp programming voltage of a microcontroller.

Pin no.5 Vss Ground of power supply.

Pin no.6 RB0 Zero pin on port B. Interrupt input is an additional function.

Pin no.7 RB1 First pin on port B. No additional function.

Pin no.8 RB2 Second pin on port B. No additional function.

Pin no.9 RB3 Third pin on port B. No additional function.

Pin no.10 RB4 Fourth pin on port B. No additional function.

23

Pin no.11 RB5 Fifth pin on port B. No additional function.

Pin no.12 RB6 Sixth pin on port B. 'Clock' line in program mode.

Pin no.13 RB7 Seventh pin on port B. 'Data' line in program mode.

Pin no.14 Vdd Positive power supply pole.

Pin no.15 OSC2 Pin assigned for connecting with an oscillator.

Pin no.16 OSC1 Pin assigned for connecting with an oscillator.

Pin no.17 RA2 Second pin on port A. No additional function.

Pin no.18 RA1 First pin on port A. No additional function.

The PIC16F84A belongs to the mid-range family of the PICmicro microcontroller devices

[28]. A block diagram of the device is shown in Fig.2.11

Fig.2.14; The block diagram of PIC16F84A microcontroller

24

2.5.3.2 MEMORY ORGANIZATION

There are two memory blocks in the PIC16F84A.These are the program memory and the data

memory. Each block has its own bus, so that access to each block can occur during the same

oscillator cycle.sss

2.5.3.3 MICROCONTROLLER BOARD

The simplest way of making microcontroller board is by connecting power supply, reset

circuit and oscillator circuit to PIC 16F84A. Such a configuration can be shown as:

Fig.2.15; PIC16F84A microcontroller board arrangement

25

2.5.3.4 PIC 16F84A LEAST CIRCUIT

Least circuit connected to PIC 16F84A are:

OSCILLATOR CIRCUIT

For simplicity, an RC oscillator can be used. However, if timer needs accurate time then use

crystal oscillator. Connection for RC oscillator is at pin 15 while crystal oscillator use both

pin 15 and 16. In this case only the input of the microcontrollers clock oscillator is used,

which means that the clock signal with the Fosc/4 frequency will appear on the OSC2 pin.

This frequency is the same as the operating frequency of the microcontroller, i.e. represents

the speed of instruction execution [26][27].

In applications where great time precision is not necessary, resonant frequency of RC

oscillator depends on supply voltage rate, resistance R, capacity C and working temperature.

It should be mentioned here that resonant frequency is also influenced by normal variations in

process parameters, by tolerance of external R and C components, etc.

Fig.2.16; PIC16F84A RC oscillator connection

Fig.2.13 shows how RC oscillator is connected with PIC16F84. With value of resistor R

being below 2.2k, oscillator can become unstable, or it can even stop the oscillation. With

very high value of R (ex.1M) oscillator becomes very sensitive to noise and humidity. It is

recommended that value of resistor R should be between 3 and 100k. Even though oscillator

will work without an external capacitor (C=0pF), capacitor above 20pF should still be used

for noise and stability.

No matter which oscillator is being used, in order to get a clock that microcontroller works

upon; a clock of the oscillator must be divided by 4. Oscillator clock divided by 4 can also be

obtained on OSC2/CLKOUT pin, and can be used for testing or synchronizing other logic

circuits.

26

RESET CIRCUIT

Reset circuit is important for after on power circuit, stabilize voltage source at fix length of

time and stabilize voltage for PIC 16F84A [26][25].

Reset is used for putting the microcontroller into a 'known' condition. That practically means

that microcontroller can behave rather inaccurately under certain undesirable conditions. In

order to continue its proper functioning it has to be reset, meaning all registers would be

placed in a starting position. Reset is not only used when microcontroller does not behave the

way we want it to, but can also be used when trying out a device as an interrupt in program

execution, or to get a microcontroller ready when loading a program.

In order to prevent from bringing a logical zero to MCLR pin accidentally (line above it

means that reset is activated by a logical zero), MCLR has to be connected via resistor to the

positive supply pole. Resistor should be between 5 and 10K. This kind of resistor, whose

function is to keep a certain line on a logical one as a preventive, is called a pull up.

Fig.2.17; PIC16F84A reset circuit.

2.5.3.5 MICROCONTROLLER PIC16F84A KNOWS SEVERAL SOURCES OF

RESETS:

a) Reset during power on, POR (Power-On Reset)

b) Reset during regular work by bringing logical zero to MCLR microcontroller's pin.

c) Reset during SLEEP regime

d) Reset at watchdog timer (WDT) overflow

e) Reset during at WDT overflow during SLEEP work regime.

27

The most important reset sources are a) and b). The first one occurs each time a power supply

is brought to the microcontroller and serves to bring all registers to a starting position initial

state. The second one is a product of purposeful bringing in of a logical zero to MCLR pin

during normal operation of the microcontroller. This second one is often used in program

development [17].

During a reset, RAM memory locations are not being reset. They are unknown during a

power up and are not changed at any reset. Unlike these, SFR registers are reset to a starting

position initial state. One of the most important effects of a reset is setting a program counter

(PC) to zero (0000h), which enables the program to start executing from the first written

instruction.

While the 12805/12509 microcontroller family has an internal 4MHz oscillator, other PICs

require external circuitry before they will spring to life. In situation where timing is non-

crucial, the simple resistor-capacitor oscillator suffices. In fact these two components are

probably the simplest way of getting a 16F84A started.

2.5.3.6 THE REGISTERS

A register is a place inside the PIC that can be written to, read from or both. Table 2.2 below

shows the register file map inside the PIC16F84A [14].

28

Table 2.2; Register file map of PIC16F84A

Address Bank 0 Bank 1 Address

00h INDF INDF 80h

01h TMR0 OPTION 81h

02h PCL PCL 82h

03h STATUS STATUS 83h

04h FSR FSR 84h

05h PORTA TRISA 85h

06h PORTB TRISB 86h

07h 87h

08h EEDATA EECON1 88h

09h EEADR EECON2 89h

0Ah PCLATH PCLATH 8Ah

0Bh INTCON INTCON 8Bh

0Ch GPR

registers

68 bytes

8Ch

Notice that some SFRs, such as the STATUS register and INTCON register, appear in both

banks and can be accessed from either Bank 0 or Bank 1.

2.5.3.7 PROGRAMMING THE MICROCONTROLLER.

The registers are split into two; Bank0 and Bank1. Bank1 is used to control the actual

operation of the PIC, for example to tell the PIC which bits of PORTA are inputs and which

are output. Bank0 is used to manipulate data. An example is as follows: Let us say we want to

make one bit on PORTA high. First we go to Bank1 to set the particular bit, or pin, on

PORTA as output. We then come back to Bank0 and send a logic high (1) to that pin.

The most common registers in Bank1 that are going to be used are the STATUS, TRISA,

TRISB. The first allows us to select which pins on PORTA are outputs and which are inputs,

29

TRISB allows us to select which pins on PORTB are output and which are input. The

STATUS (SELECT) register in Bank0 allows us to switch to Bank1.

2.5.3.7 STATUS REGISTER.

Table 2.3; The status register of PIC16F84A

R=Readable bit

W=Writable bit

To change from Bank 0 to Bank 1, we tell STATUS register on address03h of the register.

We do this by setting bit 5 of the STATUS register to 1. To switch to Bank 0, we set bit 5

of the STATUS register to 0.

This is one of the most important registers within a PIC chip in relation to programming. The

bits 0 to bit 2 are the status results from the ALU (Arithmetic Logic Unit), bits 3.4 are reset

status, and the remaining 3 relate to the bank selection [17].

The C flag, bit 0, is set to 1 whenever the results of an operation results in a carry from the

MSB.

The DC flag, bit 1 is set to 1 if the Z flag, bit 2, is set in the result of arithmetic or logical

operation results in all bits being 0 and vice versa for 1, using BTFSS and BTFSC

respectively for a it test.

2.5.3.8 TRISA AND TRISB.

These are located at addresses 85h and 86h respectively. To program a pin to be an output an

input, we simply send a 0 or a 1 to the relevant bit in the register. Now, this can either be

done in binary, or hex.

So, on Port A we have 5 pins, and hence 5 bits. If I wanted to set one of the pins to input, I

send a 1 to the relevant bit. If I wanted to set one of the pins to an output, I set the relevant

30

bit to 0. The bits are arrange in exactly the same way as the pins, in other words bit 0 is

RA0, bit 1 is RA1, bit 2 is RA2 and so on. Lets take an example. If I wanted to set RA0,

RA3 and RA4 as outputs, and RA1 and RA2 as inputs, I send this: 00110 (06h). Note that

bit zero is on the right, as shown in table 2.4 below [17][26].

Table 2.4; graphical illustration of port A

Port A Pin RA4 RA3 RA2 RA1 RA0

Bit Number 4 3 2 1 0

Binary 0 0 1 1 0

The same goes for TRISB

2.5.3.9 PORTA AND PORTB.

To send one of our output pins high, we simply send a 1 to the corresponding bit in our

PORTA or PORTB register. The same format follows as for the TRISA and TRISB

registers. To read if a pin is high or low on our port pins, we can perform a check to see if

the particular corresponding bit is set to high (1) or set to low (0).

Before we go further, explanation will be given on two more register; W and f registers.

W-Register.

The W register is a general register in which you can put any value that you wish. Once you

have assigned a value to W, you can add it to another value, or move it. If you assign another

value to W, its contents are overwritten. On the other hand, f is any memory location

(PORTA or B).

Several languages are used when it comes to programming a microcontroller such programs

are the C-language, BASIC and Assembly language e.t.c. But for the sake of the project, the

Assembly language will be used to program the microcontroller.

Recall earlier as stated that Bank1 of the PIC register is used for operation while Bank0 is

used for data manipulation. Below are introduction of some couple of instructions along the

way. PORTA will be set up as input.

31

First, we need to switch from Bank 0 to Bank 1. We do this by setting the STATUS register,

which is at address 03h, bit 5 to 1.

BSF 03h, 5

The BSF Means Bit Set F. The letter F means that we are going to use a memory location, or

register. We are using two numbers after this instruction 03h, which is the STATUS

register address, and the number 5 which corresponds to the bit number. So, what we are

saying is Set bit 5 in address 03h to 1.

We are now in Bank 1.

MOVLW b11111

We are putting the binary value 11111 (the letter b means the number is in binary) into our

general purpose register W. We could of course have done this in hex, in which case our

instruction would be:

MOVLW 1Fh

Or

MOVLW 0X1F

Either works. The MOVLW means Move Literal Value into W, which in English means

put the value that follows directly into the W register.

Now we need to put this value onto our TRISA register to set up the port:

MOVWF 85h

This instruction means Move the Contents of W into the Register Address That Follows, in

this case the address points to TRISA.

Our TRISA register now has the value 11111 or shown graphically:

32

Table.2.5; Graphical illustration of TRISA

Port A Pin RA4 RA3 RA2 RA1 RA0

Binary 1 1 1 1 1

Input/output 1 1 I I 1

Now we have set up our Port A pins, we need to come back to Bank 0 to manipulate any

data.

BCF 03h, 5

This instruction does the opposite of BSF. It means Bit Clear F. The two numbers that

follow are the address of the register, in this case the STATUS register, and the bit number, in

this case bit 5. So what we have done now is set bit 5 on our STAUS register to 0

We are now back in Bank 0.

Here is the code in a single block:

BSF 03h,5 ; Go to Bank 1

MOVLW 1Fh ; Put 11111 into W

MOVWF 85h ; Move 00110 onto TRISA

BCF 03h, 5 ; Come back to Bank 0

2.5.3.10 PROGRAM STRUCTURE FOR PIC 16F84A

The Assembly language programming has four fields which are the Label field (e.g.

START), Operand field, operation-code(op-code) and Comment fields [28].

Labels

Labels provide the easiest way of controlling the program flow. They are used to mark

particular lines in the program where jump instruction and appropriate subroutine are to be

executed.

33

Comments: Explain the purpose of the program, type of chip, clock type and

frequency, Date and authors name, etcBe descriptive about program but

not too lengthy.

;---------------------------------------------------*

; Description *

; *

;---------------------------------------------------*

Header: Header contain instruction information of the type of chip and the

base of number system

;---------------------------------------------------*

List p=16f84

Radix hex

;---------------------------------------------------*

Initialization: Here, you define ports, variables

;--------------------------------------------------*

Porta equ 0x05

Portb equ 0x06

;--------------------------------------------------*

The above code tell the chip that porta is define at Hex address 05 and portb at 06

Program: You insert your program codes here

;----------------------------------------------------------------*

34

Start movlw 0xff; load w (working register) with 0xff

Movwf porta; teach portb to be an input

Movlw 0x00; load w (working register) with 0x00

Movwf portb; teach portb to be an output

;----------------------------------------------------------------*

Even though most books do this in the program section. I prefer initialize the

port in the Initialization section.

End: The end statement tell the assembler that this is the end of the program

;----------------------------------------------------------------*

End

;----------------------------------------------------------------*

The symbol ; will tell the assembler to ignore everything after it on that

particular line.

2.5.3.11 PROGRAMMING CONCEPTS

The Instruction Set

The complete instruction for the PIC16F84A microcontroller comprises 35

instructions. For project purpose, the used instruction sets and the

interpretation of their subsets are given in the following table:

35

Table 2.6; the used instruction set for the Project.

Mnemonic Operands Description

Bcf f, b Clear bit b of file f

Bsf f, b Set bit b of file f

Btfss f, b Test bit b of file f, skip the next instruction if the bit is set.

This is a conditional branch instruction.

Clrf F Clear file f

Goto K Unconditional branch to label k

Movf f, d Move file f (to itself if d = 1, or to working register if d = 0)

Movlw K Move the literal k to the working register

Movwf F Move working register to file f

Xorwf f, d Exclusive OR logic function between the w register and the

file register f. If d is 0 the result is stored back to the file

register. The status affected from this is the zero. The logic

operation performed is: Destination(d)=W.XOR.f

Where 0

36

2.5.3.12 PIC INSTRUCTION SET BTFSS

Bit Test f Register Skip if Set. If the result of the test is 0 then the next instruction is carried

out, else it is replaced with NOP.

Let us assume we have an operation that would normally result in the carry bit being set in

the status register. We could use the BTFSS instruction to test the Status Register

Loop -

-

BTFSS 03, 0 ; testing bit0 of SR

Goto Loop ; Come out of loop if Carry is not Set

The alternative is the BTFSC which skips if the bit is clear.

Also, it is used to compare data in a program. For example, let say you want to check if your

input data from porta is arrange in binary base is equal to hex base 5. You would first read

move data from porta to Working Register. Then subtract this from hex number 5. Then we

will check the zero flag in the Status Register to see if it is set or not. If it is set, this mean

that result from the calculation is zero. Therefore out number is hex 5 [28][26].

Movf porta, w ; load porta to Working register and then

Sublw 0x05 ; subtract 5 from it. Result in W

Btfss Status, 2 ; check if zero flag is set

Goto code here ; If not

Goto code here ; if Yes...

Here is a useful table of to check for flag in the Status Register.

For this code:

Sublw N ; subtract N from W

http://pic-chip.co.uk/nop.aspxhttp://pic-chip.co.uk/btfsc.aspx

37

Table 2.8; Flag check in the Status Register

Test For Flag Tested

W = N Z Set

W /= N

(not Equal)

Z Clear

W N C Clear

2.5.3.13 THE XOR-TRICK

The XOR instruction can perform amazing results in a single instruction but it must be fully

understood before using it.

Most of the 35 instructions for the micros are easy to understand and it's easy to see a result,

but the XOR function must be checked by "by-hand" to prove what is happening. If you don't

do this, it will be impossible to find a problem, if a problem arises in a program [19].

Table 2.9; XOR Truth Table

Input A Input B A^B

0 0 0

0 1 1

1 0 1

1 1 0

The XOR function is used to detect a MATCH between two files.

To find out if two numbers are the same, they are XOR-ed together. Since each binary digit

(bit) will be the same, the result will be (0000 0000). For example, if we have two files:

b00010011' and b'0001 0011' bit 0 in each file is '1' bit1 in each file is "1" bit 2 in each

file is "0" etc. In fact all bits are the same. When all bits are the same, this will SET the zero

flag in the Status (03) file and by testing bit 2 (the z flag) you can include an instruction in

38

your program to skip the next instruction when the z bit is set.

Here are the instructions for matching two files:

match movlw 0Ch ; load 0Chex into w

xorwf motor ; see if "motor" file holds 0Chex

btfss status,2 ; test the z flag to see if it is SET

goto not same ;z flag is not set

goto same ;z flag set = files are both 0Chex

2.5.3.14 POWER REQUIREMENTS

The PIC16F84A requires a 5-volt supply; actually, any voltage from 4.0 to 6.0 volts will do

fine, so you can run it from three 1.5-volt cells. The PIC consumes only 1 mA even less, at

low clock speeds but the power supply must also provide the current flowing through

LEDs or other high-current devices that the PIC may be driving [25][28].

2.5.4 SEVEN SEGMENT DISPLAY UNIT

There are many types of displays and some of them are composed of several dozen built-in

diodes which can display different symbols. Nevertheless, the most commonly used display is

a 7-segment display. It is composed of 8 LEDs. Seven segments of a digit are arranged as a

rectangle to display symbols, whereas the additional segment is used to display a decimal

point [21].

In order to simplify connection, anodes or cathodes of all diodes are connected to one single

pin so that there are common anode displays and common cathode displays, respectively.

Segments are marked with letters from a to g, plus dp, as shown in figure 2.15. When

connecting an LED display, each diode is treated separately, which means that each one must

have its own current limiting resistor

39

.

Fig.2.18; LED display anatomy.

Here are a few things that you should pay attention to when using LED displays:

As mentioned above, depending on whether anodes or cathodes are connected to the

common pin, there are common anode displays and common cathode displays. There

is no difference between them at all in their appearance so you are advised to double

check which one is to be used prior to installing it.

Maximum current that each microcontroller pin can receive or give is limited.

Therefore, if several displays are connected to the microcontroller then so called Low

current LEDs limited to only 2mA should be used.

Display segments are usually marked with letters from a to g, but there is no fast rule

indicating display pins they are connected to. For this reason, it is very important to

check connection prior to start writing a program or designing a device.

A multi digit number must be split into units, tens etc. in a specialized subroutine.

Then each of these digits must be stored in a specific byte. Digits get recognizable

appearance for humans by performing a simple procedure called masking.

In other words, a binary number is replaced with a different combination of bits. For

example, digit 8 (0000 1000) is replaced with the binary number 0111 1111 in order

to activate all LEDs displaying this digit. The only diode remaining inactive here is

reserved for the decimal point [21][27].

2.6 LM7805 VOLTAGE REGULATOR.

This is the most common voltage regulator that is still used in embedded system designs.

LM7805 voltage regulator is a linear regulator that reduces input DC voltage to 5v which is

used to power the microcontroller and the entire circuit [29].

40

Features

Output Current up to 1A

Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V

Thermal Overload Protection

Short Circuit Protection

Output Transistor Safe Operating Area Protection

They can come in several types of packages. For output current up to 1A there may be two

types of packages: TO-220 (vertical) and D-PAK (horizontal).

Fig.2.19; pin-out of Lm7805 voltage regulator

With proper heat sink these LM7805 types can handle even more than 1A current. They also

have Thermal overload protection, Short circuit protection.

41

CHAPTER THREE

DESIGN AND ANALYSIS

The digital underground cable fault locator is made up of four building blocks;

Detector circuit

Analogue-to-Digital converter/input.

Microcontroller

Digital display unit/output.

Fig.3.1; Block diagram of digital underground cable fault locator (DUCLF)

The method of testing employed is called sectionalisation method. This type of test is best

known and cheapest of all the various methods and is used principally to locate underground

cable faults in sheathed-cable.

Detector Circuit

42

3.1 DETECTOR CIRCIUT.

Fig.3.2; Detector circuit

cable About 100R

U6LM7805CT

LINE VREG

COMMON

VOLTAGE V19 V

GND

R2100

R1

GND

Vo

GND

Ref

T2

T1

43

The locator circuit which operates on voltage divider rule is made up of two probe terminals

T1 and T2 as shown in figure 3.2 and is fed by an input Vs.

By voltage divider rule VO =2

2+1

Where, Rx is the resistance of the faulty conductor from the probe terminal to the location of

the fault. The design is base on an underground cable with cross-sectional area of 35Sq.mm

having the resistivity of 3.5 10-4m.The device is also design to locate fault using

sectionalisation technique every 10m length.

The resistance of a cable is proportional to its length, L and inversely proportional to its

cross-sectional area.

i.e. =

=3.5 104 10

35 1062= 100

3.2 ANALOGUE TO DIGITAL CONVERTER.

This comprises the switch circuit of the comparator and the ACTIVE LOW resistor

arrangement which induces a logic state of HIGH (1) and LOW (0) as shown in figure 3.3.

The principle of operation is based on the comparator operation to determine whether or not

to turn ON or remain OFF. At the point where the reference voltage (V1, V2, V3) exceeds or

equal the analogue input, the comparator output will not be switched ON. Similarly, at the

point where the analogue input, Vo exceeds the reference voltage (V1, V2, V3) the

comparators output will be turned ON. In other words, the comparator is acting like a switch

[24].

44

Fig.3.3; Analogue to digital converter of the fault locator

By voltage divider technique

1 =5(3)

4= 3.75

2 =5 (2)

4= 2.50

3 =5 (1)

4= 1.25

cable About 100R

R2

100

R1

GND

Ref

U2A

LMV393M

3

2

8

1

4

U3A

LMV393M

3

2

8

1

4

U4A

LMV393M

3

2

8

1

4

R3

1k

R41k

R51k

R61k

R71k

R81k

R91k

GND

GND

GNDGND

VCC

5V

A2

A3

A4

45

When Rx is short circuited

= 0

= 5

Vo > V1; A2= Switch ON

Vo >V2; A3= Switch ON

Vo> V3; A4= Switch ON

When Rx is open circuited

=

= 0

Vo< V1; A2= Switch OFF

Vo< V2; A3= Switch OFF

Vo< V3; A4= Switch OFF

When Rx is normal or correct

= 100

= 2.50

Vo< V1; A2= Switch OFF

Vo=V2; A3= Switch OFF

Vo>V3; A4= Switch ON.

With respect to explanations given so far, to get the project started, it will have the following

circuitry.

46

Fig.3.4; The primary configuration of the project microcontroller

Connections on pin 15 and 16 form the RC oscillator for the circuit and pin4 connections are

for the microcontroller reset. The VDD (pin14) serves as the supply source for the

microcontroller while VSS (pin5) serves as the ground of the microcontroller.

3.3 INPUTS

Inputs to a PIC have the same 5V logic requirements, and just as outputs can be 'sink' or

'source', so the inputs can be active 'high' or active 'low'. Basically this is just a variation on

the same theme - but, depending on the actual input device, you may be forced to use a

particular method [17].

U1

PIC16F84A

RA21

RA32

RA4T0CKI3

MCLR4

VSS5

RB0INT6

RB17

RB28

RB39

RB410

RB511

RB612

RB713

VDD14

OSC2CLKOUT15

OSC1CLKIN16

RA017

RA118

R110k

R210k

R310k

C11F

C230pF

S1

Key = Space

GND GNDGND

VCC

5V

47

Active LOW Active HIGH

Fig.3.5; Active Low/High arrangements

These are the two basic input alternatives, the 'Active LOW' example is the most common,

and simply because the Microchip PIC's usually have selectable 'weak pull-ups' on some

ports. The resistor R1 'pulls' the PIC input high (logic '1') - when you press the switch it pulls

the voltage down to zero, changing the input to logic '0'.

The 'Active HIGH' example works in the opposite way, R2 is a 'pull down' resistor, holding

the PIC input at logic 0; pressing the switch connects the PIC input to the 5V rail, forcing it

to logic '1'.

Figure 3.5 shows switches (S1 and S2), but these could just as easily be a switching

transistor, or the output from an IC, or simply an output pin from another PIC.

On the 16F84, only PORTB has internal pull-up resistors. The above circuit would have to

be used when switches are to be used as inputs on PORTA. For the sake of the project

design, the input arrangement takes the following circuitry.

48

Fig.3.6; Input Circuitry to Port A of PIC16F48A

The LM393 Comparator act as a switch for this ACTIVE LOW circuitry for A2,A3 and

A4 INPUTS of PORTA while input ports AO and A1 will be at logic 1 because the resistors

pulls the PIC input high [18].

3.4 Output

The project design is to locate underground cable fault and we have adopted

sectionalization method of test for the probe terminals I the range of every 10m. Open and

Short circuit fault will likely be encountered. In respect to this, the design is to make the

output to indicate Open, Short and Correct or Complete circuit condition by a sign of O,

S, and C respectively.

U1

PIC16F84A

RA21

RA32

RA4T0CKI3

MCLR4

VSS5

RB0INT6

RB17

RB28

RB39

RB410

RB511

RB612

RB713

VDD14

OSC2CLKOUT15

OSC1CLKIN16

RA017

RA118

U2A

LMV393M

3

2

8

1

4

U4A

LMV393M

3

2

8

1

4

U3A

LMV393M

3

2

8

1

4

cable About 100R

GND

GND

GND

GND

R71k

R81kR10

1k

R121k R13

1k

U6LM7805CT

LINE VREG

COMMON

VOLTAGEV19 V

GND