Copyright

By

Shehzad Akram Hashmi

2015

The thesis Supervisor for Shehzad Akram Hashmi

Certifies that this is the approved version of the following thesis

UNDERGROUND ELECTRIFICATION

SYSTEM

APPROVED BY

Supervisor: _______________________________

Engineer Hamid Raza Lecturer (Preston University)

UNDERGROUND ELECTRIFICATION

SYSTEM

By

Shehzad Akram Hashmi(Reg. No 14P2-214001)

Presented to the Faculty of Preston University in Partial Fulfillment of the

Requirements for the Degree of

B. TECH (Honors)Electrical

PRESTON UNIVERSITYIslamabad

August, 2015

DEDICATION

I dedicate this report to my parents, teachers and all my colleagues.

Page ii

ACKNOWLEDGEMENTS

I especially thank to Engineer Hamid Raza, Lecturer Preston University, who

guided me wherever I suffered from hurdles academically as well as morally. It was

he who under his kind supervision directed me towards successful accomplishment of

goals of thesis and to overcome any stumbling blocks faced. I also thank Mr. Tanvir

Asghar, Manager (Electrical), Habib Rafiq Ltd. (HRL), who gave me technical

support and provided access to the valuable technical data. He also shared with me his

practical expertise and knowledge especially related to cable laying, jointing and

termination. I am really grateful to and appreciate contribution of Mr. Mudassar

Hussain Hashmi, Senior Engineer, Pakistan Atomic Energy Commission (PAEC),

who guided me regarding technical report writing and provided me very useful tips

and trick. At the end I would like to thank all my colleagues and family members who

supported me morally, ethically and financially owing to which the successful

completion of the thesis was possible.

Page iii

ABSTRACT

Communication, electricity and transport lines often require the installation of

underground cables in order to obtain high-level performance and low impact on the

environment. Underground cables need stringing equipment capable of operating

under any condition, always providing good performance, high power and working

accuracy. This report specifies the methods and requirements for the safe and efficient

installation of underground cables in Pakistan.

With proper consideration of the many factors related to design, specification,

manufacturing, installation, and commissioning, underground cable systems can be a

viable alternative to overhead lines where the use of cable is warranted because of

rights-of-way constraints, sensitive areas along the planned route, specialized

obstacles (waterways, bridges, etc.) that must be crossed, concerns about weather

effects and reliability affecting overhead lines, or clearance limitations to get into a

congested substation. Though the material and installation costs of underground

power cables are higher than comparable capacity overhead lines, factors such as real

estate, permitting and constructability can often make underground the preferred

alternative as a complete underground system or portions of a hybrid underground and

overhead circuit.

This report provides an introduction to the cable system types and presents an

overview into considerations for using underground cable systems. The discussion is

focused on transmission cables but also has relevance for distribution cable

applications.

The report starts with standard methods and requirements of cable laying in

trenches. In subsequent section the details of cable preparation, termination, jointing,

and tools used for the purpose are detailed. At the end the methods for underground

cable fault detection and diagnosis are presented.

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TABLE OF CONTENTS

CHAPTER 1. UNDERGROUND CABLES...........................................................1

1.1 INTRODUCTION..........................................................................................1

1.2 TYPES OF UNDERGROUND POWER CABLES......................................1

1.3 XLPE CABLES (Cross-Linked Polyethylene)..............................................2

1.3.1 Construction of XLPE:...............................................................................2

1.3.2 Properties and Advantages of XLPE:........................................................4

1.3.3 Applications...............................................................................................5

1.4 SELECTION OF CABLE SIZES..................................................................5

1.5 CABLE STORAGE AND HANDLING.......................................................6

1.5.1 Storage........................................................................................................6

1.5.2 Handling.....................................................................................................7

CHAPTER 2. CABLE LAYING.............................................................................8

2.1 GENERAL.....................................................................................................8

2.1.1 Route..........................................................................................................8

2.1.2 Proximity to Communication Cables.........................................................9

2.1.3 Railway Crossing.......................................................................................9

2.1.4 Way Leave...............................................................................................10

2.2 CABLE LAYING EQUIPMENT................................................................10

2.2.1 Cable Laying Trailer................................................................................10

2.2.2 Cable Pulling Winches.............................................................................11

2.2.3 Cable Roller.............................................................................................12

2.3 LAYING DIRECT IN GROUND................................................................13

2.3.1 General.....................................................................................................13

2.3.2 Trenching.................................................................................................13

2.3.3 Cable Installation Depths.........................................................................18

2.3.4 Back Filling..............................................................................................20

2.3.5 Route Markers..........................................................................................21

2.3.6 Cable Identification Tags.........................................................................22

CHAPTER 3. JOINTING AND TERMINATION..............................................23

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3.1 CABLE JOINTING.....................................................................................23

3.1.1 Joints Pits.................................................................................................23

3.1.2 Safety Precaution.....................................................................................24

3.1.3 Jointer.......................................................................................................24

3.1.4 Characteristics of Jointing Material.........................................................24

3.2 PROCEDURE OF CABLE JOINTING.......................................................25

3.2.1 Cable Overlap...........................................................................................25

3.2.2 Cable Preparation.....................................................................................25

3.2.3 Core Preparation.......................................................................................26

3.2.4 Completion of the Joint............................................................................30

3.3 CABLE TERMINATION............................................................................35

3.3.1 Characteristics of Termination Material..................................................35

3.4 PROCEDURE OF CABLE TERMINATION.............................................36

3.4.1 Cable Preparation for Termination...........................................................36

CHAPTER 4. TESTING OF THE UNDERGROUND CABLES......................41

4.1 INSULATION RESISTANCE TESTING...................................................41

4.1.1 Insulation Testing Components................................................................41

4.1.2 Various Insulation Tests...........................................................................43

4.2 CONTINUITY TEST...................................................................................46

4.2.1 Tools & Equipments Required.................................................................46

4.2.2 Test Procedure..........................................................................................47

CHAPTER 5. CABLE FAULT LOCATING TESTS.........................................48

5.1 BLAVIER TEST..........................................................................................48

5.2 MURRAY LOOP TEST..............................................................................49

5.3 VARLEY LOOP TEST................................................................................51

5.4 FISHER LOOP TEST..................................................................................53

CHAPTER 6. CABLE FAULT LOCATING INSTRUMENTS........................55

6.1 SURGE GENERATOR...............................................................................55

6.2 CABLE FAULT PINPOINTING EQUIPMENT........................................56

6.2.1 Surge Detector..........................................................................................56

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6.2.2 Electromagnetic Impulse Detector...........................................................56

6.3 SURGE DETECTION PROCEDURE........................................................57

CHAPTER 7. UNDERGROUND DISTRIBUTION NETWORKS...................59

7.1 TYPES OF THE U/G DISTRIBUTION NETWORKS...............................59

7.1.1 Ring Distribution System.........................................................................59

7.1.2 Radial Distribution System......................................................................60

7.1.3 Interconnected System.............................................................................61

7.2 SWITCHGEARS USE IN UNDERGROUND DISTRIBUTION SYSTEM

62

7.2.1 Ring Main Unit (RMU)............................................................................62

7.2.2 Padmount Transformer.............................................................................65

7.2.3 Padmount 3 Phase Distribution Panel......................................................66

7.2.4 Street Light Control Panel........................................................................67

APPENDIX A: Arrangement of Cables in Trenches.........................................69

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LIST OF TABLES

Table 3-1: Crimp Lug...................................................................................................38

Table 3-2: Mechanical Lug BLMT..............................................................................38

Table 4-1: Recommended test voltages for routine maintenance insulation-resistance tests of equipment rated to 4,160V and above.............................................................42

Table 4-2: Listing of Conditions of Insulation as Indicated by Dielectric Absorption Ratios............................................................................................................................45

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LIST OF FIGURES

Figure 1-1: Atypical XLPE Cable..................................................................................3

Figure 1-2: Cable Drum.................................................................................................6

Figure 2-1: Cable Laying Trailer.................................................................................10

Figure 2-2: Cable Pulling Winch.................................................................................11

Figure 2-3: A typical Heavy Duty Cable Roller..........................................................12

Figure 2-4: Cable Laying in Open Trench and Pulling................................................14

Figure 2-5: Excavation of Trenches.............................................................................15

Figure 2-6: Typical Direct Laid Configuration with Single Cable..............................21

Figure 2-7: CC Marker.................................................................................................22

Figure 3-1: A Typical Cable Joint................................................................................23

Figure 3-2: Joint Pits....................................................................................................23

Figure 3-3: Caution Board...........................................................................................24

Figure 3-4: Cable Overleap..........................................................................................25

Figure 3-5: Cable Preparation......................................................................................26

Figure 3-6: Folding Screen Wire..................................................................................26

Figure 3-7: Applying Mastic Tape...............................................................................27

Figure 3-8: Folding Screen Wires Back.......................................................................27

Figure 3-9: Applying Red Mastic Tape.......................................................................27

Figure 3-10: Applying Black Tube..............................................................................28

Figure 3-11: Bending Screen Wires Back....................................................................28

Figure 3-12: Removal of Insulation Screen.................................................................28

Figure 3-13: Removal of Insulation.............................................................................29

Figure 3-14: Wrapping the Void Filler........................................................................29

Figure 3-15: Sliding the Stress Control Tubing...........................................................29

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Figure 3-16: Completion of the Joint...........................................................................30

Figure 3-17: Conductor Fitting....................................................................................30

Figure 3-18: Fitting Connector into the Conductor......................................................31

Figure 3-19: Filling the Hollows over the Sheared-off Bolts......................................31

Figure 3-20: Removal of Release Paper from the Stress Grading Patch.....................31

Figure 3-21: Positioning the Screened Insulating Sleeve............................................32

Figure 3-22: Wrapping Half-Lapped Layer of Tinned Copper Mesh..........................32

Figure 3-23: Gathering the Screen Wires Together and Cutting Them Centrally.......33

Figure 3-24: Inserting the Screen Wires into the Shear Head Connector....................33

Figure 3-25: Wrapping Second Layer of Tinned Copper Mesh..................................33

Figure 3-26: Abrading, Cleaning and Degreasing.......................................................34

Figure 3-27: Wrapping Red Sealant Tape....................................................................34

Figure 3-28: Shrinking in the Center...........................................................................34

Figure 3-29: Cooling of the Joint.................................................................................34

Figure 3-30: Medium Voltage Termination System....................................................35

Figure 3-31: 1) Cutting of Cable & Sheath Removal; 2) Sealant Tape Wrapping......37

Figure 3-32: 3) Removal of Filler; 4) Cutting the Conductive Tubing........................37

Figure 3-33: 5) Positioning the Conductive Tubing; 6) Pulling Breakout 7) Cutting Insulation......................................................................................................................38

Figure 3-34: 8) Removing Release Paper; 9) Applying Sealant Mastic; 10) Preheating Cable Lug.....................................................................................................................39

Figure 3-35: 11) Indoor Termination; 12) Outdoor Termination.................................40

Figure 4-1: Schematic of a Megohmmeter...................................................................42

Figure 4-2: Connection for Testing a Transformer Bushing........................................43

Figure 4-3: Insulation Resistance Test of Motor..........................................................44

Figure 4-4: Insulation Resistance Test of Cable..........................................................44

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Figure 4-5: Schematic of Continuity Test....................................................................46

Figure 5-1: Blavier's Test to Find the Cable’s Fault Location.....................................49

Figure 5-2: Circuit Connection of Murray Loop Test for Ground Fault......................50

Figure 5-3: Circuit Connection of Murray Loop Test for Short Circuit Fault.............50

Figure 5-4: Circuit Connection of Varley Loop Test for Ground Fault.......................52

Figure 5-5: Circuit Connection of Varley Loop Test for Short Circuit Fault..............52

Figure 5-6: Circuit Connection (A) for Fisher Loop Test to Locate the Cable Fault. .53

Figure 5-7: Circuit Connection (B) for Fisher Loop Test to Locate the Cable Fault...53

Figure 6-1: Surge Generator.........................................................................................55

Figure 6-2: A) Surge Detector, SD-3000; B) Electromagnetic Impulse Detector.......56

Figure 6-3: Acoustic/Electromagnetic Pinpointing......................................................58

Figure 6-4: SD-3000 Display at Positions 1, 2, And 3.................................................58

Figure 7-1: Operation of Ring Distribution System.....................................................59

Figure 7-2: Radial Distribution System.......................................................................61

Figure 7-3: Single Line Diagram of Interconnected System.......................................62

Figure 7-4: Ring Main Unit (RMU).............................................................................63

Figure 7-5: Single-Line Representation of a Typical RMU Configuration.................63

Figure 7-6: An Example of Distribution Network With Ring Main Units..................64

Figure 7-7: Padmount Transformer..............................................................................65

Figure 7-8: Padmount 3 Phase Distribution Panel.......................................................66

Figure 7-9: A Typical Street Light Panel.....................................................................67

Page xi

CHAPTER 1.UNDERGROUND CABLES

1.1 INTRODUCTION

Where general appearance, economics, congestion, or maintenance conditions

make overhead construction inadvisable, underground construction is specified. While

overhead lines have been ordinarily considered to be less expensive and easier to

maintain, developments in underground cables and construction practices have

narrowed the cost gap to the point where such systems are competitive in urban and

suburban residential installations, which constitute the bulk of the distribution

systems. The conductors used underground are insulated for their full length and

several of them may be combined under one outer protective covering. The cables

may be buried directly in the ground, or may be installed in ducts buried in the

ground. Concrete or metal markers are often installed at intervals to show the location

of the cables.

The installation of cable directly in the ground saves the cost of building

conduits and manholes and allows the use of long sections of cable, thereby

eliminating the necessity for a number of splices. The cable may be buried alone in a

trench, or may be buried in a trench together with other facilities, including telephone

cables, gas mains, water, or sewer systems. Sharing the cost of installing such

facilities jointly contributes greatly to the economy of underground distribution

systems. The width and depth of the trench are dictated by the number and type of

facilities to be installed and by National Electric Safety Code (NESC) requirements.

This chapter covers the requirements for the selection, installation and jointing

of Power cables for low, medium and high voltage applications up to and including

33KV.

1.2 TYPES OF UNDERGROUND POWER CABLES

The cables for applications for low and medium voltage (up to and including

1.1KV) supply shall be one of the following:

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i. PVC insulated and PVC sheathed

ii. Cross linked polyethylene insulated, PVC sheathed (XLPE)

The cables for applications for high voltage (above 1.1KV but up to and

including 11KV supply) supply shall be one of the following:

i. PVC insulated and PVC sheathed

ii. Paper insulated, lead sheathed (PILCA)

iii. Cross linked polyethylene (XLPE) insulated, PVC sheathed

The cables for applications above 11KV but up to and including 33KV supply

shall be one of the following:

i. Paper insulated lead sheathed (PILCA)

ii. Cross linked, polyethylene insulated (XLPE)

The cables shall be with solid or stranded aluminum conductors, as specified.

Copper conductors may be used, only in special applications, where use of aluminum

conductor is not technically acceptable. Where paper insulated cables are used in

predominantly vertical situation, these shall be of non-draining type. Mostly use

XLPE cable in underground high voltage power system, so now we discuss about

XLPE cable.

1.3 XLPE CABLES (Cross-Linked Polyethylene)

A typical XLPE cable is shown in Figure 1-1.

1.3.1 Construction of XLPE:

An XLPE Cable normally consists of following components.

1) Conductor:

The conductors made from electrical purity aluminum wires, are stranded

together and compacted. All sizes of conductors of single or three core cables are

circular in shape. Conductor construction and testing comply to IS 8130 - 1984.

Cables with copper conductor can also be offered.

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Figure 1-1: Atypical XLPE Cable

2) Insulation:

High quality XLPE unfilled insulating compound of natural color is used for

insulation. Insulation is applied by extrusion process and is chemically cross linked by

continuous vulcanization process.

3) Shielding

All XLPE cables rated above 3.3 kV are provided with both conductor

shielding and insulation shielding. Both conductor and insulation shielding consists of

extruded semi conducting compound. Additionally, insulation is provided with semi-

conducting tape and non-magnetic metallic tape screen over the extruded insulation.

Conductor shielding XLPE insulation and insulation shielding are all extruded in one

operation by a special process. This process ensures perfect bonding of inner and

outer shielding with insulation.

4) Inner Sheath (Common Covering)

In case of multi-core cables, cores are stranded together with suitable non-

hygroscopic fillers in the interstices and provided with common covering of plastic

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tape wrapping. As an alternative to wrapped inner sheath, extruded PVC inner sheath

can also be provided.

5) Armouring

Armouring is applied over the inner sheath and normally comprises of flat

steel wires (strips) for multi core cables. Alternatively, round steel wire armouring can

also be offered. Single core armoured cables are provided with non-magnetic armour

consisting of hard drawn flat or round aluminum wires.

6) Outer Sheath :

A tough outer sheath of heat resisting Tropodur (PVC) compound (Type ST2

as per IS 5831) is extruded over the armouring in case of armoured cables or over

non-magnetic metallic tape covering the insulation or over the non-magnetic metallic

part of insulation screening in case of un-armoured single core cables. This is always

black in color for best resistance to outdoor exposure. The outer sheath is embossed

with the voltage grade and the year of manufacture. The embossing repeats every

300/350 mm along the length of the cable.

1.3.2 Properties and Advantages of XLPE:

Given below are some outstanding features of XLPE cables.

1. High Continuous Current Rating: Its ability to withstand higher operating

temperature of 90oC enables much higher current ratings than those of PVC or

PILC cables.

2. High Short Circuit Rating: Maximum allowable conductor temperature

during short circuit of 250oC is considerably higher than for PVC or PILC

cables resulting in greater short circuit withstand capacity.

3. High Emergency Load Capacity: XLPE cables can be operated even at 1300

C during emergency, therefore in systems where cables are installed in

parallel, failure of one of two cables will not bring down the system capacity

because the remaining cables can carry the additional load even for longer

duration until repairs / replacements are carried out.

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4. Low Dielectric Losses: XLPE cables have low dielectric loss angle. The

dielectric losses are quadratically dependent on the voltage. Moreover, these

losses occur continuously in every charged cable whether it carries load or not.

Hence use of XLPE cable at higher voltages would result in considerable

saving in costs.

5. Charging Currents: The charging currents are considerably lower permitting

close setting of protection relays.

6. Easy Laying and Installation: Low weight and small bending radii make

laying and installation of cable very easy. The cable requires less supports due

to low weight.

7. High Safety: High safety against mechanical damage and vibrations

1.3.3 Applications

Because of the excellent mechanical and electrical properties XLPE cables are

being used extensively in all power stations and in industrial plants. They are ideally

suited for chemical and fertilizer industries where cables are exposed to chemical

corrosion or in heavy industries where cables are exposed to chemical corrosion or in

heavy industries where severe load fluctuations occur and for systems where there are

frequent over voltages.

Cables can also be used at higher ambient temperature on account of their

higher operating temperature. There excellent installation properties permit the cable

to be used even under most difficult cable routing conditions and also in cramped

conditions e.g. City distribution network. Single core cables due to their excellent

installation properties are used in power stations, sub stations and industrial plants

with advantage.

1.4 SELECTION OF CABLE SIZES

The cable sizes shall be selected by considering the voltage drop in the case of

MV (distribution) cables and Current carrying capacity in the case of HV (feeder)

cables. Due consideration should be given for the Prospective short circuit current and

Page 5

the period of its flow, especially in the case of HV cables. While deciding upon the

cable sizes, de-rating factors for the type of cable and depth of laying, grouping,

ambient temperature, ground temperature, and soil resistivity shall be taken into

account.

1.5 CABLE STORAGE AND HANDLING

1.5.1 Storage

i. The cable drums shall be stored on a well drained, hard surface so that

the drums do not sink in the ground causing rot and damage to the cable

drums (see Figure 1-2). Paved surface is preferred, particularly for long

term storage.

ii. The drums shall always be stored on their flanges, and not on their flat

sides.

iii. Both ends of the cables especially of PILCA cables should be properly

sealed to prevent ingress/ absorption of moisture by the insulation during

storage.

iv. Protection from rain and sun is preferable for long term storage for all

types of cables. There should also ventilation between cable drums.

Figure 1-2: Cable Drum

Page 6

v. During storage, periodical rolling of drums once in, say, 3 months

through 90 degrees shall be done, in the case of paper insulated cables.

Rolling shall be done in the direction of the arrow marked on the drum.

vi. Damaged battens of drums etc. should be replaced as may be necessary.

1.5.2 Handling

i. When the cable drums have to be moved over short distances, they

should be rolled in the direction of the arrow marked on the drum.

ii. For manual transportation over long distances, the drum should be

mounted on cable drum wheels, strong enough to carry the weight of the

drum and pulled by means of ropes. Alternatively, they may be mounted

on a trailer or on a suitable mechanical transport.

iii. For loading into and unloading from vehicles, a crane or a suitable lifting

tackle should be used. Small sized cable drums can also be rolled down

carefully on a suitable ramp or rails, for unloading, provided no damage

is likely to be caused to the cable or to the drum.

Page 7

CHAPTER 2.CABLE LAYING

In this chapter method of cable laying and precautionary measures to be taken

during laying of cables is discussed.

2.1 GENERAL

Following measures should be taken while cable laying.

i. Cables with kinks, straightened kinks or any other apparent defects like

defective armouring etc. shall not be installed.

ii. Cables shall not be bent sharp to a small radius either while handing or

in installation. The minimum safe bending radius for PVC/XLPE (MV)

cables shall be 12 times the overall diameter of the cable. The minimum

safe bending radius for PILCA/XLPE (HV) cables shall be as given in

Table-II. At joints and terminations, the bending radius of individual

cores of a multi core cable of any type shall not be less than 15 times its

overall diameter.

iii. The ends of lead sheathed cables shall be sealed with solder immediately

after cutting the cables. In case of PVC cables, suitable sealing

compound/tape shall be used for this purpose, if likely exposed to rain in

transit storage. Suitable heat shrinkable caps may also be used for the

purpose.

2.1.1 Route

i. Before the cable laying work is undertaken, the route of the cable shall

be decided by the Engineer-in-Charge considering the following.

ii. While the shortest practicable route should be preferred, the cable route

shall generally follow fixed developments such as roads, foot paths etc.

with proper offsets so that future maintenance, identification etc. are

rendered easy. Cross country run merely to shorten the route length shall

not be adopted.

Page 8

iii. Cable route shall be planned away from drains and near the property,

especially in the case of LV/MV cables, subject to any special local

requirements that may have to be necessarily complied with.

iv. As far as possible, the alignment of the cable route shall be decided after

taking into consideration the present and likely future requirements of

other services including cables enroute, possibility of widening of

roads/lanes etc.

v. Corrosive soils, ground surrounding sewage effluent etc. shall be

avoided for the routes.

vi. Route of cables of different voltages.

a. Whenever cables are laid along well demarcated or established

roads, the LV/MV cables shall be laid farther from the kerb line

than HV cables.

b. Cables of different voltages, and also power and control cables

shall be kept in different trenches with adequate separation.

Where available space is restricted such that this requirement

cannot be met, LV/MV cables shall be laid above HV cables.

c. Where cables cross one another, the cable of higher voltage

shall be laid at a lower level than the cable of lower voltage.

2.1.2 Proximity to Communication Cables

Power and communication cables shall as far as possible cross each other at

right angles. The horizontal and vertical clearances between them shall not be less

than 60cm.

2.1.3 Railway Crossing

Cables under railway tracks shall be laid in spun reinforced concrete, or cast

iron or steel pipes at such depths as may be specified by the railway authorities, but

not less than 1m, measured from the bottom of the sleepers to the top of the pipe.

Inside railway station limits, pipes shall be laid up to the point of the railway station

Page 9

limits, pipes shall be laid up to a minimum distance of 3m from the center of the

nearest track on either side.

2.1.4 Way Leave

Way leave for the cable route shall be obtained as necessary, from the

appropriate authorities, such as, Municipal authorities, Department of

telecommunication, Gas Works, Railways, Civil Aviation authorities, Owners of

properties etc.

2.2 CABLE LAYING EQUIPMENT

2.2.1 Cable Laying Trailer

Cable Trailer (see Figure 1-1) is use for cable laying. It is also use for cable

Drum handling, cable drum move from one place to another very easy and safe. The

typical diagram of cable Trailer and its specification is given below.

Figure 2-3: Cable Laying Trailer

2.2.1.1 Specification

Axle Type: Multi Axle

Payload (Approx): 50,000Kgs

Page 10

Gross Vehicle Weight: 57,000Kgs

Maximum Drum Width: 37000mm

Drum Diameter: 5000mm Max, 2900mm Min

Spindle Diameter: 152mm

Trailer Width: 5200mm

Towing Speed: 10kmh

Types Main Axle: 14.00x24

2.2.2 Cable Pulling Winches

A range of trailer mounted cable pulling winches is required for the

installation of power cables where high pulling tensions are required. All of the units

are designed with the operator in mind.

A typical cable pulling winch is shown in Figure 2-4. Simple in operation and

robustly engineered, these units are fitted with heavy duty diesel engines powering

hydraulic transmission for smooth controlled pulling, and are fitted with a console

mounted line tension indicator so that at all times the operator has a visual display of

the pulling force applied. All of the Winches can be quickly pre-set so they do not

exceed the required pulling tension.

Figure 2-4: Cable Pulling Winch

Page 11

This is our 3 tonne capacity single capstan winch and is a great asset to any

Multi Utility business. It is fitted with a free spooling payout system which makes it a

one man only operation. The unit is fitted with a pull load indicator with read out via

dial gauge.

2.2.2.1 Technical Specification

Power unit diesel engine, rated at 13kw, at 3600 rpm, main drive system single

capstan.

Rewind drum drive hydraulic driven for rope tension.

Cable pay out free spooling

Pull load indicator dial gauge for pressure / tonnes pull

Oil cooler fitted with thermostat fan.

Rewind drum 400 mtrs. 10mm dia rope. (mounted under rewind drum)

Rope pull/speed 0.5 tonne 30 mtr/min 1.5 tonne 22mtr/min 3.0 tonne 7

mtr/min capacity (max)

Rope layering free moving layering arm

2.2.3 Cable Roller

The main feature of the heavy duty cable roller (see Figure 2-5) is the full

rounded edges of the frame that protect the cable in the event of the roller falling or

flipping over. This is why it is known as the 'SAFE' roller.

Figure 2-5: A typical Heavy Duty Cable Roller

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2.2.3.1 Technical Specification

Size: 41cm Long x 20.5cm Wide x 22cm High.

Steel Roller Diameter: 110mm Diameter.

Cable Capacity: 130mm diameter.

Weight: 3.5Kg

Finish: Bright Zinc plated.

2.3 LAYING DIRECT IN GROUND

2.3.1 General

This method shall be adopted where the cable route is through open ground,

along roads/lanes, etc. and where no frequent excavations are likely to be encountered

and where re-excavation is easily possible without affecting other services. The

general method of cable laying in open trenches is show in .Figure 2-6

2.3.2 Trenching

Method of trenching is discussed in following sections.

i. Width of Trench

The width of the trench shall first be determined on the following basis

a. The minimum width of the trench for laying a single cable shall

be 35cm.

b. Where more than one cable is to be laid in the same trench in

horizontal formation, the width of the trench shall be increased

otherwise specified, shall be at least 20cm.

c. There such that the inter-axial spacing between the cables,

except where shall be a clearance of at least 15cm between axis

of the end cables and the sides of the trench.

Page 13

Figure 2-6: Cable Laying in Open Trench and Pulling

Page 14

ii. Excavation of Trenches

a. The trenches shall be excavated in reasonably straight lines.

Wherever there is a change in the direction, a suitable curvature

shall be adopted complying with the requirements of clause

2.6.1(ii).

b. Where gradients and changes in depth are unavoidable, these shall

be gradual.

c. The bottom of the trench shall be level and free from stones, brick

bats etc. (see Figure 2-7)

Figure 2-7: Excavation of Trenches

d. The excavation should be done by suitable means-manual or

mechanical. The excavated soil shall be stacked firmly by the side

of the trench such that it may not fall back into the trench.

e. Adequate precautions should be taken not to damage any existing

cable(s), pipes or any other such installations in the route during

excavation. Wherever tricked, tiles or protective covers or bare

cables are encountered, further excavation shall not be carried out

without the approval of the Engineer-in-Charge.

f. Existing property, if any, exposed during trenching shall be

temporarily supported adequately as directed by the Engineer-in-

Page 15

Charge. The trenching in such cases shall be done in short lengths,

necessary pipes laid for passing cables therein and the trench

refilled in accordance with clause 2.6.7.4.

g. It there is any danger of a trench collapsing or endangering

adjacent structures, the sides may be left in place when back filling

the trench.

h. Excavation through lawns shall be done in consultation with the

Department concerned

2.3.2.1 Laying of Cable in Trench

Steps are below:

i. Sand cushioning

The trench shall then be provided with a layer of clean, dry sand cushion

of not less than 8cm in depth, before laying the cables therein. However,

sand cushioning need not be provided for MV cables, where there is no

possibility of any mechanical damage to the cables due to heavy or shock

loading on the soil above. Such stretches shall be clearly specified in the

tender documents. Sand cushioning shall however be invariably provided

in the case of HV cables.

ii. Testing before laying

All the time of issue of cables for laying, the cables shall be tested for

continuity and insulation resistance.

iii. The cable drum shall be properly mounted on jacks, or on a cable wheel at

a suitable location, making sure that the spindle, jack etc. are strong

enough to carry the weight of the drum without failure, and that the spindle

is horizontal in the bearings so as to prevent the drum creeping to one side

while rotating.

iv. The cable shall be pulled over on rollers in the trench steadily and

uniformly without jerks and strain. The entire cable length shall as far as

possible be laid off in one stretch. PVC/XLPE cables less than 120sq.mm.

Page 16

Size may be removed by “Flaking” i.e. by making one long loop in the

reverse direction.

Note:

For short runs and sizes upto 50sq.mm. of MV cables, any other suitable

method of direct handing and laying can be adopted without strain or

excess bending of the cables.

v. After the cable has been so uncoiled, it shall be lifted slightly over the

rollers beginning from one and by helpers standing about 10m apart and

drawn straight. The cable shall then be lifted off the rollers and laid in a

reasonably straight line.

vi. Testing Before Covering

The cables shall be tested for continuity of cores and insulation resistance

and the cable length shall be measured, before closing the trench. The

cable end hall be sealed /covered.

vii. Sand covering

Cables laid in trenches in a single tier formation shall have a covering of

dry sand of not less than 17cm above the base cushion of sand before the

protective cover is laid. In the case of vertical multi-tier formation, after

the first cable has been laid, a sand cushion of 30cm shall be provided over

the base cushion before the second tier is laid.

viii. Extra loop cable

a. At the time of original installation, approximately 3m of

surplus cable shall be left on each terminal end of the cable and

on each side of the underground joints. The surplus cable shall

be left in the form of a loop. Where there are long runs of

cables such loose cable may be left at suitable intervals as

specified by the Engineer-in-Charge.

b. Where it may not be practically possible to provide separation

between cables when forming loops of a number of cables as in

Page 17

the case of cables emanating from a substation, measurement

shall be made only to the extent of actual volume of excavation,

sand filling etc. and paid for accordingly.

ix. Mechanical protection over the covering

a. Mechanical protection to cables shall be laid over the covering

b. Unless otherwise specified, the cables shall be protected by

second class brick of nominal size 22cmX11.4cmX7 cm or

locally available size, placed on top of the sand (or, soil as the

case may be). The bricks shall be placed breadth-wise for the

full length of the cable. Where more than one cable is to be laid

in the same trench, this protective covering shall cover all the

cables and project at least 5cm over the sides of the end cables.

c. Where bricks are not easily available, or are comparatively

costly, there is no objection to use locally available material

such as tiles or slates or stone/cement concrete slabs. Where

such an alternative is acceptable, the same shall be clearly

specified in the tender specifications.

d. Protective covering as per need not be provided only for MV

cables, in exceptional cases where there is normally no

possibility of subsequent excavation. Such cases shall be

particularly specified in the Tender specifications.

2.3.3 Cable Installation Depths

All cables shall be installed to the following minimum depths of cover, where

the depth is measured to the top surface of either the cable or the duct containing the

cable. Cables and ducts should never be installed at increased depths unless there is

no other alternative, as increased depth reduces a cables rating and precludes easy

access to the asset in the future.

In instances where these minimum depths cannot be achieved, the cables shall

be installed with additional mechanical protection, in the form of either steel plates or

ducts. In this case, all steel plates shall be a minimum of 1000mm x150mm x 6mm

Page 18

and painted with a single coat of Bitumastic paint. Steel pipes shall be of the same

internal diameter as the plastic ducts, normally used for the type of cable to be

installed. All steel plates and ducts shall be protected with a suitable cable protection

tape. Typical arrangements of cables in trenches are shown in Appendix A.

2.3.3.1 Cable Protection Tapes and Covers

All cables and ducts shall be protected by a cable protection tape or a

‘stokboard’ (heavy duty plastic tile). The tape used depends upon the highest voltage

to be protected, which are available from Power Networks Supply Chain:

LV Service = Tile tape (40m × 200mm × 2.5mm)

LV Mains = Tile tape (40m × 200mm × 2.5mm)

11kV = Tile tape (40m × 200mm × 2.5mm)

22kV = Tile tape (40m × 200mm × 2.5mm)

33kV = Stokboard (1000mm × 244mm × 9mm)

66kV = Stokboard (1000mm × 244mm × 9mm)

132kV=Stokboard (1000mm × 244mm × 9mm)

2.3.3.2 Low Voltage Service and Mains Cables

All Low Voltage (LV) cables shall be installed to the following minimum depths,

whether they are laid direct or installed in suitable ducts.

Footways, grass verges or private property = 450mm.

Carriageways (including road crossings) = 600mm.

Normal agricultural land (not subject to deep ploughing) = 1050mm.

Agricultural land subject to deep ploughing = 1200mm

2.3.3.3 11kV and 20kV Cables

All 11 and 20kV cables shall be installed to the following minimum depths, whether

they are laid direct or installed in suitable ducts:

Footways, grass verges or private property=600mm.

Carriageways (including road crossings) = 750mm.

Normal agricultural land (not subject to deep ploughing) = 1050mm.

Agricultural land subject to deep ploughing = 1200mm

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2.3.3.4 33kV Cables

All 33kV cables shall be installed to the following minimum depths, whether

they are laid direct or installed in suitable ducts:

Footways, grass verges or private property = 900mm.

Carriageways (including road crossings) = 900mm.

Normal agricultural land (not subject to deep ploughing) = 1050mm.

Agricultural land subject to deep ploughing = 1200mm.

2.3.3.5 66 and 132kV Cables

All 66 and 132kV cables shall be installed to the following minimum depths, whether

they are laid direct or installed in suitable ducts:

Footways, grass verges or private property = 900mm.

Carriageways (including road crossings) = 900mm.

Normal agricultural land (not subject to deep ploughing) = 1050mm.

Agricultural land subject to deep ploughing = 1200mm.

2.3.4 Back Filling

The trenches shall be then back-filled (see Figure 2-8) with excavated earth,

free from stones or other sharp ended debris and shall be rammed and watered, if

necessary in successive layers not exceeding 30cm depth. Unless otherwise specified,

a crown of earth not less than 50mm and not exceeding 100mm in the center and

tapering towards the sides of the trench shall be left to allow for subsidence. The

crown of the earth however, should not exceed 10 cm so as not to be a hazard to

vehicular traffic. The temporary re-statements of roadways should be inspected at

regular intervals, particularly during wet weather and settlements should be made

good by further filling as may be required.

After the subsidence has ceased, trenches cut through roadways or other paved

areas shall be restored to the same density and materials as the surrounding area and –

re-paved in accordance with the relevant building specifications to the satisfaction of

the Engineer-in-Charge.

Page 20

Where road beams or lawns have been cut out of necessity, or kerb stones

displaced, the same shall be repaired and made good, except for turfing /asphalting, to

the satisfaction of the Engineer-in-Charge and all the surplus earth or rock shall be

removed to places as specified.

Figure 2-8: Typical Direct Laid Configuration with Single Cable

2.3.5 Route Markers

Route markers shall be provided along the runs of cables at locations approved

by the Engineer-in-Charge and generally at intervals not exceeding 100m. Markers

shall also be provided to identity change in the direction of the cable route and at

locations of underground joints.

a. Plate Type Marker

Route markers shall be made out of 100mm × 5mm GI/Aluminum plate

welded/bolted on 35mm × 35mm × 6mm angle iron, 60cm long. Such plate markers

shall be mounted parallel to and at about 0.5m away from the edge of the trench

Page 21

b. CC Marker

Alternatively, cement concrete 1:2:4 (1 cement:2 coarse sand: 4 graded stone

aggregate of 20mm in size) as shown in Figure 2-9 shall be laid flat and centered over

the cable. The concrete markers, unless otherwise instructed by the Engineer-in-

Charge, shall project over the surrounding surface so as to make the cable route easily

identifiable.

Figure 2-9: CC Marker

c. Inscription

The words ‘BURIED-MV/HV CABLE’ as the case may be, shall be inscribed on the

marker.

2.3.6 Cable Identification Tags

Whenever more than one cable is laid / run side by side, marker tags as

approved, inscribed with cable identification details shall be permanently attached to

al the cables in the manholes / pull pits / joint pits / entry points in buildings / open

ducts etc. These shall also be attached to cables laid direct in ground at specified

intervals, before the trenches are backfilled.

Page 22

CHAPTER 3.JOINTING AND TERMINATION

3.1 CABLE JOINTING

Before laying a cable, proper locations for the proposed cable joints, if any,

shall be decided, so that when the cable is actually laid, the joints are made in the

most suitable places. As far as possible, water logged locations, carriage ways,

pavements, proximity to telephone cables, gas or water mains, inaccessible places,

ducts, pipes, racks etc. shall be avoided for locating the cable joints. A typical cable

joint is shown in Figure 3-10.

Figure 3-10: A Typical Cable Joint

3.1.1 Joints Pits

Joint pits shall be of sufficient dimensions as to allow easy and comfortable

working (see Figure 3-11). The sides of the pit shall be well protected from loose

earth falling into it. It shall also be covered by a tarpaulin to prevent dust and other

foreign matter being blown on the exposed joints and jointing materials. Sufficient

ventilation shall be provided during jointing operation in order to disperse fumes

given out by fluxing.

Page 23

Figure 3-11: Joint Pits

3.1.2 Safety Precaution

i. A caution board indicating “WARNING – ELECTRICAL JOINTING WORK

IN PROGRESS” shall be displayed to warn the public and traffic where

necessary (as shown in Figure 3-12).

ii. Before jointing is commenced, all safety precautions like isolation,

discharging, earthing, display of caution board on the controlling switchgear

etc. shall be taken to ensure that the cable would not be inadvertently charged

from live supply. Metallic armour and external metallic bonding shall be

connected to earth.

Figure 3-12: Caution Board

3.1.3 Jointer

Jointing work shall be carried out by a licensed/ experienced (where there is

no licensing system for jointers) cable jointer.

3.1.4 Characteristics of Jointing Material

3.1.4.1 Electrical Stress Control

The stress control tubing and the patch have a precisely defined impedance

characteristic which smoothes the electrical field over the connector and cable screen

ends.

Page 24

3.1.4.2 Insulation and Screen

The elastomeric sleeve provides the correct thickness of insulation (red) in one

step. The insulation screen is provided by the outer wall of the sleeve, which is of heat

shrink able conductive polymer (black). This technique saves installation time and

ensures a flawless bond between joint insulation and screen.

3.1.4.3 Metallic Shielding

Copper mesh and roll springs ensure the correct screen connection across the

joint area and make electrical contact with the outer screen of the joint.

3.1.4.4 Outer Sealing and Protection

The heat used to shrink the outer sleeve causes the pre-coated adhesive to melt

and flow, resulting in a lasting moisture and corrosion barrier on the cable oversheath.

The outer sleeve provides mechanical impact and chemical resistance as expected

from cable oversheaths.

3.2 PROCEDURE OF CABLE JOINTING

Complete procedure of cable jointing is given in following sections.

3.2.1 Cable Overlap

Overlap the cables to be jointed as shown in Figure 3-13. Mark the reference line.

Figure 3-13: Cable Overleap

3.2.2 Cable Preparation

Page 25

i. Remove the outer layer of the composite sheath for 215mm, from the

reference line. Remove the inner layer of the composite sheath to the

dimension as shown in Figure 3-14.

ii. Remove the water swellable tapes (if any) level with the inner layer of

composite sheath cut.

iii. Abrade the outer layer of the composite sheath for a distance of 200 mm from

the outer layer composite sheath cut. Abrade the inner layer of the composite

sheath.

iv. Clean and degrease the inner and outer layer of composite sheath using the

cleaning tissue provided in the kit.

Note:

On the side of the joint where the outer sealing sleeve is intended to be parked, clean

the outer layer of the composite sheath for a distance of 1 metre using the cleaning

tissue provided in the kit.

Figure 3-14: Cable Preparation

3.2.3 Core Preparation

Fold the screen wires back onto the cable sheath. Do not bend them into

position at this stage. Do not cut the cables (see Figure 3-15).

Page 26

Figure 3-15: Folding Screen Wire

Apply one layer of red mastic tape (20mm wide) over the insulation screen; 10

mm from the cable sheath cut (see Figure 3-16).

Figure 3-16: Applying Mastic Tape

Fold the screen wires back over the insulation screen (see Figure 3-17). Apply

one layer of red mastic tape (20mm wide) over the screen wires, 10 mm from the

cable sheath cut (i.e. over the mastic applied in step 5).

Figure 3-17: Folding Screen Wires Back

Apply red mastic tape (50mm wide) to equally overlap the inner and outer

layers of the composite sheath (see Figure 3-18).

Page 27

Figure 3-18: Applying Red Mastic Tape

Slide the 150 mm long tube (black) over the cable, centered over the outer

sheath cutback. Shrink tube into position, applying additional heat over the area where

the red mastic tape was applied (see Figure 3-19).

Figure 3-19: Applying Black Tube

Bend the screen wires back over the tube and onto the cable sheath. Secure the

screen wires to the cable using PVC tape (see Figure 3-20).

Figure 3-20: Bending Screen Wires Back

Cut the cables at 140mm as shown using a hacksaw or a suitable power driven

saw to prevent deformation of the conductor strands (see Figure 3-21). Thoroughly

remove the insulation screen to the dimension shown, so that the insulation surface is

free from all traces of conductive material.

Page 28

Figure 3-21: Removal of Insulation Screen

Remove the insulation from the cables to for a length of 30mm (see I in

Figure 3-22). Clean and degrease the insulation using the cleaning tissues provided in

the kit. Use a wiping action from the exposed conductor towards the insulation screen.

Do not use a cleaning tissue that has previously been in contact with the insulation

screen.

Figure 3-22: Removal of Insulation

Remove the yellow void filling tape from the aluminum foil packet. Remove

the release papers from the yellow void filling tape with the pointed ends. Wrap the

void filler around the insulation screen starting 20 mm from the end of the insulation

screen and continuing onto the insulation for 10 mm (see Figure 3-23). Stretch the

tape to half of its original width to achieve a fine, thin edge around the insulation.

Finish on the insulation screen.

Figure 3-23: Wrapping the Void Filler

Page 29

Slide the stress control tubing (black) over the cable level with the end of the

insulation cut. Start shrinking from the insulation cut towards the cable sheath (see

Figure 3-24). Apply additional heat over the area where the void filling tape was

applied.

Figure 3-24: Sliding the Stress Control Tubing

3.2.4 Completion of the Joint

Slide a combined tubing set over the cable. 1 – Screened insulation sleeve (black and

red). 2 – Outer sealing sleeve (black) (see Figure 3-25).

Figure 3-25: Completion of the Joint

The connector is supplied with either half shell inserts or centralizing inserts

for use on smaller conductor cross sections. Check if each of the conductors will fit

with the respective half shell or centralizing insert installed. If the conductor fits,

leave the half shell or centralizing insert fitted. The centralizing insert is a tight fit in

the connector and requires complete insertion. If the conductor does not fit with the

half shell insert or centralizing insert installed, remove and discard the half shell or

centralizing insert from that side of the connector (see Figure 3-26).

Page 30

Figure 3-26: Conductor Fitting

Fit the conductors into the connector. There should be no gap left between the

connector and the insulation. Take up the tension equally on all shear bolts with a tee

bar spanner (do not shear the heads at this stage). Starting at the connector ends and

working towards the middle (following the number sequence indicated), tighten the

bolts until the heads shear off. If a proud edge remains after removal of the bolt heads,

this edge should be filed to obtain a smooth finish (see Figure 3-27).

Figure 3-27: Fitting Connector into the Conductor

Re-align the cables if necessary. The numbers indicate the bolt tightening

sequence. Clean and degrease the cable cores and connector using the cleaning

tissues provided in the kit (see Figure 3-28). Using the clay pack supplied in the kit,

fill the hollows over the sheared off bolts in the connector to obtain a smooth finish.

Figure 3-28: Filling the Hollows over the Sheared-off Bolts

Page 31

Remove the release paper from the stress grading patch (black). Position the

patch centrally over the connector area (see Figure 3-29).

Figure 3-29: Removal of Release Paper from the Stress Grading Patch

The start of the patch should just cover the shear bolts to ensure two layers of

stress grading will be applied over the shear bolt area. Apply the long side of the

patch across the connector. Wrap the patch over the connector. Do not stretch the

patch. Position the screened insulating sleeve (black and red) centrally over the

connector area (see Figure 3-30).

1. Start shrinking the sleeves in the centre (1).

2. Continue shrinking by working towards one side (2), stopping 50 mm from the

end.

3. Shrink the other half in the same way (3).

4. Shrink down the first end (4) and finally the second (5).

Page 32

Figure 3-30: Positioning the Screened Insulating Sleeve

Wrap one half-lapped layer of tinned copper mesh around the cable and across

the full length of the joint (see Figure 3-31). Cover 80 mm of the 150mm long black

tube on the side of the joint with the short screen wires. Fix the screen wires with a

copper wire binder at the end of the tinned copper mesh.

Figure 3-31: Wrapping Half-Lapped Layer of Tinned Copper Mesh

For cable side with long screen wires bend the screen wires back over the joint

area. For cable side with short screen wires bend the screen wires back over the joint

area close to the tinned copper mesh. Gather the screen wires together and cut them

centrally above the 80 mm tinned copper mesh overlap on the cable sheath (see Figure

3-32).

Page 33

Figure 3-32: Gathering the Screen Wires Together and Cutting Them Centrally

Insert the screen wires into the shear head connector supplied. Tighten the

shear heads until the heads shear off. Rotate the screen wire connector so that the

screws of the screen wire connector do not puncture the outer sealing sleeve or the

screened insulating sleeve (see Figure 3-33).

Figure 3-33: Inserting the Screen Wires into the Shear Head Connector

Wrap a second layer of tinned copper mesh around the joint with a 50%

overlap. Cover the complete joint area including the mechanical screen wire

connector (see Figure 3-34).

Figure 3-34: Wrapping Second Layer of Tinned Copper Mesh

Abrade, clean and degrease the cable sheath for a distance of 150mm either

side of the tinned copper mesh (see Figure 3-35).

Page 34

Figure 3-35: Abrading, Cleaning and Degreasing

Wrap one layer of red sealant tape (50mm wide) around the composite sheath

sealing sleeve (Step 9) starting at 20mm from the ends of the mesh on both sides of

the joint (see Figure 3-36).

Figure 3-36: Wrapping Red Sealant Tape

Centre the outer sealing sleeve (black) over the copper mesh area. Start shrinking in

the centre, working towards the ends (see Figure 3-37).

Figure 3-37: Shrinking in the Center

The joint is completed. Allow the joint to cool before applying any mechanical strain

(see Figure 3-38).

Figure 3-38: Cooling of the Joint

Important Note: It must be ensured that a buried joint is surrounded with soft

bedding material up to a depth of 100mm above the joint.

Page 35

3.3 CABLE TERMINATION

3.3.1 Characteristics of Termination Material

A typical Medium Voltage Termination System is shown in Figure 3-39.

Figure 3-39: Medium Voltage Termination System

3.3.1.1 Moisture sealing

Durable sealing is achieved by special Raychem sealants on the inside of non-

tracking, weather-resistant components. At the same time as the installer heats the

tubing, the shrinking action causes the sealant to melt and flow into place. In case of

three core cables, a sealant-lined heat-shrinkable breakout installed over the cores and

cable crutch provides a sealed and weather-resistant surface from the connecting lugs

to the oversheath.

3.3.1.2 Compact and versatile stress control

To meet the need for space-saving, flexible termination design, adaptable to

different types of compact equipment, we developed a Raychem material with

carefully controlled non-linear impedance based on ceramic semiconductor

technology (ZnO), which is applied in the form of a coating inside the tubing. When

the tubing is shrunk, the stress control coating is softened by the applied heat and

conforms and bonds to even irregular insulation surfaces to ensure a void free contact.

Details of electrical stress control in Raychem terminations can be found on page 10.

3.3.1.3 Non-tracking insulation tubing

The superior non-tracking characteristics and long-term erosion resistance of

Raychem terminations have been exhaustively demonstrated in comparative tests at

major independent laboratories and Raychem’s own extensive development facilities.

Page 36

These results are borne out by the continuing performance of over a million

units installed in tropical, desert, arctic and industrially polluted climates, confirming

that Raychem terminations do not track even in severe service conditions and

verifying their exceptional erosion resistance and reliability.

3.3.1.4 Yellow void filler

The semi-conducting void filler is easily applied in form of a short adhesive

tape. It ensures that, independent of the type of semi-conductive screen or removal

method, no air voids can cause discharges in the high stress area of the screen end.

3.3.1.5 Earthing

Earthing wires or braids are imbedded in the sealing mastic to prevent any

corrosion by moisture ingress. For cables with tape screen or metal sheaths with

armour solder less earthing systems are either provided within the termination kit or

can be ordered separately.

3.4 PROCEDURE OF CABLE TERMINATION

3.4.1 Cable Preparation for Termination

Steps are below (see Figure 3-40 to Figure 3-44):

1. Cut the cable and remove both the layers of composite sheath for 320mm

minimum. Remove the outer layer of composite sheath for 40mm. Leave

enough length to set the cores into their final position. Degrease and clean the

end of the oversheath for about 100 mm using the cleaning tissue provided in

the kit.

2. Wrap one layer of red sealant tape (50mm wide) with slight tension starting at

the end of the inner sheath cutback, downwards for 80 mm. Bend the shielding

wires from each core back onto the oversheath. Avoid crossing individual

wires. Temporarily fix the shielding wires to the oversheath.

Page 37

Figure 3-40: 1) Cutting of Cable & Sheath Removal; 2) Sealant Tape Wrapping

3. Remove any filler up to the end of the inner sheath cutback. Bend and shape

the cores into their final position. Cut the cores to the required length.

Thoroughly remove the core screen according to dimension a (see Error:

Reference source not found for crimp lugs. For mechanical lugs see Table 3-

2). The surface of the insulation should be free from all traces of conductive

material. Smooth out any irregularities.

4. Wrap one layer of red sealant tape (50mm wide) over the screen wires for

80mm. Mark the core screen 40 mm below the screen cut. Measure the

distance c of each individual core and cut the conductive tubing accordingly.

Figure 3-41: 3) Removal of Filler; 4) Cutting the Conductive Tubing

Page 38

Table 3-1: Crimp Lug

95 to 24

a

[mm]

11kV indoor/outdoor 200

Table 3-2: Mechanical Lug BLMT

BLMT(range mm2)

95 to 240

a

[mm]

11kV indoor/outdoor 200

5. Position the conductive tubing over the cores 40 mm below the end of the core

screen. Shrink each tubing into place by starting on top and continue shrinking

them down towards the cable crotch. Allow the tubing to cool before

continuing.

6. Pull the breakout down the crotch as far as possible. Shrink the breakout into

place starting at the centre. Work first towards the lower end and then shrink

the turrets onto the cores. The numbers in the drawing indicate the shrinking

sequence.

7. Cut back the insulation according to K = depth of cable lug barrel hole + 5

mm for crimp lug. BLMT = depth of the cable lug + 0 mm. Install the cable

lug. Remove any sharp edges. Degrease and clean the lug and the insulation

using the cleaning tissue provided in the kit.

Figure 3-42: 5) Positioning the Conductive Tubing; 6) Pulling Breakout 7) Cutting Insulation

Page 39

Note: Do not use cable lugs with barrel holes deeper than 110 mm.

8. Remove the release paper and wrap the void filling strip (yellow) around the

end of the core screen. Stretch the strip to half of its original width to achieve

a fine, thin edge onto the insulation. Cover 20 mm of the core screen and

continue onto the insulation for 10 mm.

9. Apply one complete turn of red sealant mastic (20mm wide) around the barrel

of the lugs, at the tab end of the lug barrels as depicted covering the top

fastener of compressed area as applicable. Use the remaining sealant to fill in

the space between the core insulation and the cable lug to leave a smooth

transition.

10. Preheat the cable lug slightly before placing the tubing over the core so that

the top of the tubing covers the crimping area or the top fastener of the cable

lug. Shrink the tubing down starting at the screen cut using a soft yellow

flame. Heat the area well but avoid scorching of surface. Continue shrinking

towards the cable lug. Finally shrink down the bottom end of the tubing. The

numbers in the drawing indicate the shrinking sequence.

Figure 3-43: 8) Removing Release Paper; 9) Applying Sealant Mastic; 10) Preheating Cable Lug

Page 40

11. Indoor termination completed.

Note: After installation the termination must be post-heated as well as the

palm of the cable lug until a bead of sealant (green) appears around the top of

the tubing. Allow the termination to cool before applying any mechanical

strain. Tie the shielding wires or the earth leads with a wire binder to the

oversheath below the breakout. Gather the shielding wires together to form an

earth lead.

12. For outdoor terminations:

Shrink the skirts into place at the position shown in the drawings on the next

page. Start with the first skirt on the lowest position.

Figure 3-44: 11) Indoor Termination; 12) Outdoor Termination

Page 41

CHAPTER 4.TESTING OF THE UNDERGROUND

CABLES

Three types of fault may occur in underground cabling i.e. open circuit fault, short

circuit fault and earth fault. To avoid these faults there are some tests carried out at

site before laying and after laying of each cable span.

Before Laying

Insulation Resistance Test

Continuity Test

After Laying

Insulation resistance test

Continuity test

DC pressure test

(For 15 minutes, if not possible than test for 01 minute with 1000 V Megger

for LT cable and with 2500/5000 V Megger for HT cable)

4.1 INSULATION RESISTANCE TESTING

Insulation starts to age as soon as it's made. As it ages, its insulating

performance deteriorates. Any harsh installation environment, especially with

temperature extremes and/or chemical contamination, accelerates this process. This

deterioration can result in dangerous conditions in power reliability and personnel

safety. As such, it's important to identify this deterioration quickly so that corrective

steps can be taken. One of the simplest tests and its required test instrument are not

universally understood. To help eliminate this lack of understanding, let's discuss in

detail Insulation Resistance (IR) testing and the megohmmeter.

4.1.1 Insulation Testing Components

Insulation testing components are below.

Page 42

4.1.1.1 The Megohmmeter

A basic megohmmeter hook-up schematic is shown in Figure 4-45. The

megohmmeter is similar to a multimeter, when the latter is in its ohmmeter function.

There are differences, however.

Figure 4-45: Schematic of a Megohmmeter

First, the megohmmeter's output is much higher than that of a multimeter.

Voltages of 100, 250, 500, 1,000, 2500, 5,000, and even 10,000V are used (Table 4-

3). The most common voltages are 500V and 1,000V. Higher voltages are used to

stress insulation to a greater degree and thus obtain more accurate results.

Table 4-3: Recommended test voltages for routine maintenance insulation-resistance tests of equipment rated to 4,160V and above.

Equipment AC Rating DC Test Voltage

 Up to 100V  100V and 250V

 440V to 550V  500V and 1,000V

 2,400V  1,000V to 2,000V and higher

 4,160V and above  1,000V to 5,000V or higher

 Second, the range of a megohmmeter is in megohms, as its name implies,

instead of ohms as in a multimeter. Third, a megohmmeter has a relatively high

internal resistance, making the instrument less hazardous to use in spite of the higher

voltages.

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Testing Connections: A megohmmeter usually is equipped with three terminals. The

"LINE" (or "L") terminal is the so-called "hot" terminal and is connected to the

conductor whose insulation resistance you are measuring. Remember: These tests are

performed with the circuit de-energized. The "EARTH" (or "E") terminal is connected

to the other side of the insulation, the ground conductor.

The "GUARD" (or "G") terminal provides a return circuit that bypasses the

meter. For example, if you are measuring a circuit having a current that you do not

want to include, you connect that part of the circuit to the "GUARD" terminal.

Figure 4-46, Figure 4-47 and Figure 4-48 show connections for testing three

common types of equipment. Figure 4-46 shows a connection for testing a

transformer bushing, without measuring the surface leakage. Only the current through

the insulation is measured, since any surface current will be returned on the

"GUARD" lead.

Figure 4-46: Connection for Testing a Transformer Bushing

4.1.2 Various Insulation Tests

Basically, there are three different tests that can be done using a megohmmeter.

4.1.2.1 Insulation Resistance (IR) Test

This is the simplest of the tests. After the required connections are made, you apply

the test voltage for a period of one min. (see Figure 4-47). (The one-min interval is an

industry practice that allows everyone to take the reading at the same time. In this

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way, comparison of readings will be of value because, although taken by different

people, the test methods are consistent.)

Figure 4-47: Insulation Resistance Test of Motor

During this interval, the resistance should drop or remain relatively steady. Larger

insulation systems will show a steady decrease, while smaller systems will remain

steady because the capacitive and absorption currents drop to zero faster on smaller

insulation systems. After one min, read and record the resistance value.

Figure 4-48: Insulation Resistance Test of Cable

Note that IR is temperature sensitive. When the temperature goes up, IR goes

down, and vice versa. Therefore, to compare new readings with previous readings,

you need to correct the readings to some base temperature. Usually, 20°C or 40°C are

used as comparison temperatures; tables are available for any correction. However, a

common rule of thumb is that IR changes by a factor of two for each 10°C change.

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For example, suppose we obtained an IR reading of 100 megohms with an insulation

temperature of 30°C. The corrected IR (at 20°C) would be 100 megohms times 2, or

200 megohms.

Also note that acceptable values of IR will depend upon the equipment.

Historically, field personnel have used the questionable standard of one megohm per

kV plus one. The international Electrical Testing Assoc. (NETA) specification

NETA MTS-1993,Maintenance Testing Specifications for Electrical Power

Distribution Equipment and Systems, provides much more realistic and useful values.

Test results should be compared with previous readings and with readings

taken for similar equipment. Any values below the NETA standard minimums or

sudden departures from previous values should be investigated.

4.1.2.2 Dielectric Absorption Ratio

This test recognizes the fact that "good" insulation will show a gradually

increasing IR after the test voltage is applied. After the connections are made, the test

voltage is applied, and the IR is read at two different times: Usually either 30 and 60

sec, or 60 sec and 10 min. The later reading is divided by the earlier reading, the result

being the dielectric absorption ratio. The 10 min./60 sec. ratio is called the

polarization index (PI).

For example, let's assume we apply the megohmmeter as described earlier

with the appropriate test voltage impressed. The one min. IR reading is 50 megohms,

and the 10 min. IR reading is 125 megohms. Thus, the PI is 125 megohms divided by

50 megohms, or 2.5. Various sources have tables of acceptable values of dielectric

absorption ratios (see Table 4-4 below). The values must be considered tentative and

relative, subject to experience with the time-resistance method over a period of time.

Table 4-4: Listing of Conditions of Insulation as Indicated by Dielectric Absorption Ratios.

 Insulation Condition 60/30-sec Ratio10/1-min Ratio

(Polarization Index)

Dangerous - Less than 1

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Questionable 1.0 to 1.25 1.0 to 2*

Good 1.4 to 1.6 2 to 4

Excellent Above 1.6** Above 4**

*These results would be satisfactory for equipment with very low capacitance,

such as short runs of house wiring.

**In some cases with motors, values approximately 20% higher than shown

here indicate a dry, brittle winding that may fail under shock conditions or during

starts. For preventative maintenance, the motor winding should be cleaned, treated,

and dried to restore winding flexibility.

4.1.2.3  Step Voltage Test

This test is particularly useful in evaluating aged or damaged insulation not

necessarily having moisture or contamination. A dual voltage test instrument is

required here. After the connections are made, the IR test is done at a low voltage, say

500V. The test specimen then is discharged and the test is done again, this time at a

higher voltage, say 2500V. If more than a 25% difference exists between the two IR

readings, age deterioration or damaged insulation should be suspected.

4.2 CONTINUITY TEST

Schematic of a continuity test is illustrated in Figure 4-49.

Figure 4-49: Schematic of Continuity Test

4.2.1 Tools & Equipments Required

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1. Multimeter 

2. Wire Nipper 

3. Screw Driver Set

4. Box Spanner

4.2.2 Test Procedure

This test is carried out to confirm that the core under test is either showing break

between both ends or continuous. Testing can be commenced as per the following

procedure:

Set the knob of multimeter to check resistance at 200 ohm range.

(At Location A)

Connect one probe of multimeter to earth and other probe to the end of the

cable conductor to be tested, as shown in above figure.

Instruct staff at the other end (at Location B) to connect earth to same

conductor of the cable.

If earth is light at both ends, connect earth to armour also at both the ends.

Deflection of multimeter needle shows that the conductor under test is OK;

otherwise there is a break in the conductor.

Then test continuity of all other conductors with respect to

this tested conductor. For example to test conductor 2, connect the one probe

of the multimeter to conductor 2 and other probe to the tested conductor (at

Location A). Instruct the staff at other end (at Location B) to short conductor 2

with the tested conductor. Test continuity of all other conductors as above.

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CHAPTER 5.CABLE FAULT LOCATING TESTS

Following tests are used to locate fault in an underground cable.

5.1 BLAVIER TEST

Blavier’s test is used to find the earth fault location in an underground cable.

The two ends faulty cable are mentioned as sending end and far end respectively as

shown in Figure 5-50. In this test, the sending end of the cable must be open and

isolated and the resistance between sending end and earth point is measured by

keeping the far end isolated from earth and then it measured keeping far end of the

faulty cable, shorted to the ground.

Suppose, we get, resistance values R1 and R2 in these two said measurements

respectively. In the fault location, the conductor is shorted to ground, because of fault.

Thus, this short circuit may have some resistance that is mentioned as ‘g’. In Blavier’s

test the total line resistance is supposed to be mentioned as ‘L’. The resistance

between the sending end to the fault end is mentioned as ‘x’ and the resistance

between the fault end to the far end is denoted as ‘y’. So, the total resistance ‘L’ is

equals to the addition of ‘x’ and ‘y’ resistances.

Now, the total resistance of the ‘x’ and ‘g’ loop is nothing but ‘R1’ – the

conductor resistance between sending end and earth by keeping far end open,

The total resistance of the entire loop of the above circuit is nothing but R2 – the

conductor resistance between sending end and earth by keeping far end earthed.

By solving the above three equation and eliminating g and y;

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This expression gives the resistance from the sending end to the fault location. The

corresponding distance is calculated by known resistance per unit length of the cable.

A practical difficulty in Blavier's test is that the resistance to ground ‘g’ is variable,

being influenced by the amount of moisture present in the cable and the action of the

current at the fault condition. Also, the resistance ‘g’ may be so high that it exerts

very little shunting action when y is placed in parallel with it by grounding the far end

of the line.

Figure 5-50: Blavier's Test to Find the Cable’s Fault Location

5.2 MURRAY LOOP TEST

This test is used to find the fault location in an underground cable by making

one Wheatstone bridge in it and by comparing the resistance we shall find out the

fault location. But we should use the known length of the cables in this experiment.

The necessary connection of the Murray loop test is shown in Figure 5-51 & Figure 5-

52. The Figure 5-51 shows that the circuit connection for finding the fault location

when the ground fault occurs and the Figure 5-52 shows that the circuit connections

for finding the fault location when the short circuit fault occurs.

In this test, the faulty cable is connected with sound cable by a low resistance

wire, because that resistance should not affect the total resistance of the cable and it

should be able to circulate the loop current to the bridge circuits without loss.

The variable resistors R1 and R2 are forming the ratio arms. Balance of the

bridge is achieved by adjusting the variable resistors. ‘G’ is the galvanometer to

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indicate the balance. [R3 + RX] is the total loop resistance formed by the sound cable

and the faulty cable.

Figure 5-51: Circuit Connection of Murray Loop Test for Ground Fault

Figure 5-52: Circuit Connection of Murray Loop Test for Short Circuit Fault

At the balance condition,

When the cross section area of the both sound cable and faulty cable are equal,

then the resistances of the conductors are directly proportional to their lengths. So, if

LX represents the length between test end to the fault end of the faulty cable and if L

represents the total length of the both cables, then the expression for LX is as follows;

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The above test is only valid when the lengths of the cables are known. In

Murray Loop Test, the fault resistance is fixed and it may not be varied. Also it is

difficult to set the bridge as balance. Thus, the determination of the fault position is

not accurate. Then the current circulation through the cable would cause temperature

rises due to high voltage or high current. If the resistance varies according to the

temperature, then the balance collapses. So, we need to apply less voltage or less

current to this circuit.

5.3 VARLEY LOOP TEST

This test is used to find the fault location in an underground cable by making

one Wheatstone bridge in it and by comparing the resistance we shall find out the

fault location instead of calculating it from the known lengths of the cable. The

necessary connection of the Varley loop test is shown in Figure 5-53 & Figure 5-54.

The Figure 5-53 shows that the circuit connection for finding the fault location when

the ground fault occurs and the Figure 5-54 shows that the circuit connections for

finding the fault location when the short circuit fault occurs.

In this test, the faulty cable is connected with sound cable by a low resistance

wire, because that resistance should not affect the total resistance of the cable and it

should be able to circulate the loop current to the bridge circuits without loss. A single

pole double through switch ‘S’ is used in this circuit. There would be a variable

resistor ‘R’ which is used to balance the bridge circuit during the working period.

If the switch ‘S’ is in position 1, then we need to adjust the variable resistance ‘R’ to

balance the circuit. Let us assume that the present ‘R’ value as ‘RS1’. At this position,

the expressions are as follows;

This expression gives the value of [R3 + RX], if the value of R1, R2 and RS1

are known. If the switch ‘S’ is in position 2, then again we need to adjust the variable

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resistance ‘R’ to balance the bridge circuit. Let us assume that the new ‘R’ value as

‘RS2’. At this position, the expressions are as follows;

By solving the equation (1) and (2),

Therefore, the unknown resistance RX is,

Figure 5-53: Circuit Connection of Varley Loop Test for Ground Fault

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Figure 5-54: Circuit Connection of Varley Loop Test for Short Circuit Fault

Varley Loop Test is valid only when the cable sections are uniform throughout

the loop. The current flowing through the cable would cause the temperature effect.

Due to this temperature effect, the resistance of the cable would change. Thus, we

need to apply less current to this circuit to carry out the experiment.

5.4 FISHER LOOP TEST

In this Fisher Loop Test, there must be two healthy sound cables which must have the

same length and same cross sectional area as the faulty cable. As per the Figure 5-55

& Figure 5-56 circuit diagram, all the three cables are connected by a low resistance

wire.

Figure 5-55: Circuit Connection (A) for Fisher Loop Test to Locate the Cable Fault

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Figure 5-56: Circuit Connection (B) for Fisher Loop Test to Locate the Cable Fault

In the Figure 5-55 circuit connection, the bridge connection is connected to

ground. Now, the bridge arms are RA, RB, RX and [RS1 + RY]. In the Figure 5-56

circuit connection, the bridge connection is connected to ‘Sound Cable 2’. Now, the

bridge arms are RA', RB', RS2 and [RX + RY]. Here [RS1 = RS2]. Two balancing are

necessary as per the two different circuits. Let, for the first balance, the expressions

are as follows;

For the second balance, the expressions are as follows;

From the expression (1) and (2),

In this two circuits, if the bridge arm resistors are equal (or) if [(RA + RB) =

(RA' + RB')], then the expression (3) can be modified as,

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So, when the resistance per unit length of the conductor is uniform in all

conditions, then the fault location LX is as follows;

Here ‘L’ if the total length of the faulty cable. But practically, this is not

possible. There would be fractional changes in the bridge arms. Thus, the fault

location LX is as follows;

This is about the working principle of “Fisher Loop Test”.

CHAPTER 6.CABLE FAULT LOCATING

INSTRUMENTS

Following instruments are used for location of cable faults.

6.1 SURGE GENERATOR

The surge generator is a lightweight and compact unit that impulses at 3, 6, 9,

12, or 15-kV. It offers benefits that ensure efficient and effective fault locating. Figure

6-57 shows a simple surge generator.

Features: Rugged construction — housed in a sturdy fiberglass case,

designed to withstand the rigors of field operation.

Convenient transport/storage — its light weight allows one

person to carry unit easily. Its compact size allows for easy

storage and transport in tight locations.

Noiseless discharge — SCR provides discharge with no air gap

or moving parts. Quiet discharge is extremely desirable in

confined locations.

Simple operation — minimum of controls and instrumentation

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to reduce operation complexity.

Figure 6-57: Surge Generator

6.2 CABLE FAULT PINPOINTING EQUIPMENT

6.2.1 Surge Detector

The surge detector is designed to locate faults in shielded, direct buried cables

by detecting both the electromagnetic and acoustic pulses emitted from an arcing fault

when it is surged. Either single or dual detector configurations are available. A typical

surge detector (Model SD-3000) is shown in Figure 6-58 (A).

Features: Determines distance and direction to the fault

Operates in all weather conditions

Selectable acoustic frequency band

6.2.2 Electromagnetic Impulse Detector

The Electromagnetic Impulse Detector is used primarily to localize faults on cable in

duct or conduit. The instrument is comprised of an amplifier module, a sheath coil and

a carrying case. Electromagnetic Impulse Detector is shown in Figure 6-58 (B).

Features: Indicates direction of fault

Works under all weather conditions

Converts to voltage gradient tester with optional earth frame

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Figure 6-58: A) Surge Detector, SD-3000; B) Electromagnetic Impulse Detector

6.3 SURGE DETECTION PROCEDURE

Some surge detectors combine both electromagnetic and acoustic pickups to

efficiently pinpoint the fault. The working principle of Model SD-3000 Surge

Detector is shown in Figure 6-59. A pickup in the receiver detects the magnetic field

produced by the current impulse and also displays its magnitude on a bar graph

display every time the thumper discharges. The indicated magnitude of the impulse

will decrease if the fault has been passed or if the receiver is no longer over the cable

route. After detecting an impulse, an acoustic pickup placed on the ground listens for

a thump as a result of the discharge. The detected impulse starts a timer in the receiver

and when an audible thump is sensed, the timer is stopped. This measurement is the

time it takes the sound wave produced at the fault to travel to the acoustic pickup and

is displayed in milliseconds.

As the fault is approached, this time interval decreases to a minimum directly

over the fault and increases again as the fault is passed. The time never goes to zero

because there is always the depth of the cable between the pickup and the fault. This

technique relies on the elapsed time between the two events, not simply the loudness

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of the sound and thereby eliminates the problems of accurate pinpointing even under

difficult conditions.

If two acoustic pickups are used, the receiver makes dual measurements and

indicates with an arrow on the display which direction to move toward the fault. As

the fault is approached, the tail on the arrow becomes shorter until the fault is passed

when the direction of the arrow reverses. At this point, small movements of the

pickups are made. When they actually straddle the fault, two arrowheads appear

pointed toward each other. Once the instrument “hears” the thump, headphones are no

longer necessary and the measurements will lead the operator directly to the fault. The

receiver also measures and displays a digital value of the sound level, which typically

increases as the fault is approached. By using the Save function in the unit, two sets of

time and sound level values can be saved and displayed while observing the current

values which confirm that the direction being taken is correct.

Figure 6-59: Acoustic/Electromagnetic Pinpointing

The SD-3000 provides information on its liquid crystal display, which will efficiently

and quickly guide the operator to within inches of the exact fault location:

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Features: Intensity of the surge impulse.

Elapsed time between the impulse and thump.

Magnitude of the sound.

Direction and relative distance to the fault.

Figure 6-60 shows typical displays of the SD-3000.

Figure 6-60: SD-3000 Display at Positions 1, 2, And 3

CHAPTER 7. UNDERGROUND DISTRIBUTION

NETWORKS

7.1 TYPES OF THE U/G DISTRIBUTION NETWORKS

Distribution system is a circuit of users linked to a generating station and

substations that is typically arranged in either a radial or interconnected manner.

Local distribution systems transport power within a building. All distribution of

electrical energy is done by constant voltage system. In practice, the following

distribution circuits are generally used

7.1.1 Ring Distribution System

The loop or ring system of distribution starts at the substation and is connected to or

encircles an area serving one or more distribution transformers or load centre. The

conductor of the system returns to the same substation. The loop system (Figure 7-61)

is more expensive to build than the radial type, but it is more reliable.

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Figure 7-61: Operation of Ring Distribution System

It may be justified in an area where continuity of service is of considerable

importance, for example a medical centre. In the loop system, circuit breakers

sectionalize the loop on both sides of each distribution transformer connected to the

loop. The two primary feeder breakers and the sectionalizing breakers associated with

the loop feeder are ordinarily controlled by pilot wire relaying or directional over

current relays. Pilot wire relaying is used when there are too many secondary

substations to obtain selective timing with directional over current relays.

A fault in the primary loop is cleared by the breakers in the loop nearest the

fault, and power is supplied the other way around the loop without interruption to

most of the connected loads. Because the load points can be supplied from two or

more directions, it is possible to remove any section of the loop from service for

maintenance without causing an outage at other load points.

If a fault occurs in a section adjacent to the distribution substation, the entire

load may have to be fed from one side of the loop until repairs are made. Sufficient

conductor capacity must be provided in the loop to permit operation without excessive

voltage drop or overheating of the feeder when either side of the loop is out of

service. If a fault occurs in the distribution transformer, it is cleared by the breaker in

the primary leads; and the loop remains intact.

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7.1.1.1 Advantages of Ring Distribution System

i. Less copper is required as each part of the ring carries less current than

that in radial system.

ii. Less voltage fluctuations.

iii. It is more reliable. In the event of fault on any one section the continuity of

supply to all consumers can be maintained by isolating the faulty section

iv. Disadvantages of Ring Distribution System

v. High cost of maintenance

vi. It only used in urban place.

7.1.2 Radial Distribution System

A representative schematic of a radial distribution system is shown in Figure

7-62. You should note that the independent feeders branch out to several distribution

centers without intermediate connections between feeders.

The most frequently used system is the radial distribution system because it is

the simplest and least expensive system to build. Operation and expansion are simple.

It is not as reliable as most systems unless quality components are used. The fault or

loss of a cable, primary supply, or transformer will result in an outage on all loads

served by the feeder. Furthermore, electrical service is interrupted when any piece of

service equipment must be de-energized to perform routine maintenance and service.

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Figure 7-62: Radial Distribution System

Service on this type of feeder can be improved by installing automatic circuit

breakers that will reclose the service at predetermined intervals. If the fault continues

after a predetermined number of closures, the breaker will lock out until the fault is

cleared and service is restored by hand reset.

7.1.3 Interconnected System

When the feeder ring is energized by two or more than two generating stations or

substations, it is called inter-connected system. The Figure 7-63 shows the single line

diagram of interconnected system where the closed feeder ring ABCD is supplied by

two substation S1 and S2 point D and C respectively. Distributors are connected to

points O, P, Q and R of the feeder ring through distribution transformers.

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Figure 7-63: Single Line Diagram of Interconnected System

The interconnected system has the following advantages:

i. It increases the service reliability.

ii. Any area fed from one generating station during peak load hours can be

fed from the other generating station. This reduces reserve power capacity

and increases efficiency of the system.

7.2 SWITCHGEARS USE IN UNDERGROUND

DISTRIBUTION SYSTEM

7.2.1 Ring Main Unit (RMU)

Ring Main Unit as an important part of Secondary Distribution Substations

(Figure 7-64) A Ring Main Unit (RMU) is a totally sealed, gas-insulated compact

switchgear unit. The primary switching devices can be either switch disconnectors or

fused switch disconnectors or circuit breakers.

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Figure 7-64: Ring Main Unit (RMU)

Different combinations of these primary switching devices within the unit are

commonly used. In case a circuit breaker is the switching device, it is also equipped

with protective relaying, either with a very basic self-powered type or a more

advanced one with communication capabilities. A single-line representation of a

typical RMU configuration is show in Figure 7-65.

Figure 7-65: Single-Line Representation of a Typical RMU Configuration

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The rated voltage and current ranges for RMUs typically reach up to 24

kV and 630 Arespectively. With many of the manufacturers of RMUs, the basic

construction of the unit remains the same for the whole of the voltage range. An

example of distribution network with Ring Main Units is shown in Figure 7-66.

Figure 7-66: An Example of Distribution Network With Ring Main Units

The increase in rated voltage is handled by an increase in the insulating gas

pressure. The figure above shows a typical RMU configuration where load

disconnectors are the switching devices for the incoming cable feeders and circuit

breaker works as the switching device for distribution transformer feeder.

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7.2.2 Padmount Transformer

A padmount or pad-mounted transformer (Figure 7-67) is a ground mounted

electric power distribution transformer in a locked steel cabinet mounted on a

concrete pad. Since all energized connection points are securely enclosed in a

grounded metal housing, a padmount transformer can be installed in places that do not

have room for a fenced enclosure. Padmount transformers are used with

underground electric power distribution lines at service drops, to step down the

primary voltage on the line to the lower secondary voltage supplied to utility

customers. A single transformer may serve one large building, or many homes.

Figure 7-67: Padmount Transformer

Pad-mounted transformers are made in power ratings from around 75 to

around 5000 kVA and often include built-in fuses and switches. Primary power cables

may be connected with elbow connectors, which can be operated when energized

using a hot stick and allows for flexibility in repair and maintenance.

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7.2.3 Padmount 3 Phase Distribution Panel 

This marine-grade aluminum distribution panel is a padmount enclosure generally

used for highway intersection lighting control. It has a built in meter base with a

viewing window, and ample room for cable termination (see Figure 7-68).

Figure 7-68: Padmount 3 Phase Distribution Panel

7.2.3.1 Features/Specifications

i. Marine-grade aluminum construction

ii. Removeable lifting ears

iii. Powder coated ASA 61 Grey

iv. NEMA 3R rated

v. All doors have a padlockable stainless steel 3-point latch

vi. mechanism and gas shock door stays

vii. Designed to fit precast base

viii. Deadfront wireways (conduit extensions not required)

ix. Built in meter base

x. Customer specified breaker panel

xi. Used in 3 phase 347/600V applications

xii. Service entrance rated main breaker

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7.2.4 Street Light Control Panel

This stainless steel lighting control panel is specifically designed for areas where

corrosion is an issue. The compact form makes it easy to install and maintain, and the

pad-lockable door offers added security (see Figure 7-69). This multi-purpose lighting

control panel offers extreme protection in a compact footprint

Figure 7-69: A Typical Street Light Panel

7.2.4.1 Features

i. Stainless steel construction

ii. NEMA 3R rated

iii. Service entrance rated

iv. Branch circuits

v. Lighting contactors in control

vi. section

vii. Pole mount

viii. Test switch

ix. Pad-lockable cover

x. Top photocell access

xi. Mounting hardware and required fittings included

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CONCLUSIONWith proper consideration of the many factors related to design, specification,

manufacturing, installation, and commissioning, underground cable systems can be a

viable alternative to overhead lines where the use of cable is warranted because of

rights-of-way constraints, sensitive areas along the planned route, specialized

obstacles (waterways, bridges, etc.) that must be crossed, concerns about weather

effects and reliability affecting overhead lines, or clearance limitations to get into a

congested substation. Though the material and installation costs of underground

power cables are higher than comparable capacity overhead lines, factors such as real

estate, permitting and constructability can often make underground the preferred

alternative as a complete underground system or portions of a hybrid underground and

overhead circuit.

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APPENDIX A: ARRANGEMENT OF CABLES IN TRENCHES

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BIBLIOGRAPHY

[1]

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VITA

The author of the thesis is currently an employee of Habib

Rafiq (Pvt.) Ltd., HRL a multinational construction company

since 2004. He is resident of Mandi Bahauddin, Punjab, Pakistan.

He completed his technical education in 2004 and started his

career in Speciallists Group Incorporated (SGI) as a technician.

Then he joined Habib Rafiq (Pvt.) Ltd. as an electrical technician

and was promoted to electrical supervisor due to his meritorious performance in the

said field. After completion of B. Tech (Pass), he was given the charge of assistant

electrical engineer in 2009. Since then he is working in the aforementioned

organization and has solved many practical problem in the field of underground cable

laying, jointing and termination.

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