Cable

Cable

1. Introduction

Electric wires and cables have become such an important part of everyday life that without

them the world as we know it would simply not exist. For without wires and cables the existence

and operation of conveniences such as electric lights, telephones, computers and a host of other

household appliances would not have been possible. Moreover, as the standard of living rises, so

does the demand for those types of products. Consequently, there has been an incredible increase

in the demand for electric wire and cable. As developing nations around the world continue to

develop, this demand will undoubtedly continue to rise.

For example, Taiwan, the Republic of China, a country with a population of only 19 million,

has more than one hundred factories successfully producing electric wire and cable to satisfy the

needs of the domestic market. According to an estimate made in 1984,the total production of

electric wire and cable had reached a level of 200,000 tons per year. Furthermore, the vast

majority of this cable was purchased domestically by retailers, manufacturers, construction

contractors, and the government owned power and telephone companies. Clearly, the

establishment of an electric wire and cable making plant is a project worthy of investment.

The wire and cable making plant described in this particular proposal is designed for the

production of wire and low voltage (below 600V) power cable. It is not intended to be used for

the production of telecommunication or high voltage power cable, as the plants capable of

producing these types of cable are considerably more expensive and require a higher level of

technical knowledge to set up. The types of wire and cable which can be produced by the

machinery outlined in this proposal are classified as follows:

I. Single wire: PVC insulated single copper conductors.

II. Multiple wires: PVC insulated copper conductors consisting of 7-61 stranded wires.

III. Flexible wire: single or twin core, PVC insulated cords consisting of 20-100 fine copper

wires.

IV. Flat twin-cord wire: twin core, PVC insulated single or multiple copper conductors,

sheathed with PVC layers.

V. Power cables: three of four cores of PVC insulated multiple copper conductors assembled

together.

VI. Armour cable: power cables consisting of three or four round and shaped cores armored

with steel wires and sheathed with PVC layers.

2. General Process Information

2.1. Process Description

(1) Drawing

Bulk copper is formed into wire of varying diameters by drawing it through a series of dies.

(2) Annealing

Since the drawing process causes the copper to become hard and brittle, it should be annealed.

(3) Stranding

Anywhere from 20-100 (very fine copper conductor wires) are twisted into cords which will be

used in making flexible wire and cable.

(4) Twisting

Layers of wires (1+6+12+18+24 etc.) are stranded together to make copper conductors. The

maximum nominal cross-section area of a power cable core is 500m㎡.

(5) Insulating

The copper conductors, whether they are single wire or multiple stranded wire, are covered by

PVC for current insulation.

(6) Lay-up

Three or four of these PVC insulated copper conductors are assembled into power cables.

(7) Sheathing

Complete cables are formed by sheathing twin-core or multiple-core PVC insulated copper

conductors with PVC.

(8) Armoring

Special purpose power cables must be surrounded with steel wires in order to increase the cable

structure strength.

*For Special Purpose Power Cable Only.

2.2. Flow Chart

3. Plant Description

3.1. Production capacity

According to investor’s different situations, we classify the plant within three sizes.

Plant scale Copper Production

consumption capacity

Mini plant 30 Tons/month 45 Tons/month

Small plant 60 Tons/month 90 Tons/month

Medium

plant120 Tons/month 180 Tons/month

3.2. Raw materials

- - PVC grain (30% of the product weight)

- - 8.0mm or 2.6mm diameter copper rod (70% of the product weight)

In order to save money for the beginning investors, it is more economical to purchase 8mm (or

2.6mm) diameter copper rod and PVC grains as raw materials. These raw materials can be

supplied by Taiwan. For the detail information, please contact the supplier in Taiwan.

3.3. Manpower

Job

Classification

Manpower Required

Mini

plant

Small

plant

Medium

plant

Staff 2 4 6

Technician 2 3 4

Skilled operator 8 10 16

Unskilled labor 8 13 24

Total 20 30 50

3.4. Machinery and equipment

(1) Machinery

Name of Machinery

Plant scale

Min

i

Smal

l

Mediu

m

8mmØ copper rod breakdown machine 0 1 1

Medium Wire drawing machine

Fine Wire drawing machine

Annealing machine

Flexible wire twisting machine

7-Wire stranding machine

19-Wire stranding machine

37 wire stranding machine

3-4 core cable lay-up machine

70mm PVC insulation machine

90mm PVC insulation and sheathing

machine

120mm PVC sheathing machine

1

2

1

1

1

1

0

0

1

1

0

1

4

1

2

2

0

1

1

2

1

0

2

8

2

4

2

1

1

1

2

2

1

(2) Accessory equipment

- Copper wire drawing dies

- Copper wire welding devices

- Fork Lift

- Air Compressor

- Cooling systems for drawing lubrication oil

- Drums and bobbins

(3) Accessory equipment

- Lathe

- Milling machine

- Drilling machine

- Grinding machine

- Grinding machine

- Welding machine

3.5. Inspection and testing equipment

(1) Mega meter

(2) Tensile tester

(3) Heat deformation tester

(4) Dielectric withstanding tester

(5) Spark tester

(6) Aging tester

(7) Drawing dies polishing machine

3.6. Utilities

(1) Requirement of water (KL/month)

Mini

plant

Small

plant

Medium

plant

1,500 2,200 3,000

(2) Power requirement capacity (KW)

Mini

plant

Small

plant

Medium

plant

1,000 1,500 2,500

3.7. Plant site planning

(1) Abundant electric power supply and water supply

(2) Abundant labor

(3) Convenient transportation

(4) Lower humidity and far from the seashore

3.8. Area of land and plant building (square meter)

ItemMini

plant

Small

plant

Medium

plant

Land

Plant

building

2,500

1,500

3,200

2,000

5,000

3,000

3.9. Plant layout

(1) Mini size plant layout Legend:

1. QC room

2. Conference room

3. Lobby

4. Office

5. Manager room

6. Raw material storage

7. Product storage

8. Packing area

9. Annealing machine

10. Medium drawing machine

11. 19B stranding machine

12. Fine drawing machine

13. Twisting machine

14. 7B stranding

15. Maintenance room

16. 70mm PVC insulation machine

17. 90mm PVC insulation and sheathing machine

(2) Small size plant layout Legend:

1. QC room

2. Conference room

3. Lobby

4. Office

5. Manager room

6. Raw material storage

7. Products storage

8. Packing area

9. Breakdown machine







16. Cable lay-up machine


18. 90mm PVC sheathing machine


(3) Medium size plant layout Legend:

1. QC room

2. Conference room

3. Lobby

4. Office

5. Manager rooms

6. Raw materials storage

7. Products storage

8. Packing are

9. Break down machine









18. Cable lay-up machine


20. 90mm PVC insulation & sheathing machine


4. Suppliers Information

U Gear Automatic Machinery Co.,LTD .

5F,No,10,Lane315,chungshan Rd.,Sec.2, Chungho, Taipei Hsien, Taiwan,R. O. C.

TEL:886-2-2249-0999 FAX:886-2-2240-5083

URL: http://www.A1A1A.com

E-mail: [email protected]

Introduction

In order to transmit high voltage power, there is a need to use a cable that has the necessary

qualities for the transmission of large quantities of electricity. Overhead power line entails

transmission of electricity using towers. Moreover, another way to transmit electricity is through

utility poles. Overhead transmission method tends to be the most commonly used way of

transmitting high voltage power because most of the insulation is provided by air. This method is

less costly especially when transmitting large quantities of electricity. In order to accomplish this

obligation, the most efficient cable to use is aluminum. This paper explores aluminum as the best

cable to use in high voltage transmission lines and the process used in making the cable.

Aluminum Cable

Properties of Aluminum

Physical and chemical properties of aluminum

Some of the physical properties include that aluminum is a slivery white metal. The metal is also

reflective to heat. Moreover, aluminum metal can be produced to various different forms with the

help of machines. This means that the metal can have various surface finishes. Another physical

property of aluminum is that it is easily recyclable. One of the chemical properties of aluminum

is that it is resistance to oxidation. The other chemical property is that the metal is created using

an electronic method.

Good conductor of electricity

Aluminum cable tends to be the best to use in high voltage transmission lines compared to cables

made from other metals. One of the advantages of using aluminum cable is that it is a good

conductor of electricity (Warne, 2005). This means that the metal has a low electrical resistivity.

The reason why aluminum is a good conductor of electricity is that it has 3+ charges. This means

that aluminum has three delocalized electrons that tend to move freely in each atom of the metal.

The relationship between of electricity and aluminum is that when there is an electrical field

applied on the metal, every loose electron is able to move freely. This translates that all the loose

electrons will move towards the positive terminal where there is presence of an electric field.

Eventually aluminum ends up being a good conduct of electricity hence eligible to use in

transmission lines (Warne, 2005). The metal is also a very good thermal conductor.

Light in weight

The other significant importance that makes aluminum to be the best cable while using in high

voltage transmission lines is its lightweight. Comparing aluminum with other metals like copper,

nickel and brass, aluminum tends to be less in weight to about one third of the others. The other

aspect in relation to weight is that aluminum has a specific gravity of 2.7. This means that the

metal is very light in weight. Moreover, aluminum cables being of lightweight makes them most

efficient for overhead transmission of electricity. Another significant importance of aluminum

being lighter is that cables made from the metal require little support.

Economical

Another significant feature about aluminum is that it is economical. Aluminum cables poses as

the most economical compared to other cables form other metals. Moreover, the production of

aluminum is also economical. Most of the production sites of aluminum tend to be near the

sources areas and therefore costs related to transportation are less. Another significant issue

related to less cost in aluminum is that the metal is recyclable. Moreover, aluminum being of low

cost enables cables made from this metal to move for a longer distance (Vargel, 2004).

Corrosion resistant

The other reason behind choosing aluminum as the best cable is that it is resistant to corrosion.

Aluminum is able to resist corrosion because of presence of a thin layer on its surface. The thin

layer lining is made of aluminum oxide. Moreover, using technology, this particular layer can be

made stronger by anodizing the metal. Anodizing refers converting the metal to anode. This is

done in electrolysis of dilute sulphuric acid. However, in order to accomplish this step, there is a

need to first etch the aluminum with sodium hydroxide solution. This is done in order to remove

the existing oxide layer. After electrolyzing the aluminum article, a thick film of oxide is build

up that is highly resistance to corrosion.

Ductility and Malleability

The other feature associated with aluminum is that it is highly ductile. Apart from being highly

ductile, aluminum has a high aspect of malleability. Both ductility and malleability are properties

related to how the possibility of deformation can occur on a particular metal. Aluminum holds

both of these properties and therefore seems to be the best while using as electrical cables. In all

metals, aluminum is the second malleable one. While in the aspect of being ductile, aluminum

holds the sixth position in all metals. Both of the two properties are of significant importance

when using in electrical cables. This is because; malleability refers to the ability of a metal to be

deformed by compression. Moreover, this process ought to occur without cracking without or

rupturing. This feature also translates that it is possible to roll aluminum into several sheets.

Aluminum holds a high percentage on this specific feature. Ductile means that aluminum has the

ability to be deformed plastically. This process ought to occur without fracture under tensile

force. This feature also illustrates that aluminum can be easily drawn into wires. Aluminum also

tends to have high percentage on this feature. Both of these properties make aluminum to be the

perfect metal for drawing large cables that are recommendable for overhead lines.

Others

Aluminum has another significant property of being highly strength (Fraden, 2010). This makes

aluminum cables to be the best in making overhead lines because the high strength helps the

cables not to creep. Aluminum metal is also non-magnetic. This property makes aluminum to be

the best in making cables because they cannot attach to each other in case they are swung by

wind.

Characteristics of Aluminum cables

One of the characteristics of aluminum cables is that they tend to lose some of their strength

during the high temperatures. However, even in during these periods, ductility of the metal

remains the same as in low temperatures. This feature makes aluminum to the best in cabling

even in cold regions. The other characteristic of aluminum cables is that they are able to form a

layer of oxide. The importance of these layers is that they are corrosion resistant. When

repeatedly used, the cables made from aluminum tend to lose their strength. Therefore, they

require extra care when handling.

Process and Manufacturing the Power Transmission Lines Using Aluminum Cables

High voltage electric transmission entails transfer of energy form an electric source to various

substations. Most of the substations are located near residential places. This network of

transferring electricity from the source to the final consumer is generally known as distribution

system. The importance of using overhead line transmission is because the method is less costly.

Most of the aluminum conductors were being manufactured from pure aluminum (ECAL) in the

early period. In order to make the overhead transmission lines, most of the manufactures used

wire rods. The wire rods were made through the process of hot rolling. The wire rods were also

made through extrusion methods. However, nowadays, in order to process and manufacture

transmission lines using aluminum cables, a set systematic system is used to make sure that the

best product is made.

Material and properties

The most important materials include the recommended composite conductor. The conductor

ought to have a number of zirconium strands that are made from aluminum. These specific

strands ought to be of high temperatures. Another significant property required is reinforced

composite wires that have to be of aluminum oxide. The reinforced wires are covered by the

zirconium strands. The significant importance of both the composite wires that make the

composite core and outer core aluminum-zirconium (Al-Zr) is that they help in making the

transmission lines to have the overall conductor strength and conductivity.

Composite core

The composite core or the inner strands are made of aluminum composite wires. Most of the

wires depend on the conductor size and the wire diameters. In most of the time, the wire

diameters range from 0.073” (1.9mm) to 0.114 (209mm). One significant feature of the core

wires is that they have the stiffness and strength of steel. However, the core wires have little

weight and higher conductivity compared to materials made from steel. This is an advantage to

of the aluminum materials in making strong and reliable transmission wires. Each of the core

wire is composed of a large numbers of aluminum oxide fibers. The fibers are small in diameters

and they are of ultra-high strength. The other aspect of the composite core is that the ceramic

fibers are continuous. They are oriented in the direction of the wire. The ceramic wires are also

embedded with high-purity aluminum. The composite wires are different from aluminum itself in

strength. Moreover, the wires tend to exhibit various mechanical and physical properties that are

of more degree compared to that aluminum.

Outer strands

The outer strands compromise of a temperature resistance alloy. The specific alloy is aluminum-

zirconium. This particular alloy is of made of hard aluminum. The other significant feature is

that this particular alloy is designed to maintain high strength. This in many cases occurs after

high temperatures. The following figure shows how the outer strands of the transmission wire

ought to be.

Tensile strength

After finding the necessary and recommended materials, the other process is making laboratory

tensile tests. The test strength is made in a gauge facility that is 10ft in length. During this

process, there is a need to take considerable care when handling the materials. Moreover, there is

also significant advantage in cutting and preparing the materials in order to ensure that the wires

http://livepaperhelp.com/blog/wp-content/uploads/2012/04/1.jpg

did not slacken. Disadvantage of slacken is that the wires might decrease their strength values.

The other issue that is determined while checking the tensile strength is the breaking load. This is

usually done by pulling a conductor to a 1000-lb load. Then the load is further loaded to failure

at 10000 lbs/min. After the testing the tensile strength, the breaking load ought to reach the

recommended Rated Breaking Strength (RBS).

Stress strain behavior

Another significant issue addressed during the manufacturing process of the transmission lines is

the stress-strain behavior. The behavior is determined according to the set standards by the

Aluminum Association. The stress-strain behavior is test is started at 1000lbs (4.4KN). During

this process, the strain measurement is set at zero. The load is then incrementally increased to a

percentage of between 30 and 75 of RBS. Moreover, curve fitting is then applied to the

transmission lines (Kaufman, Rooy & American Foundry Society, 2004).

Short Circuit Behavior

This test is conducted in order to determine whether the aluminum cable is able to sustain the

compression that might occur in case of short circuits. The following figure demonstrates the

consequences that might occur during a short circuit while using cables other than aluminum.

Axial Impact strength

This is a test usually done to investigate whether there is slippage in conductor terminations. The

other significant importance of this test is to investigate whether there is a possibility to sustain

the high shock loads (>100% RBS). In most cases, the loads ought to be sustained by the 795-


kcmil Composite Conductor. Moreover, this is done under high rate axial loading. Through this

particular process, the shock load is comparable to various loading rates. Some the loading rates

include those experienced in certain situations like during ice jumps and galloping events. The

following figure demonstrates an axial impact.

Crush strength

The crush strength test is usually done to test the full strength retention of the aluminum cable.

The test is done on a 795-kcmil Composite Conductor. The main reason for conducting the test is

to simulate the possible damage that might occur during the process of shipping and installation.

An example of this process is crushing a section of the aluminum cable between 6-inch steel

plates for a period of one minute. If the aluminum cable shows any detection of damage, then the

cable it is not fit for application.

Lightening Resistance

This is a significant process usually undertaken during the process of manufacturing overhead

transmission lines. A lightening arc is struck across the aluminum cable to determine whether it

can be able to resist very severe strikes (Smith, 2008). The following figures demonstrate two

types of cables where one is able to resist occurrence of lightening while the other one is not.


Conclusion

After the aluminum cable, is able to pass through all the above manufacturing processes, then it

eligible for application in overhead transmission lines. Aluminum tends to be the best metal for

making transmission cables as illustrated above. Transmission of high-voltage electricity

requires cables that are able to resist various manufacture and environmental unhealthy

conditions. Through various developments in technology, it is possible to make aluminum cables

in an easy way compared to the methods that were used in the past. Moreover, through

technology there is a possibility of extra advancement in making the overhead aluminum cable

lines. Aluminum cables poses to be the best compared with other metals.



Cable Construction

Contents

[hide]

1 Introduction

2 Cable Parts

o 2.1 Conductor

o 2.2 Conductor Screen

o 2.3 Insulation

o 2.4 Insulation Screen

o 2.5 Conductor Sheath

o 2.6 Filler

o 2.7 Bedding / Inner Sheath

o 2.8 Individual Screen (Instrument Cables)

o 2.9 Drain Wire (Instrument Cables)

o 2.10 Overall Screen (Instrument Cables)

o 2.11 Armour

o 2.12 Outer Sheath

o 2.13 Termite Protection

o 2.14 Conductor Protection (Appendix)

2.14.1 Non-Metallic

2.14.2 Metallic

3 Low Voltage Power and Control Cables

4 Low Voltage Instrumentation Cables

5 Medium / High Voltage Power Cables

o 5.1 Teck Cables

o 5.2 Shielded Cables

o 5.3 Concentric Neutral Cables

o 5.4 Paper-Insulated Lead-Covered Cables (PILC)

o 5.5 Submarine Cables

o 5.6 Mining Cables

o 5.7 Aluminum-Sheathed Cables

6 Trivia

7 References

Introduction

This article gives a brief exposition on the construction of typical low voltage, medium / high

voltage and instrumentation cables. The focus is on thermoplastic and thermosetting insulated

cables, however the construction of other cables are similar. Although there is more than one

way to construct a cable and no one standard to which all vendors will adhere, most cables tend

to have common characteristics.

Low voltage power and control cables pertain to electrical cables that typically have a

voltage grade of 0.6/1 kV or below.

Low voltage instrumentation cables pertain to cables for use in instrument applications

and typically have a voltage grade of 450/750 V or below.

Medium / High voltage cables pertain to cables used for electric power transmission at

madium and high voltage (usually from 1 to 33 kV are medium voltage cables and those

over 50 kV are high voltage cables).

Cable Parts

Here, we will take a short overview of the main and the most typical cable construction parts:

Conductor

Usually stranded copper (Cu) or aluminium (Al). Copper is densier and heavier, but more

conductive than aluminium. Electrically equivalent aluminium conductors have a cross-sectional

area approximately 1.6 times larger than copper, but are half the weight (which may save on

material cost).

Annealing – is the process of gradually heating and cooling the conductor material to make it

more malleable and less brittle.

Coating – surface coating (eg. tin, nickel, silver, lead alloy) of copper conductors is common to

prevent the insulation from attacking or adhering to the copper conductor and prevents

deterioration of copper at high temperatures. Tin coatings were used in the past to protect against

corrosion from rubber insulation, which contained traces of the sulfur used in the vulcanising

process.

Conductor Screen

A semi-conducting tape to maintain a uniform electric field and minimise electrostatic stresses

(for MV/HV power cables).

Insulation

Commonly thermoplastic (eg. PVC) or thermosetting (eg. EPR, XLPE) type materials. Mineral

insulation is sometimes used, but the construction of MI cables are entirely different to normal

plastic / rubber insulated cables. Typically a thermosetting(eg. EPR, XLPE) or paper/lead

insulation for cables under 22kV. Paper-based insulation in combination with oil or gas-filled

cables are generally used for higher voltages.

Plastics are one of the more commonly used types of insulating materials for electrical

conductors. It has good insulating, flexibility, and moisture-resistant qualities. Although there are

many types of plastic insulating materials, thermoplastic is one of the most common. With the

use of thermoplastic, the conductor temperature can be higher than with some other types of

insulating materials without damage to the insulating quality of the material. Plastic insulation is

normally used for low- or medium-range voltage.

The designators used with thermoplastics are much like those used with rubber insulators. The

following letters are used when dealing with NEC type designators for thermoplastics:

T - Thermoplastic

H - Heat-resistant

W - Moisture-resistant

A - Asbestos

N - Outer nylon jacket

M - Oil-resistant

Paper has little insulation value alone. However, when impregnated with a high grade of mineral

oil, it serves as a satisfactory insulation for extremely high-voltage cables. The oil has a high

dielectric strength, and tends to prevent breakdown of the paper insulation. The paper must be

thoroughly saturated with the oil. The thin paper tape is wrapped in many layers around the

conductors, and then soaked with oil.

Enamel: the wire used on the coils of meters, relays, small transformers, motor windings, and so

forth, is called magnet wire. This wire is insulated with an enamel coating. The enamel is a

synthetic compound of cellulose acetate (wood pulp and magnesium). In the manufacturing

process, the bare wire is passed through a solution of hot enamel and then cooled. This process is

repeated until the wire acquires from 6 to 10 coatings. Thickness for thickness, enamel has

higher dielectric strength than rubber. It is not practical for large wires because of the expense

and because the insulation is readily fractured when large wires are bent.

Mineral-insulated (MI) cable was developed to meet the needs of a noncombustible, high heat-

resistant, and water-resistant cable. MI cable has from one to seven electrical conductors. These

conductors are insulated in a highly compressed mineral, normally magnesium oxide, and sealed

in a liquidtight, gastight metallic tube, normally made of seamless copper.

Silk and Cotton: in certain types of circuits (for example, communications circuits), a large

number of conductors are needed, perhaps as many as several hundred. Because the insulation in

this type of cable is not subjected to high voltage, the use of thin layers of silk and cotton is

satisfactory.

Silk and cotton insulation keeps the size of the cable small enough to be handled easily. The silk

and cotton threads are wrapped around the individual conductors in reverse directions. The

covering is then impregnated with a special wax compound.

Insulation Screen

A semi-conducting material that has a similar function as the conductor screen (ie. control of the

electric field for MV/HV power cables).

Conductor Sheath

A conductive sheath / shield, typically of copper tape or sometimes lead alloy, is used as a shield

to keep electromagnetic radiation in, and also provide a path for fault and leakage currents

(sheaths are earthed at one cable end). Lead sheaths are heavier and potentially more difficult to

terminate than copper tape, but generally provide better earth fault capacity.

Filler

The interstices of the insulated conductor bundle is sometimes filled, usually with a soft polymer

material.

Bedding / Inner Sheath

Typically a thermoplastic (eg. PVC) or thermosetting (eg. CSP) compound, the inner sheath is

there to keep the bundle together and to provide a bedding for the cable armour.

Individual Screen (Instrument Cables)

An individual screen is occasionally applied over each insulated conductor bundle for shielding

against noise / radiation and interference from other conductor bundles. Screens are usually a

metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in

instrument cables, but not in power cables.

Drain Wire (Instrument Cables)

Each screen has an associated drain wire, which assists in the termination of the screen.

Typically used in instrument cables, but not in power cables.

Overall Screen (Instrument Cables)

An overall screen is applied over all the insulated conductor bundles for shielding against noise /

radiation, interference from other cables and surge / lightning protection. Screens are usually a

metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in

instrument cables, but not in power cables.

Armour

For mechanical protection of the conductor bundle. Steel wire armour or braid is typically used.

Tinning or galvanising is used for rust prevention. Phosphor bronze or tinned copper braid is also

used when steel armour is not allowed.

SWA - Steel wire armour, used in multi-core cables (magnetic),

AWA - Aluminium wire armour, used in single-core cables (non-magnetic).

When an electric current passes through a cable it produces a magnetic field (the higher the

voltage the bigger the field). The magnetic field will induce an electric current in steel armour

(eddy currents), which can cause overheating in AC systems. The non-magnetic aluminium

armour prevents this from happening.

Outer Sheath

Applied over the armour for overall mechanical, weather, chemical and electrical protection.

Typically a thermoplastic (eg. PVC) or thermosetting(eg. CSP) compound, and often the same

material as the bedding. Outer sheath is normally colour coded to differentiate between LV, HV

and instrumentation cables. Manufacturer’s markings and length markings are also printed on the

outer sheath.

Termite Protection

For underground cables, a nylon jacket can be applied for termite protection, although sometimes

a phosphor bronze tape is used.

Conductor Protection (Appendix)

Wires and cables are generally subject to abuse. The type and amount of abuse depends on how

and where they are installed and the manner in which they are used. Cables buried directly in the

ground must resist moisture, chemical action, and abrasion. Wires installed in buildings must be

protected against mechanical injury and overloading. Wires strung on crossarms on poles must

be kept far enough apart so that the wires do not touch. Snow, ice, and strong winds make it

necessary to use conductors having high tensile strength and substantial frame structures.

Generally, except for overhead transmission lines, wires or cables are protected by some form of

covering. The covering may be some type of insulator like rubber or plastic. Over this, additional

layers of fibrous braid or tape may be used and then covered with a finish or saturated with a

protective coating. If the wire or cable is installed where it is likely to receive rough treatment, a

metallic coat should be added.

The materials used to make up the conductor protection for a wire or cable are grouped into one

of two categories: non-metallic or metallic.

Non-Metallic

The category of non-metallic protective coverings is divided into three areas. These areas are:

(1) according to the material used as the covering,

(2) according to the saturant in which the covering was impregnated, and

(3) according to the external finish on the wire or cable.

These three areas reflect three different methods of protecting the wire or cable. These methods

allow some wire or cable to be classified under more than one category. Most of the time,

however, the wire or cable will be classified based upon the material used as the covering

regardless of whether or not a saturant or finish is applied.

Many types of non-metallic materials are used to protect wires and cables. Fibrous braid is by far

the most common and will be discussed first.

Fibrous Braid

Fibrous braid is used extensively as a protective covering for cables. This braid is woven over

the insulation to form a continuous covering without joints. The braid is generally saturated with

asphalt, paint, or varnish to give added protection against moisture, flame, weathering, oil, or

acid. Additionally, the outside braid is often given a finish of stearin pitch and mica flakes, paint,

wax, lacquer, or varnish depending on the environment where the cable is to be used.

Woven Covers

Woven covers, commonly called loom, are used when exceptional abrasion-resistant qualities are

required. These covers are composed of thick, heavy, long-fibered cotton yarns woven around

the cable in a circular loom, much like that used on a fire hose. They are not braids, although

braid covering are also woven; they are designated differently.

Rubber and Synthetic Coverings

Rubber and synthetic coverings are not standardized. Different manufactures have their own

special compounds designated by individual trade names. These compounds are different from

the rubber compounds used to insulate cable. These compounds have been perfected not for

insulation qualities but for resistance to abrasion, moisture, oil, gasoline, acids, earth solutions,

and alkalies. None of these coverings will provide protection against all types of exposure. Each

covering has its own particular limitations and qualifications.

Jute and Asphalt Coverings

Jute and asphalt coverings are commonly used as a cushion between cable insulation and

metallic armour. Frequently, they are also used as a corrosive-resistant covering over a lead

sheath or metallic armour. Jute and asphalt coverings consist of asphalt-impregnated jute yarn

heli-wrapped around the cable or of alternate layers of asphalt-impregnated jute yarn. These

coverings serve as a weatherproofing.

Unspun Felted Cotton

Unspun felted cotton is commonly used only in special classes of service. It is made as a solid

felted covering for a cable.

Metallic

Metallic protection is of two types: sheath or armour. As with all wires and cables, the type of

protection needed will depend on the environment where the wire or cable will be used.

Metallic Sheath

Cables or wires that are continually subjected to water must be protected by a watertight cover.

This watertight cover is either a continuous metal jacket or a rubber sheath molded around the

cable.

Lead-sheathed cable is one of three types currently being used: alloy lead, pure lead, and

reinforced lead. An alloy-lead sheath is much like a pure lead sheath but is manufactured with 2-

percent tin. This alloy is more resistant to gouging and abrasion during and after installation.

Reinforced lead sheath is used mainly for oil-filled cables where high internal pressures can be

expected. Reinforced lead sheath consists of a double lead sheath. A thin tape of hard-drawn

copper, bronze, or other elastic metal (preferably nonmagnetic) is wrapped around the inner

sheath. This tape gives considerable additional strength and elasticity to the sheath, but must be

protected from corrosion. For this reason, a second lead sheath is applied over the tape.

Metallic Armour

Metallic armour provides a tough protective covering for wires and cables. The type, thickness,

and kind of metal used to make the armour depend on three factors:

(1) the use of the conductors,

(2) the environment where the conductors are to be used, and

(3) the amount of rough treatment that is expected.

1. Wire-braid armour

Wire-braid armour, also known as basket-weave armour, is used when light and flexible

protection is needed. Wire braid is constructed much like fibrous braid. The metal is woven

directly over the cable as the outer covering. The metal used in this braid is galvanized steel,

bronze, copper, or aluminum. Wire-braid armour is mainly for shipboard use.

2. Steel tape

A second type of metallic armour is steel tape. Steel tape covering is wrapped around the cable

and then covered with a serving of jute. There are two types of steel tape armour. The first is

called interlocking armour. Interlocking armour is applied by wrapping the tape around the cable

so that each turn is overlapped by the next and is locked in place. The second type is flat- band

armour. Flat-band armour consists of two layers of steel tape. The first layer is wrapped around

the cable but is not overlapped. The second layer is then wrapped around the cable covering the

area that was not covered by the first layer.

3. Wire armour

Wire armour is a layer of wound metal wire wrapped around the cable. Wire armour is usually

made of galvanized steel and can be used over a lead sheath (see view C of the figure above). It

can be used with the sheath as a buried cable where moisture is a concern, or without the sheath

when used in buildings.

4. Coaxial cable

Coaxial cable is defined as two concentric wires, cylindrical in shape, separated by a dielectric of

some type. One wire is the center conductor and the other is the outer conductor. These

conductors are covered by a protective jacket. The protective jacket is then covered by an outer

protective armour.

Coaxial cables are used as transmission lines and are constructed to provide protection against

outside signal interference.

Low Voltage Power and Control Cables

Low voltage power and control cables pertain to electrical cables that typically have a voltage

grade of 0.6/1 kV or below.

http://www.openelectrical.org/wiki/index.php?title=File:LV_Cable.jpg

Armoured FAS Cable

An important item that is under the grouping known as 'Low Voltage Cables', is Type FAS (Fire

Alarm & Signal Cable). This 300-volt cable, is specifically designed for the interconnection of

security system elements, including fire protection signalling devices such as smoke and fire

detectors, fire alarms, and two-way emergency communications systems.

Fire alarm installations in non-combustible buildings require mechanical protection, consisting

of interlock armour, metallic conduit, non-metallic conduit embedded in concrete or installed

under-ground. Armoured FAS Cable provided with an interlocking aluminum armour, may be

expected to have an appreciable cost advantage, compared with cables installed in rigid conduit.

Other common cables are LVT (Low Voltage Thermoplastic) and ELC (Extra Low Voltage

Control), which are frequently used in residential installations for such items as door bells and

thermostats.

Low Voltage Instrumentation Cables

http://www.openelectrical.org/wiki/index.php?title=File:Fig11-1-.jpg

http://www.openelectrical.org/wiki/index.php?title=File:Fig11-1-.jpg

Instrumentation cables

Low voltage instrumentation cables pertain to cables for use in instrument applications and

typically have a voltage grade of 450/750 V or below.

Instrumentation Cables rated at 300 volts have copper conductors 0.33 mm2 (#22 AWG) to

2.08 mm2 (#14 AWG), while those rated at 600 volts have 0.82 mm2(#18 AWG) to 5.26

mm2(#10 AWG), and unarmoured and armoured types are available. The cables may be an

assembly of single conductors, pairs, triads or quads. The conductors are stranded seven-wire

tinned or bare copper. The insulation is usually a PVC compound chosen dependant on the

environment for which it is intended. Insulated conductors are paired with staggered lays to

prevent electromagnetic coupling and crosstalk. When individual shielding is specified, each pair

is aluminum/polyester shielded with drain wire to eliminate electrostatic interference.

Armoured cables have an interlocked aluminum or galvanized steel armour. The armouring is

applied over an inner PVC jacket, followed by a PVC outer jacket. Armoured cables are suitable

for installation on cable trays in dry, damp and wet locations, or direct earth burial.

Unarmoured Instrumentation Cables are intended for installation in raceways/conduit (except

cable trays) in dry, damp or wet locations, or direct earth buried. Unarmoured Cable with Type

TC (Tray Cable) designation, may be installed in cable trays.

Thermocouple Extension Cables

http://www.openelectrical.org/wiki/index.php?title=File:6a-1-.jpg

http://www.openelectrical.org/wiki/index.php?title=File:6a-1-.jpg

Thermocouple Extension Cables have a 300 volt rating, and are of similar construction to

Instrumentation Cables, but the metals/alloys used for the conductors are different.

Thermocouples measure temperature using the electric current created when heat is applied to

two dissimilar metals/alloys. The cable assembles may consist of as many as 50 pairs, depending

on the number of locations being temperature monitored.

Medium / High Voltage Power Cables

Medium or High Voltage power cables have voltage grade greater than 1 kV. Medium voltage

usually goes up to 46 kV and High voltage is considering all voltage levels above 46 kV.

http://www.openelectrical.org/wiki/index.php?title=File:Inst_Cable.jpg

Medium Voltage distribution systems begin at substations and supply electricity to a wide

spectrum of power consumers. When selecting a cable, the basic aim is to safely provide

adequate electrical power, with continuous, trouble-free operation, in a system that is able to

withstand unexpected demands and overload conditions. Each installation has particular

requirements that must be considered. There are distinct benefits from specifying a copper-

conductor cable that has been manufactured under rigid specification and quality control

procedures. It will provide maximum performance with minimum maintenance. There are seven

types different by construction for medium voltage copper power cables in the 1 kV to 46 kV

range. Most are available in single- and multi-core configurations. There are ranges of sizes and

design variations for each type.

The MV cable types are:

Teck Cables,

Shielded Cables,

Concentric Neutral Cables,

Paper-Insulated Lead-Covered Cables,

Submarine Cables,

Mining Cables,

Aluminum-Sheathed Cables.

In the cable descriptions a number of insulation and sheath (jacket) materials have been

abbreviated as follows:

Cross-Linked Polyethylene - XLPE,

Ethylene-Propylene Rubber - EPR,

Polyvinyl Chloride - PVC,

Polyethylene - PE,

Tree-Retardant Cross-Linked Polyethylene - TR-XLPE.

Teck Cables

Teck Cables were originally developed for use in mines, but they are now widely used in

primary and secondary industries, chemical plants, refineries and general factory environments.

They are also used in multi-storey and commercial buildings. They are flexible, resistant to

mechanical abuse, corrosion resistant, compact and reliable. A modified Teck Cable construction

may be used for vertical installations, such as in mine shafts and multi-storey buildings, where

the armour is locked-in-place to prevent slippage of the inner core. There are many different

combinations of conductor size, voltage rating, armour type and so forth, available in Teck

Cables to meet the requirements of particular installations. Annealed, bare, copper is used for the

conductor (s), and they are usually compact stranded to reduce diameter. In multi-conductor

cables, the insulated conductors are cabled together, including the bare copper bonding

(grounding) conductor. In shielded multi-conductor cables, the bonding (grounding) conductor is

positioned to contact the copper shields. A PVC outer jacket which may be colour-coded

depending on the rating of the cable is applied.

Shielded Cables

Shielded Power Cable may be single-or three-conductor. The basic construction begins with a

conductor of annealed, bare, solid or concentric-stranded copper, which may be compact or

compressed. This is followed by a semi-conducting conductor shield, insulation, and then a semi-

conducting insulation shield. Metallic shielding follows, which is usually either gapped or lapped

copper tape. Other types of metallic shielding are available, including concentric wires and

longitudinally corrugated copper tape. The outer jacket is either PVC or PE.

Concentric Neutral Cables

These power cables may be used in dry or wet locations, for a wide variety of types of

installations, and may be single- or three-conductor. The two standard constructions are

Unjacketed and Jacketed, the latter being most frequently used. The conductor is typically

annealed, bare, stranded copper, but tin-coated wire and solid conductors are also available. The

concentric neutral conductor, from which the cable derives its name, is bare or tin-coated copper

wire, applied helically over the insulation shield. These wires act as the metallic component of

the shield and the neutral, at the same time.

Paper-Insulated Lead-Covered Cables (PILC)

PILC cables are used in power distribution and industrial applications, and they may be installed

exposed, in underground ducts or directly buried. Their design begins with annealed, bare copper

conductor(s) which may be round, concentric, compressed or compact stranded, compact sector,

and in larger sizes … Type M segmental stranded. An example of compact sector conductors is

shown in the illustration. The insulated cable core is impregnated with a medium viscosity

polybutene-based compound. The combination of the excellent electrical and mechanical

characteristics of the liquid and the paper has resulted in a reliable and economic insulation,

which now claims a history of almost 100 years. It is little wonder why so many utilities and

power-consuming industries, still continue to specify PILC. To prevent the ingress of moisture, a

seamless lead-alloy sheath is applied. The outer jacket may be PVC or PE, and if required by the

application, armour is available.

Submarine Cables

For submarine installations, usually Self-Contained Liquid-Filled Cables (SCLF), or Solid

Dielectric Cables are selected, depending on voltage and power load. SCLF Cables are capable

of handling very high voltages. However, for medium-voltage installations, a Solid Dielectric

Cable can easily fulfil the electrical demands of the system. A submarine Solid Dielectric Cable

is shown in the illustration. Its construction begins with a compact stranded, annealed, bare

copper conductor, followed by a semi-conducting conductor shield. A copper tape shield is

helically applied, followed by a lead-alloy sheath. Due to the severe environmental demands

placed on submarine cables, a lead-alloy sheath is often specified because of its compressibility,

flexibility and resistance to moisture and corrosion. The sheath is usually covered by a number of

outer layers, comprising a PE or PVC jacket and metal wire armouring.

Mining Cables

A number of different types of cables are used in mines. There are fixed mining cables and

portable mining cables, the latter being described here. The key requirements of portable cables

are flexibility, and resistance to mechanical abrasion and damage. Due to the additional demands

put on portable mining cables used for reeling and dereeling applications, special design may be

required. There are many types of portable mining cables. They are available in ratings up to 25

kV, and may have as many as five conductors. An example of SHD-GC Cable, is shown in the

illustration. It has three insulated, shielded conductors, two bare ground wires, a ground check

wire, and an overall jacket. The conductors for this cable are annealed, bare or tinned copper

wires. The braided shield may be tin-coated wires, or a tin-coated copper wire/textile composite.

The grounding conductor(s) annealed, bare or tinned, stranded copper wires, and the ground

check conductor is annealed, bare, stranded copper wires with EPR insulation and nylon braid,

elastomeric jacket holds the conductor assembly firmly in place, to minimize snaking and cork-

screwing during reeling and dereeling.

Aluminum-Sheathed Cables

These power cables are used for exposed and concealed wiring, in wet and dry locations, and

where exposed to the weather. They may be installed in ventilated, unventilated and ladder-type

cable-troughs, and ventilated flexible cableways. Aluminum-Sheathed Power Cables may be

single-,two-,three- or four-conductor, the conductor(s) being annealed, bare, compressed-round

stranded copper. The insulated core is enclosed in a liquid- and vapour-tight solid corrugated

aluminum sheath, covered by a PVC jacket.

Teck Cable

overview

Shielded

Cable

overview

Concentric

Neutral

Cable

overview

Paper-

Insulated

Lead-

Covered

Cable

overview

Submarine

Cable

overview

Mining

Cable

overview

Aluminum-

Sheathed

Cable

overview

http://www.openelectrical.org/wiki/index.php?title=File:Teck-cable.jpg

http://www.openelectrical.org/wiki/index.php?title=File:Teck-cable.jpg

http://www.openelectrical.org/wiki/index.php?title=File:Shielded_cable_mv.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Shielded_cable_mv.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Concentric_neutral_cables.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Concentric_neutral_cables.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Paper-insulated-cable.jpg

http://www.openelectrical.org/wiki/index.php?title=File:Paper-insulated-cable.jpg

http://www.openelectrical.org/wiki/index.php?title=File:Submarine_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Submarine_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Minig_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Minig_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Alu_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:Alu_cable.JPG

http://www.openelectrical.org/wiki/index.php?title=File:HV_Cable.jpg

A high-voltage cable - also called HV cable - is used for electric power transmission at high voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, AC and DC power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent breakdown of the insulation. Often a high-voltage cable will have a metallic shield layer over the insulation, connected to earth ground and designed to equalize the dielectric stress on the insulation layer.

Segments of high-voltage cables

High-voltage cables may be any length, with relatively short cables used in apparatus, longer cables run within buildings or as buried cables in an industrial plant or for power distribution, and the longest cables are often run as submarine cables under the ocean for power transmission.

Contents

1 Construction 2 AC power cable

o 2.1 Quality 3 HVDC cable 4 Cable terminals 5 Cable joints 6 X-ray cable 7 Testing of high-voltage cables 8 See also 9 Sources and notes

o 9.1 Notes 10 External links

Construction

http://en.wikipedia.org/wiki/File:Hochspannungskabel_110kV_400kV.JPG

http://en.wikipedia.org/wiki/File:Hochspannungskabel_110kV_400kV.JPG

A cross-section through a 400 kV cable, showing the stranded segmented copper conductor in the center, semiconducting and insulating layers, copper shield conductors, aluminum sheath and plastic outer jacket.

Like other power cables, high-voltage cables have the structural elements of one or more conductors, insulation, and a protective jacket. High-voltage cables differ from lower-voltage cables in that they have additional internal layers in the insulation jacket to control the electric field around the conductor.

For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916;[1] the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at the ends of the shield, and at splices. Stress relief cones are applied at the shield ends.

Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath. For higher voltages the oil may be kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation.

Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared paper could form satisfactory cable insulation at 11,000 volts. Previously paper-insulated cable had only been applied for low-voltage telegraph and telephone circuits. An extruded lead sheath over the paper cable was required to ensure that the paper remained absolutely dry.

Vulcanized rubber was patented by Charles Goodyear in 1844, but it was not applied to cable insulation until the 1880s, when it was used for lighting circuits.[1] Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls Power Generation project.

Mass-impregnated paper-insulated medium voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.[2] Modern high-voltage cables use polymers or polyethylene, including (XLPE) for insulation.

http://en.wikipedia.org/wiki/File:Hochspannungskabel_400kV_Querschnitt.JPG

http://en.wikipedia.org/wiki/File:Hochspannungskabel_400kV_Querschnitt.JPG

AC power cable

High voltage is defined as any voltage over 1000 volts. Cables for 3000 and 6000 volts exist, but the majority of cables are used from 10 kV and upward.[3] Those of 10 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables.

Figure 1, cross section of a high-voltage cable, (1) conductor, (3) insulation.

Modern HV cables have a simple design consisting of few parts. A conductor of copper or aluminum wires transports the current, see (1) in figure 1. (For a detailed discussion on copper cables, see main article: Copper wire and cable.)

Conductor sections up to 2000 mm2 may transport currents up to 2000 amperes. The individual strands are often preshaped to provide a smoother overall circumference. The insulation (3) may consist of cross-linked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120 °C. EPDM is also an insulation.

At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation.[4] The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that little electric discharges can arise and endanger the insulation material.[5]

The outer conductor or sheath (5) serves as an earthed layer and will conduct leakage currents if needed.

Most high-voltage cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cables may have a lead or aluminium jacket in conjunction with XLPE insulation to allow for fiber optics. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability, particularly early XLPE, resulted in a slow uptake at transmission voltages. While cables of 330 kV are commonly constructed using XLPE, this has occurred only in recent decades.

Quality

During the development of HV insulation, which has taken about half a century, two characteristics proved to be paramount. First, the introduction of the semiconducting layers.

http://en.wikipedia.org/wiki/File:HV_Cable.jpg

http://en.wikipedia.org/wiki/File:HV_Cable.jpg

These layers must be absolutely smooth, without even protrusions as small as a few µm. Further the fusion between the insulation and these layers must be absolute;[6] any fission, air-pocket or other defect - of the same micro-dimensions as above - is detrimental for the breakdown characteristics of the cable.

Secondly, the insulation must be free of inclusions, cavities or other defects of the same sort of size. Any defect of these types shortens the voltage life of the cable which is supposed to be in the order of 30 years or more.[7]

Cooperation between cable-makers and manufacturers of materials has resulted in grades of XLPE with tight specifications. Most producers of XLPE-compound specify an “extra clean” grade where the number and size of foreign particles are guaranteed. Packing the raw material and unloading it within a cleanroom environment in the cable-making machines is required. The development of extruders for plastics extrusion and cross-linking has resulted in cable-making installations for making defect-free and pure insulations. The final quality control test is an elevated voltage 50 or 60 Hz partial discharge test with very high sensitivity (in the range of 5 to 10 picoCoulombs) This test is performed on every reel of cable before it is shipped.

An extruder machine for making insulated cable

HVDC cable

A high-voltage cable for HVDC transmission has the same construction as the AC cable shown in figure 1. The physics and the test-requirements are different.[8] In this case the smoothness of the semiconducting layers (2) and (4) is of utmost importance. Cleanliness of the insulation remains imperative.

Many HVDC cables are used for DC submarine connections, because at distances over 30 km AC can no longer be used. The longest submarine cable today is the NorNed cable between Norway and Holland that is almost 600 km long and transports 700 megawatts, a capacity equal to a large power station.Most of these long deep-sea cables are made in an older construction, using oil-impregnated paper as an insulator.

Cable terminals

http://en.wikipedia.org/wiki/File:Extruder_section.jpg

http://en.wikipedia.org/wiki/File:Extruder_section.jpg

Figure 2, the earth shield of a cable (0%) is cut off, the equipotential lines (from 20% to 80%) concentrate at the edge of the earth electrode, causing danger of breakdown.

Terminals of high-voltage cables must manage the electric fields at the ends.[9] Without such a construction the electric field will concentrate at the end of the earth-conductor as shown in figure 2.Equipotential lines are shown here which can be compared with the contour lines on a map of a mountainous region: the nearer these lines are to each other, the steeper the slope and the greater the danger, in this case the danger of an electric breakdown. The equipotential lines can also be compared with the isobars on a weather map: the denser the lines, the more wind and the greater the danger of damage.

Figure 3, a rubber or elastomer body R is pushed over the insulation (blue) of the cable. The equipotential lines between HV (high voltage) and earth are evenly spread out by the shape of the earth electrode. Field concentrations are prevented in this way.

In order to control the equipotential lines (that is to control the electric field) a device is used that is called a stress-cone, see figure 3.[10] The crux of stress relief is to flare the shield end along a logarithmic curve. Before 1960, the stress cones were handmade using tape—after the cable was

http://en.wikipedia.org/wiki/File:Field_concentration.jpg

http://en.wikipedia.org/wiki/File:Field_concentration.jpg

http://en.wikipedia.org/wiki/File:HV_Terminal.jpg

http://en.wikipedia.org/wiki/File:HV_Terminal.jpg

installed. These were protected by potheads, so named because a potting compound/ dielectric was poured around the tape inside a metal/ porcelain body insulators. About 1960, preformed terminations were developed consisting of a rubber or elastomer body that is stretched over the cable end.[11] On this rubber-like body R an shield electrode is applied that spreads the equipotential lines to guarantee a low electric field.

The crux of this device, invented by NKF in Delft in 1964,[12] is that the bore of the elastic body is narrower than the diameter of the cable. In this way the (blue) interface between cable and stress-cone is brought under mechanical pressure so that no cavities or air-pockets can be formed between cable and cone. Electric breakdown in this region is prevented in this way.

This construction can further be surrounded by a porcelain or silicone insulator for outdoor use,[13] or by contraptions to enter the cable into a power transformer under oil, or switchgear under gas-pressure.[14]

Cable joints

Connecting two high-voltage cables with one another poses two main problems. First, the outer conducting layers in both cables shall be terminated without causing a field concentration,[15] similar as with the making of a cable terminal. Secondly, a field free space shall be created where the cut-down cable insulation and the connector of the two conductors safely can be accommodated.[16] These problems have been solved by NKF in Delft in 1965 [17] by introducing a device called bi-manchet.

Photograph of a section of a high-voltage joint, bi-manchet, with a high-voltage cable mounted at the right hand side of the device.

Figure 4 shows a photograph of the cross-section of such a device. At one side of this photograph the contours of a high-voltage cable are drawn. Here red represents the conductor of that cable and blue the insulation of the cable. The black parts in this picture are semi-conducting rubber parts. The outer one is at earth potential and spreads the electric field in a similar way as in a cable terminal. The inner one is at high-voltage and shields the connector of the conductors from the electric field.The field itself is diverted as shown in figure 5, where the equipotential lines are smoothly directed from the inside of the cable to the outer part of the bi-manchet (and vice versa at the other side of the device).

http://en.wikipedia.org/wiki/File:HV_Joint.jpg

http://en.wikipedia.org/wiki/File:HV_Joint.jpg

Field distribution in a bi-manchet or HV joint.

The crux of the matter is here, like in the cable terminal, that the inner bore of this bi-manchet is chosen smaller than the diameter over the cable-insulation.[18] In this way a permanent pressure is created between the bi-manchet and the cable surface and cavities or electrical weak points are avoided.

Installing a terminal or bi-manchet is skilled work. Removing the outer semiconducting layer at the end of the cables, placing the field-controlling bodies, connecting the conductors, etc., require skill, cleanness and precision.

X-ray cable

X-ray cables [19] are used in lengths of several meters to connect the HV source with an X-ray tube or any other HV device in scientific equipment. They transmit small currents, in the order of milliamperes at DC voltages of 30 to 200 kV, or sometimes higher. The cables are flexible, with rubber or other elastomer insulation, stranded conductors, and an outer sheath of braided copper-wire. The construction has the same elements as other HV power cables.

Testing of high-voltage cables

There are different causes for faulty cable insulations. Hence, there are various test and measurement methods to prove fully functional cables or to detect faulty ones. One needs to distinguish between cable testing and cable diagnosis. While cable testing methods result in a go/no go statement cable diagnosis methods allow judgement of the cables current condition. In some cases it is even possible to locate the position of the fault in the insulation. One of the favorite testing methods is VLF cable testing. Using a very low frequency voltage with frequencies in the range of 0.1 to 0.01 Hz protects the device under test from deteriorating due to the test itself, as it used to be with DC testing methods in the older days. Depending on the sort of treeing in the insulation two cable diagnostics methods are common. Water trees can be detected by tan delta measurement. Interpretation of measurement results yield the possibility to distinguish between new, strongly aged and faulty cables and appropriate maintenance and repair measures may be planned. Damages to the insulation and electrical treeing may be detected and located by partial discharge measurement. Data collected during the measurement procedure is compared to measurement values of the same cable gathered during the acceptance-test. This allows simple and quick classification of the dielectric condition of the tested cable.

http://en.wikipedia.org/wiki/File:Field_in_joint.jpg

http://en.wikipedia.org/wiki/File:Field_in_joint.jpg

Cable Construction & Manufacturing Process

NATIONAL INSTITUTE OF INDUSTRIAL ENGINEERING, MUMBAI

CABLE CONSTRUCTION & MANUFACTURING

PROCESS

UNDER THE GUIDANCE OF Dr. KVSS NARAYANA RAO

Submitted By

PRASENJIT HOJAI SARVESH HIREMATH

http://2.bp.blogspot.com/-Zbc6ie0quqg/UCYvKYAP2CI/AAAAAAAAABY/sJetyBZ4WNo/s1600/nitie-mumbai.jpg

ROLL NO-66 ROLL NO-88

Cable Construction & Manufacturing Process

fig-power cable parts

Power Cable mainly subdivided into parts-

1. Conductor2. Insulation3. Metallic Sheath4. Bedding5. Armouring6. Outersheath

ConductorCopper or Aluminium used for the Conductors obtained in the form of rods. The 8.0 mm

Copper or 9.5mm aluminium rods. After testing, rods are drawn into wires of required sizes. These wires are formed into final Conductor in the stranding machines under strict Quality Assurance Program.

Insulation

Cross linked polyethylene compound or PVC is insulated over Conductor by Extrusion process. XLPE insulated cores are cured by steam curing in vulcanizing chamber to provide thorough cross-linking.

The raw materials & thickness of Insulation are maintained under strict Quality Control and conform to B.S. 5467 / IEC 60502 Part-1 or B.S. 6346 / IEC 60502 Part-1 Standards for XLPE & PVC cables respectively.

http://3.bp.blogspot.com/-65JveA108BE/UCY0KPOy4AI/AAAAAAAAABw/14aMwZUNyf8/s1600/im1.jpg

Laying UpThe insulated cores are laid up with a right hand, or alternating left & right hand,

direction of lay in the sequence of the core numbers or colours. Where ever necessary non-hygroscopic PP / PVC Fillers & binder tape are used to form a compact and reasonably circular cable.

BeddingAll armoured cables have extruded PVC bedding. The PVC used for bedding is

compatible with the temperature of Insulation material.

ArmourWhen armouring is required, the armour consists of single layer of Galvanised steel wire.

The armour is applied helical, with a left hand direction. We also provide other armours such as steel strip, tape or tinned copper. Single core cables are armoured with Aluminium or copper wires.

OuterSheathThe standard cables are manufactured with Extruded black PVC Type-9 of B.S. 7655 or

ST-2 of IEC 60502. Outer sheath is embossed or printed with the information required by the related standards. Special FR, FRLS compounds are used for outer sheathing of cables, to suit customer’s specification requirements.

http://2.bp.blogspot.com/-qX20lSY0fb4/UCYyqtZ9skI/AAAAAAAAABo/7nFxg1fcCTI/s1600/cabl.png

Cable Manufacture

http://1.bp.blogspot.com/-5TcIiKXMj-A/UCY0iz6uswI/AAAAAAAAAB4/kRJXEwRHFQM/s1600/im3.png

An extruded cable production line is a highly sophisticated manufacturing process that must be run with great care to assure that the end product will perform reliably in service for many years. It consists of many sub processes that must work in concert with each other. If any part of the line fails to unction properly, it can create problems that will lead to poorly made cable and will potentially generate many metres of scrap cable. The process begins when pellets of insulating and semiconducting compounds are melted within the extruder. The melt is pressurised and this conveys material to the crosshead where the respective cable layers are formed. Between the end of the screw and the start of the crosshead.it is possible to place meshes or screens, which act as filters. The purpose of these screens was, in the earliest days of cable extrusion, to remove particles, or contaminants that might be present within the melt. While still used today, the clean characteristics of today’s materials minimize the need for this type of filter. In fact, if these screens are too tight, they themselves can generate contaminants in the form of scorch or precross linking.

Nevertheless, appropriately sized (100 to 200-micron hole size) filters are helpful to stabilize the melt and protect the cable from large foreign particles that most often enter from the materials handling system.

The most current technology uses a method called a true triple extrusion process where the conductor shield, insulation and insulation shield are coextruded simultaneously. The cables produced in this way have been shown to have better longevity (Figure 3) [7]. After the structure of the core is formed the cable is crosslinked to impart the high temperature performance. When a CV tube is used fine control of the temperature and residence time (linespeed) is required to ensure that the core is crosslinked to the correct level.

Jackets

In most MV, HV and EHV cable applications, the metal sheath/neutral is itself protected by a polymeric oversheath or jacket. Due to the critical performance needed from the oversheath, there are a number of properties that are required, such as good abrasion resistance, good processability, reasonable moisture resistance properties, and good stress cracking resistance. Experience has shown that the material with the best composite performance is a PE-based oversheath, though PVC, Chlorosulfonated Polyethylene and Nylon have been used as jacket materials.

Tests on XLPE cables retrieved after 10 years of operation show that the mean breakdown strength falls by almost 50% (from 20 to 11 kV/mm – HDPE & PVC, respectively) when PVC is used as a jacket material. Many utilities now specify robust PE based jackets as a result. The hardness of PE is also an advantage when protection is required from termite damage.

Jackets extend cable life by retarding the ingress of water and soluble ions from the ground, minimizing cable installation damage and mitigating neutral corrosion. Ninety three percent of investor-owned utilities in the USA specify a protective jacket. The semiconductive jacket or oversheath is recommended for high lightning incident areas or joint-use trenches where telecommunications cables co-exist with power cables.

Inspection and Test Plan for Power Cable

The inspection and test plan for power cable article provides you information about power cable test and power cable inspection in manufacturing shop.

Witnessing voltage and insulation resistance tests or alternative spark tests.

For 33Kv cable, witness dielectric power factor voltage test.

Dimensional checking on sample off-cut i.e. construction consistency, insulation thickness, external sheath screen, armours and mans of main components.

Visual checking in respect of cable formation, core and external sheath colors, marking legibility.

Testing on material sample i.e. conductor coating, insulation external jacket for elongation, heat strocle blending and characteristics of armour, metal and sheath components including zinc coating.

Third Party Inspection for Power Cable Configuration

Third party inspector checks the power cable configuration in accordance to drawing and datasheets. Following items is taken in account:

Cable type Number of conductors Conductor size Conductor color coding Insulation type and size Fillers Water stoppers Armour Shield Outer diameter Other specified elements/dimensions Cable identification

References:-

Electrical Power Cable Engineering by Bruce S. Bernstein and William A. Thu L.V. Power and Control Cables by Oman Cables Industry TR-101670 “Underground Transmission Systems Reference Book: 1992 Edition,”

Electric Power ResearchInstitute http://en.wikipedia.org http://ieeexplore.ieee.org/Xplore/home.jsp

Technical specifications of cable work1.1 SCOPEThis chapter covers the requirements for the selection, installation and jointing ofpower cables for low, medium and high voltage applications upto and including 33KV.For details not covered in these Specifications, IS:1255-1983 shall be referred to. Allreferences to BIS-Specifications and codes are for codes with amendments issued uptodate i.e. till the date of call of tender.1.2 TYPES OF CABLES1.2.1 The cables for applications for low and medium voltage (upto and including1.1KV) supply shall be one of the following: -(i) PVC insulated and PVC sheathed, conforming to IS:1554 (Part-1)- 1988(ii) Cross linked polyethylene insulated, PVC sheathed (XLPE), conformingto IS: 7098 (Part-1)- 1988.1.2.2 The cables for applications for high voltage (above 1.1KV but upto and including11KV supply) supply shall be one of the following: -(i) PVC insulated and PVC sheathed, conforming to IS:1554 (Part-2)- 1988.(ii) Paper insulated, lead sheathed (PILCA) conforming to IS:692-1973(iii) Cross linked polyethylene (XLPE) insulated, PVC sheathed conforming toIS:7098 (Part-2)- 1985.1.2.3 The cables for applications above 11KV but upto and including 33KV supplyshall be one of the following: -(i) Paper insulated lead sheathed (PILCA) conforming to IS: 692-1973.(ii) Cross linked, polyethylene insulated (XLPE) conforming to IS:7098(Part-2)-1985.1.2.4 The cables shall be with solid or stranded aluminium conductors, as specified.Copper conductors may be used, only in special applications, where use of aluminiumconductors is not technically acceptable.1.2.5 Where paper insulated cables are used in predominantly vertical situation, theseshall be of non-draining type.1.3 ARMOURING AND SERVING1.3.1 All multicore cables liable for mechanical damage and all HV cabkes(irrespective of the situation of installation) shall be armoured. Where armouring isunavoidable in dingle core cables, either the armour should be made of nonmagnetic

material, or it should be ensured that the armouring is not shorted at terminations, thuspreventing the flow of circulating currents therein.1.3.2 Short runs of cables laid in pipes, closed masonary trenches and similar protectedor secured enclosures need not be armoured.1.3.3 PVC and XLPE cables, when armoured, shall have galvanized steel wires (flat orround) for armouring.1.3.4 Paper insulated cables shall have for armouring, a double layer of steel tape fornormal applications. Steel wire armouring is preferred where the cables are liable totensile stresses in applications such as vertical runs, suspended on brackets or laid in soilthat is likely to subside.1.3.5 Serving over armouring in paper insulated cables shall consist of a complete layeror layers of suitable compounded Hessian materials.1.4 SELECTION OF CABLE SIZES1.4.1 The cable sizes shall be selected by considering the voltage drop in the case ofMV (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 the periodof its flow, especially in the case of HV cables.1.4.2 While deciding upon the cable sizes, derating factors for the type of cable anddepth of laying, grouping, ambient temperature, ground temperature, and soil resistivityshall be taken into account.1.4.3 Guidance for the selection of cables shall be served from relevant IndianStandards such as IS:3961 (Part-1)-1967 for paper insulated lead sheathed cables, IS:3961 (Part-2)-1967 for PVC insulated and PVC sheathed heavy duty cables, IS: 5819-1970 for recommended short circuit ratings of high voltage PVC cables, IS: 1255-1983on code of practice for installation and maintenance of power cables upto and including33KV rating etc.1.5 STORAGE AND HANDLING1.5.1 Storage

(i) The cable drums shall be stored on a well drained, hard surface, so that thedrums do not sink in the ground causing rot and damage to the cable drums. Pavedsurface is preferred, particularly for long term storage.(ii) The drums shall always be stored on their flanges, and not on their flatsides.(iii) Both ends of the cables especially of PILCA cables should be properlysealed to prevent ingress/ absorption of moisture by the insulation during storage.(iv) Protection from rain and sun is preferable for long term storage for alltypes of cables. There should also ventilation between cable drums.(v) During storage, periodical rolling of drums once in, say, 3 months through90 degrees shall be done, in the case of paper insulated cables. Rolling shall be done inthe 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 shouldbe rolled in the direction of the arrow marked on the drum.(ii) For manual transportation over long distances, the drum should bemounted on cable drum wheels, strong enough to carry the weight of the drum and pulledby means of ropes. Alternatively, they may be mounted on a trailer or on a suitablemechanical transport.(iii) For loading into and unloading from vehicles, a crane or a suitable liftingtackle should be used. Small sized cable drums can also be rolled down carefully on asuitable ramp or rails, for unloading, provided no damage is likely to be caused to thecable or to the drum.1.6 INSTALLATION1.6.1 General(i) Cables with kinks, straightened kinks or any other apparent defects likedefective armouring etc. shall not be installed.(ii) Cables shall not be bent sharp to a small radius either while handing or ininstallation. The minimum safe bending radius for PVC/XLPE (MV) cables shall be 12times the overall diameter of the cable. The minimum safe bending radius forPILCA/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 than15 times its overall diameter.(iii) The ends of lead sheathed cables shall be sealed with solder immediatelyafter cutting the cables. In case of PVC cables, suitable sealing compound/tape shall beused for this purpose, if likely exposed to rain in transit storage. Suitable heat shrinkablecaps may also be used for the purpose.1.6.2 RouteBefore the cable laying work is undertaken, the route of the cable shall be decidedby the Engineer-in-Charge considering the following.(i) While the shortest practicable route should be preferred, the cable routeshall generally follow fixed developments such as roads, foot paths etc. with properoffsets so that future maintenance, identification etc. are rendered easy. Cross country runmerely to shorten the route length shall not te adopted.(ii) 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 thatmay have to be necessarily complied with.(iii) As far as possible, the alignment of the cable route shall be decided aftertaking into consideration the present and likely future requirements of other servicesincluding cables enroute, possibility of widening of roads/lanes etc.(iv) Corrosive soils, ground surrounding sewage effluent etc. shall be avoidedfor the routes.(v) Route of cables of different voltages.(a) Whenever cables are laid along well demarcated or established roads, theLV/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 bekept in different trenches with adequate separation. Where available space is restrictedsuch 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 ata lower level than the cable of lower voltage.1.6.3 Proximity to communication cablesPower and communication cables shall as far as possible cross each other at rightangles. The horizontal and vertical clearances between them shall not be less than 60cm.1.6.4 Railway crossing

Cables under railway tracks shall be laid in spun reinforced concrete, or cast ironor steel pipes at such depths as may be specified by the railway authorities, but not lessthan 1m, measured from the bottom of the sleepers to the top of the pipe. Inside railwaystation limits, pipes shall be laid upto the point of the railway station limits, pipes shall belaid upto a minimum distance of 3m from the center of the nearest track on either side.1.6.5 Way LeaveWay leave for the cable route shall be obtained as necessary, from the appropriateauthorities, such as, Municipal authorities, Department of telecommunication, GasWorks, Railways, Civil Aviation authorities, Owners of properties etc. In case of privateproperty, Section 12/51 of the Indian Electricity Act shall be complied with.1.6.6 Methods of layingThe cables shall be laid direct in ground, pipe, closed or open ducts, cable trays oron surface of wall etc. The method(s) of laying required shall be specified in the tenderschedule of work.1.6.7 Laying direct in ground1.6.7.1 GeneralThis method shall be adopted where the cable route is through open ground, alongroads/lanes, etc. and where no frequent excavations are likely to be encountered andwhere re-excavation is easily possible without affecting other services.1.6.7.2 Trenching(i) Width of trenchThe width of the trench shall first be determined on the following basis(Refer figure 1)(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 horizontalformation, the width of the trench shall be increased such that the inter-axial spacingbetween the cables, except where otherwise specified, shall be at least 20cm.© There shall be a clearance of at least 15cm between axis of the end cablesand the sides of the trench.(ii) Depth of trenchThe depth of the trench shall be determined on the following basis (Refer figure 1): -

(a) Where the cables are laid in a single tier formation, the total depth oftrench shall not be less than 75cm for cables upto 1.1KV and 1.2m for cables above1.1KV.(b) When more than one tier of cables is unavoidable and vertical formationof laying is adopted, the depth of the trench in (ii) a above shall be increased by 30cm foreach additional tier to be formed.© Where no sand cushioning and protective covering are provided for thecables as per 2.6.7.3(i)(b), 2.6.7.3(vii)(c) and 2.6.7.3(ix)(d) below, the depth of the trenchas per (ii)(a) and (b) above shall be increased by 25cm.(iii) Excavation of trenches(a) The trenches shall be excavated in reasonably straight lines. Whereverthere is a change in the direction, a suitable curvature shall be adopted complying withthe requirements of clause 2.6.1(ii).(b) Where gradients and changes in depth are unavoidable, these shall begradual.© The bottom of the trench shall be level and free from stones, brick bats etc.(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 notfall 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 trickd, tilesor protective covers or bare cables are encountered, further excavation shall not becarried out without the approval of the Engineer-in-Charge.(f) Existing property, if any, exposed during trenching shall be temporarilysupported adequately as directed by the Engineer-in-Charge. The trenching in such casesshall be done in short lengths, necessary pipes laid for passing cables therein and thetrench refilled in accordance with clause 2.6.7.4.(g) It there is any danger of a trench collapsing or endangering adjacentstructures, the sides may be left in place when back filling the trench.(h)Excavation through lawns shall be done in consultation with the Departmentconcerned.1.6.7.3Laying of cable in trench(i) Sand cushioning

(a) The trench shall then be provided with a layer of clean, dry sand cushionof not less than 8cm in depth, before laying the cables therein.(b) However, sand cushioning as per (a) above need not be provided for MVcables, where there is no possibility of any mechanical damage to the cables due to heavyor shock loading on the soil above. Such stretches shall be clearly specified in the tenderdocuments.© Sand cushioning as per (a) above shall however be invariably provided inthe case of HV cables.(ii) Testing before layingAll the time of issue of cables for laying, the cables shall be tested for continuityand insulation resistance (See also clause 2.8.1)(iii) The cable drum shall be properly mounted on jacks, or on a cable wheel at asuitable location, making sure that the spindle, jack etc. are strong enough to carry theweight of the drum without failure, and that the spindle is horizontal in the bearings so asto prevent the drum creeping to one side while rotating.(iv) The cable shall be pulled over on rollers in the trench steadily and uniformlywithout jerks and strain. The entire cable length shall as far as possible be laid off in onestretch. PVC/XLPE cables less than 120sq.mm. size may be removed by “Flaking” i.e. bymaking one long loop in the reverse direction.Note: - For short runs and sizes upto 50sq.mm. of MV cables, any other suitable methodof direct handing and laying can be adopted without strain or excess bending of thecables.(v) After the cable has been so uncoiled, it shall be lifted slightly over the rollersbeginning from one and by helpers standing about 10m apart and drawn straight. Thecable shall then be lifted off the rollers and laid in a reasonably straight line.(vi) Testing before coveringThe cables shall be tested for continuity of cores and insulation resistance (Refer clause2.8.1) and the cable length shall be measured, before closing the trench. The cable endshall be sealed /covered as per clause 2.6.1 (iii)(vii) Sand covering

Cables laid in trenches in a single tier formation shall have a covering of dry sand of notless 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 sandcushion of 30cm shall be provided over the base cushion before the second tier is laid. Ifadditional tiers are formed, each of the subsequent tiers also shall have a sand cushion of30cm as stated above. Cables in the top most tiers shall have final sand covering not lessthan 17cm before the protective cover is laid.Sand covering as per (a) and (b) above need not be provided for MV cables where adecision is taken by the Engineer-in-Charge as per sub clause (i)(b) above, but the intertier spacing should be maintained as in (b) above with soft soil instead of sand betweentiers and for covering.Sand cushioning as per (a) and (b) above shall however be invariably provided in the caseof HV cables.(viii) Extra loop cable(a) At the time of original installation, approximately 3m of surplus cable shall be lefton each terminal end of the cable and on each side of the underground joints. The surpluscable shall be left in the form of a loop. Where there are long runs of cables such loosecable 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 cableswhen forming loops of a number of cables as in the case of cables emanating from asubstation, 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 in accordance with(b) and (c) below to provide warning to future excavators of the presence of the cableand also to protect the cable against accidental mechanical damage by pick-axe blows etc.

(b) Unless otherwise specified, the cables shall be protected by second class brick ofnominal 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 thecable. Where more than one cable is to be laid in the same trench, this protective coveringshall cover all the cables and project at least 5cm over the sides of the end cables.© Where bricks are not easily available, or are comparatively costly, there is noobjection to use locally available material such as tiles or slates or stone/cement concreteslabs. Where such an alternative is acceptable, the same shall be clearly specified in thetender specifications.(d) Protective covering as per (b) and (c) above need not be provided only for MVcables, in exceptional cases where there is normally no possibility of subsequentexcavation. Such cases shall be particularly specified in the Tender specifications.(e) The protective covering as per (b) and (c) above shall, however invariably beprovided in the case of HV cables.1.6.7.4 Back filling(i) The trenches shall be then back-filled with excavated earth, free fromstones or other sharp ended debris and shall be rammed and watered, if necessary insuccessive layers not exceeding 30cm depth.(ii) Unless otherwise specified, a crown of earth not less than 50mm and notexceeding 100mm in the center and tapering towards the sides of the trench shall be leftto allow for subsidence. The crown of the earth however, should not exceed 10 Cms so asnot to be a hazard to vehicular traffic.(iii) The temporary re-statements of roadways should be inspected at regularintervals, particularly during wet weather and settlements should be made good by furtherfilling as may be required.(iv) After the subsidence has ceased, trenches cut through roadways or otherpaved areas shall be restored to the same density and materials as the surrounding areaand –re-paved in accordance with the relevant building specifications to the satisfaction

of the Engineer-in-Charge.(v) Where road beams or lawns have been cut out of necessity, or kerb stonesdisplaced, the same shall be repaired and made good, except for turfing /asphalting, to thesatisfaction of the Engineer-in-Charge and all the surplus earth or rock shall be removedto places as specified.1.6.7.5 Laying of single core cables(i) Three single core cables forming one three phase circuit shall normally belaid in close trefoil formation and shall be bound together at intervals of approximately1m.(ii) The relative position of the three cables shall be changed at each joint atthe time of original installation, complete transposition being effected in every threeconsecutive cable lengths.1.6.7.6 Route markers(i) LocationRoute markers shall be provided along the runs of cables at locations approved by theEngineer-in-Charge and generally at intervals not exceeding 100m. Markers shall also beprovided to identity change in the direction of the cable route and at locations ofunderground joints.(ii) (a) Plate type markerRoute markers shall be made out of 100mm X 5mm GI/ aluminium plate welded / boltedon 35mm X 35mm X 6mm angle iron, 60cm long. Such plate markers shall be mountedparallel to and at about 0.5m away from the edge of the trench.(b) CC markerAlternatively, cement concrete 1:2:4 (1 cement:2 coarse sand: 4 graded stone aggregateof 20mm in size) as shown in figure 2 shall be laid flat and centered over the cable. Theconcrete markers, unless otherwise instructed by the Engineer-in-Charge, shall projectover the surrounding surface so as to make the cable route easily identifiable.(c) InscriptionThe words ‘CPWD-MV/HV CABLE’ as the case may be, shall be inscribed on themarker.1.6.8 Laying in pipes / closed ducts

1.6.8.1 In locations such as road crossing, entry in to buildings, paved areas etc. cablesshall be laid in pipes or closed ducts. Metallic pipe shall be used as protection pipe forcables fixed on poles of overhead lines.1.6.8.2(i) Stone ware pipes, GI, CI or spun reinforced concrete pipes shall be used forcables in general; however only GI pipe shall be used as protection pipe on poles.(ii) The size of the pipe shall not be less than 10cm in diameter for a single cable andnot less than 15cm for more than one cable.(iii) Where steel pipes are employed for protection of single core cable feeding ACload, the pipe should be large enough to contain both cables in the case of single phasesystem and all cables in the case of poly phase system.(iv) Pipes for MV and HV cables shall be independent ones.1.6.8.3(i) In the case of new construction, pipes as required (including for anticipated futurerequirements) shall be laid alongwith the civil works and jointed according to the CPWDBuilding Specifications.(ii) Pipes shall be continuous and clear of debris or concrete before cables are drawn.Sharp edges if any, at ends shall be smoothened to prevent damage to cable sheathing.(iii) These pipes shall be laid directly in ground without any special bed except for SWpipe which shall be laid over 10cm thick cement concrete 1:5:10 (1 cemtnt:5coarsesand:10 graded stone aggregate of 40mm nominal size) bed. No sand cushioning or tilesneed be used in such situations.1.6.8.4 Road crossings(i) The top surface of pipes shall be at a minimum depth of 1m from the pavementlevel when laid under roads, pavements etc.(ii) The pipes shall be laid preferably askew to reduce the angle of bend as the cableenters and leaves the crossing. This is particularly important for HV cables.(iii) When pipes are laid cutting an existing road, care shall be taken so that the soil

filled up after laying the pipes is rammed well in layers with watering as required toensure proper compaction. A crown of earth not exceeding 10cm should be left at the top.(iv) The temporary re-instatements of roadways should be inspected at regularintervals, particularly after a rain, and any settlement should be made good by furtherfilling as may be required.(v) After the subsidence has ceases, the top of the filled up trenches in roadways orother paved areas shall be restored to the same density and material as the surroundingarea in accordance with the relevant CPWD Building Specifications to the satisfaction ofthe Engineer-in-Charge.1.6.8.5 Manholes shall be provided to facilitate feeding/drawing in of cables withsufficient working space for the purpose. They shall be covered by suitable manholecovers. Sizes and other details shall be indicated in the Schedule of work.1.6.8.6 Cable entry into the buildingPipes for cable entries to the building shall slope downwards from the building. The pipesat the building end shall be suitably sealed to avoid entry of water, after the cables arelaid.1.6.8.7 Cable-grip / draw-wires, winches etc. may be employed for drawing cablesthrough pipes / closed ducts.1.6.8.8 Measurement for drawing/ laying cables in pipes/ closed duct shall be on the basisof the actual length of the pipe / duct for each run of the cable, irrespective of the lengthof cable drawn through.1.6.9 Laying in open ducts1.6.9.1 Open ducts with suitable removable covers (RCC slabs or chequered plates) aregenerally provided in sub-stations, switch rooms, plant rooms, workshops etc. for takingthe cables. The cable ducts should be of suitable dimensions for the number of cablesinvolved.1.6.9.2(i) Laying of cables with different voltage ratings in the same duct shall be avoided.

Where it is inescapable to take HV & MV cables same trench, they shall be laid with abarrier between them or alternatively, one of the two (HV &MV) cables may be takenthrough pipe(s).(ii) Splices or joints of any type shall not be permitted inside the ducts.1.6.9.3(i) The cables shall be laid directly in the duct such that unnecessary crossing ofcables is avoided.(ii) Where specified, cables may be fixed with clamps on the walls of the duct ortaken in hooks/brackets/troughs in ducts.1.6.9.4Where specified, ducts may be filled with dry sand after the cables are laid and coveredas above, or finished with cement plaster, specially in high voltage applications.1.6.10Laying on surface1.6.10.1This method may be adopted in places like switch rooms, workshops, tunnels, rising(distribution) mains in buildings etc. This may also be necessitated in the works ofadditions and/or alterations to the existing installation, where other methods of layingmay not be feasible.1.6.10.2Cables may be laid in surface by any of the following methods as specified:(a) Directly clamped by saddles or clamps,(b) Supported on cradles,(c) Laid on troughs/trays, duly clamped.1.6.10.3(i) The saddles and clamps used for fixing the cables on surface shall comply withthe requirements given in Table-III.(ii) Saddles shall be secured with screws to suitable approved plugs. Clamps shall besecured with nuts on to the bolts, grouted in the supporting structure in an approvedmanner.(iii) In the case of single core cables, the clamps shall be of non-magnetic material. Asuitable non-corrosive packing shall be used for clamping unarmoured cables to prevent

damage to the cable sheath.(iv) Cables shall be fixed neatly without undue sag or kinks.1.6.10.4The arrangement of laying the cables in cradles is permitted only in the case of cables of1.1KV grade of size exceeding 120sq.mm. In such cases, the cables may be suspended onMS flat cradles of size 50mmX5mm which in turn shall be fixed on the wall by boltsgrouted into the wall in an approved manner at a spacing of not less than 60cm.1.6.10.5All MS components used in fixing the cables shall be either galvanized or given a coat ofred oxide primer and finished with 2 coats of approved paint.1.6.11 Laying on cable tray1.6.11.1This method may be adopted in places like indoor substations, air-conditioning plantrooms, generator rooms etc. or where long horizontal runs of cables are required withinthe building and where it is not convenient to carry the cable in open ducts. This methodis preferred where heavy sized cables or a number of cables are required to be laid. Thecable trays may be either of perforated sheet type or of ladder type.1.6.11.2 Perforated type cable tray(i) The cable tray shall be fabricated out of slotted/perforated MS sheets aschannel sections, single or double bended. The channel sections shall be supplied inconvenient lengths and assembled at site to the desired lengths. These may begalvanished or painted as specified. Alternatively, where specified, the cable tray may befabricated by two angle irons of 50mmX50mmX6mm as two longitudinal members, withcross bracings between them by 50mmX5mm flats welded/bolted to the angles at 1 mspacing. 2mm thick MS perforated sheet shall be suitably welded/bolted to the base aswell as on the two sides.(ii) Typically, the dimensions, fabrication details etc. are shown in figure3A,B and C.(iii) The jointing between the sections shall be made with coupler plates of the

same material and thickness as the channel section. Two coupler plates, each of minimum200mm length, shall be bolted on each of the two sides of the channel section with 8mmdia round headed bolts, nuts and washers. In order to maintain proper earth continuitybond, the paint on the contact surfaces between the coupler plates and cable tray shall bescraped and removed before the installation.(iv) The maximum permissible uniformly distributed load for various sizes ofcables trays and for different supported span are given in Table IV. The sizes shall bespecified considering the same.(v) The width of the cable tray shall be chosen so as to accommodate all thecables in one tier, plus 30 to 50% additional width for future expansion. This additionalwidth shall be minimum 100mm. The overall width of one cable tray shall be limited to800mm.(vi) Factory fabricated bends, reducers, tee/cross junctions, etc. shall beprovided as per good engineering practice. (Details are typically shown in figure 3). Theradius of bends, junctions etc. shall not be less than the minimum permissible radius ofbending of the largest size of cable to be carried by the cable tray.(vii) The cable tray shall be suspended from the ceiling slab with the help of10mm dia MS rounds or 25mmX5mm flats at specified spacing (based on Table III). Flattype suspenders may be used for channels upto 450mm width bolted to cable trays.Round suspenders shall be threaded and bolted to the cable trays or to independentsupport angles 50mmX50mmX5mm at the bottom end as specified. These shall begrouted to the ceiling slab at the other end through an effective means, as approved by theEngineer-in-Charge, to take the weight of the cable tray with the cables.(viii) The entire tray (except in the case of galvanized type) and the suspendersshall be painted with two coats of red oxide primer paint after removing the dirt and rust,and finished with two coats of spray paint of approved make synthetic enamel paint.(ix) The cable tray shall be bonded to the earth Terminal of the switch bonds atboth ends.

(x) The cable trays shall be measured on unit length basis, along the centerline of the cable tray, including bends, reducers, tees, cross joints, etc. and paid foraccordingly.1.6.11.3 Ladder type cable tray(i) The ladder type of cable tray shall be fabricated of double bended channelsection longitudinal members with single bended channel section rungs of cross memberswelded to the base of the longitudinal members at a center to center spacing of 250cm.(ii) Alternatively, where specified, ladder type cable trays may be fabricatedout of 50mmX50mmX6mm (minimum) angle iron for longitudinal members, and30mmX6mm flat for rungs.(iii) Typical details of fabrication and dimensions of both the types of trays areshown in figure 4A,B,C and D.(iv) The maximum permissible loading, jointing of channel sections, width ofthe cable tray, provision of elbows, bends, reducers, horizontal tee/ cross junctions etc.suspension of cable tray from the ceiling slab; painting and measurement of the cable trayshall be as per sub-clauses (ii) to (x) below clause 2.6.11.2, except that the overall widthof one cable tray may be limited to 800mm.1.6.11.4 Cables laid on cable trays shall be clamped on to the tray at suitableintervals as per Table-III.1.6.12 Cable identification tagsWhenever 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 inthe manholes / pull pits / joint pits / entry points in buildings / open ducts etc. These shallalso be attached to cables laid direct in ground at specified intervals, before the trenchesare backfilled.1.7 JOINTING1.7.1 Location(i) Before laying a cable, proper locations for the proposed cable joints, ifany, shall be decided, so that when the cable is actually laid, the joints are made in themost 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.(ii) Joints shall be staggered by 2m to 3m when joints are to be done for twoor more cables laid together in the same trench.1.7.2 Joints pits(i) Joint pits shall be of sufficient dimensions as to allow easy andcomfortable working. The sides of the pit shall be well protected from loose earth fallinginto it. It shall also be covered by a tarpaulin to prevent dust and other foreign matterbeing blown on the exposed joints and jointing materials.(ii) Sufficient ventilation shall be provided during jointing operation in orderto disperse fumes given out by fluxing.1.7.3 Safety precaution(i) A caution board indicating “CAUTION – CABLE JOINTING WORK INPROGRESS” shall be displayed to warn the public and traffic where necessary.(ii) Before jointing is commenced, all safety precautions like isolation,discharging, earthing, display of caution board on the controlling switchgear etc. shall betaken to ensure that the cable would not be inadvertently charged from live supply.Metallic armour and external metallic bonding shall be connected to earth. Where“Permit to work” system is in vogue, safety procedures prescribed shall be compliedwith.1.7.4 Jointing materials(i) Jointing materials and accessories like conductor ferrules, solder, flux,insulating and protective tapes, filling compound, jointing boxes, heat shrinking joint kitetc. of right quality and correct sizes, conforming to relevant Indian Standards, whereverthey exist, shall be used.(ii) The design of the joint box and the composition of the filing compoundshall be such as to provide an effective sealing against entry of moisture in addition toaffording proper electrical characteristic to joints.(iii) Where special type of splicing connector kits or epoxy resin spliced jointsor heat shrinkable jointing kits are specified, materials approved for such applicationshall be used. Storing as well as jointing instructions of the manufacturer of suchmaterials shall be strictly followed.1.7.5 Jointer

Jointing work shall be carried out by a licensed/ experienced (where there is nolicensing system for jointers) cable jointer.1.7.6 Cable work with joints(i) About 3m long surplus cable shall be left on each side of joints as laiddown in clause 1.6.7.3 (viii).(ii) Insulation resistance of cables to be jointed shall be tested as per clause1.8.1. Unless the insulation resistance values are satisfactory, jointing shall not be done.(iii) Cores of the cables must be properly identified before jointing.(iv) Where cable is to be jointed with the existing cable, the sequence shouldbe so arranged as to avoid crossing of cores wile jointing.(v) Whenever the aluminium conductor is exposed to outside atmosphere, ahighly tenacious oxide film is formed which makes the soldering of aluminium conductordifficult. This oxide film should be removed by using appropriate type of flux.(vi) The clamps for the armour shall be clean and tight.1.7.7 Jointing procedureWhile it would be necessary to follow strictly the instructions for jointingfurnished by the manufacturers of cables and joint kits, a brief on the jointing proceduresis given for general guidance in Appendix F.1.8 TESTING1.8.1 Testing before layingAll cables, before laying, shall be tested with a 500V megger for cables of 1.1KVgrade, or with a 2500/5000V megger for cables of higher voltage. The cable cores shallbe tested for continuity, absence of cross phasing, insulation resistance from conductorsto earth / armour and between conductors.1.8.2 Testing before backfillingAll cables shall be subjected to the above mentioned tests, before covering thecables by protective covers and back filling and also before taking up any jointingoperation.1.8.3 Testing after laying(i) After laying and jointing, the cable shall be subjected to a 15 minutespressure test. The test pressure shall be as given in Table VI. DC pressure testing maynormally be preferred to AC pressure testing.(ii) In the absence of facilities for pressure testing as above, it is sufficient totest for one minute with 1000V megger for cables of 1.1KV grade and with 2500/5000V

megger for cables of higher voltages.

Cable

Business

Transcript of Cable