Introduction

Cables are basically an insulated conductors which carry / transmit electric

power, data, signals etc., from one end to another. In old days the above job was

done through over head lines with bare uninsulated conductors which poised

problems to electrical safety. As the technology developed various types of

cables have come into use and they are like Cotton / cloth covered, Rubber

Insulated, Paper Insulated, PVC Insulated, PVC sheathed, and presently XLPE

cables. Also depending upon the service various types of cables are developed

like Mining, Crane duty, Heat resistant, Shielded Telecommunication, Computer

Application, Co-axial, Welding, etc., for specific application.

Cables are also made of steel armouring to protect against mechanical

damage. Cables are also classified based on voltages it is being operated.

Various standards are drawn up for manufacturing cables depending on their

duty, voltage, environment (Hazardous, Heat zone, Mining) and also standards

for their installation in under ground installation (either directly in trenches or

buried with sand and brick bedding) or in Air.

Once the layout of electrical equipment is prepared cable selection is done

based on their duty, application, voltage etc., and necessary precautions are

taken to see that minimum voltage drop is maintained and losses are kept to

minimum after considering the due derating factors for type of laying (air /

ground), grouping, load factor etc.

Transporting power through overhead lines which is totally exposed to corrosive

dust pollution's atmosphere and rough weathers like lighting poses frequent

breakdown due to problems faced like bird faults, failures of disc insulator etc.

Though we can overcome the above problems in cables but still cable end

termination's and straight and tee joints form weak points for breakdown.

Therefore, right selection of cable end termination, straight or tee joints are to be

selected and with perfection these termination's are to be carried out to avoid any

failures. Such problems are more predominant as we go to higher voltages.

Unlike in overhead conductors where routine inspection is done to check up for

failure of hardware, sealing etc., in cables it is not necessary. Once cables are

laid and properly installed we can forget about the installation. But it is advisable

to check once in a while the heating of cable, end termination, megger values

etc.

Though initial cost is very much on higher side for cables when compared to

overhead transmission keeping in view of electricity safety, area lost under the

transmission lines, thefts of power (In case of State Electricity Board) and

aesthetic point of view, cables shall still be economical on a broader perspective.

The enclosed module gives a brief account of various aspects in selection,

laying, testing and maintenance of cables.

Terminology

2.1 DEFINITIONS OF CABLE TERMS

1. Cable

A single standard conductor with insulation and protective covering or two or more such conductors laid-up together. 2.1.2 Core

A single conductor with its insulation but not including protective covering. (Core

is called conductor in USA).

A cable may be:

- Single core or single conductor type.

- Double core or two conductor type.

- Multi core or Multi-conductor type.

2.1.3 Screened cable or shielded cable

A cable in which each conductor is separately enclosed in a conducting films in

order to ensure even distribution radial electric field surrounding the conductor. It

also limits electromagnetic field beyond the shield to low value, minimise surface

voltage stress on conductor surface & reduces shock hazards when shields are

properly grounded. These conducting films have electrical connection with each

other and with the metallic sheath/armour of the cable which is usually earthed.

1. Unshielded cable or non-shielded cable (Belted cable)

The cable does not have metallic shield or screen over insulation.

1. Earth shield (screen)

Metallic sheath immediately under the lead sheath and connected to it. Lead Sheath

A protective lead cover over the insulation for excluding moisture from conductor

and insulation. Sheath is made of commercially available pure lead or lead with

small amount of hardening alloy. Sheath should be of non-magnetic material. It

serves as a return path to fault current.

1. Tough rubber sheath

A sheathing over insulation of a cable to form outer protective cover of tough rubber mixed with hardening substance and suitably vulcanised to make it water proof and resistant to decay, mechanical abrasion, acids, alkalis and other corrosive material.

1. Armour

Metallic wrapping (usually tape or wires) over the sheath for the purpose of mechanical protection. In single core cable, armour should be of non-magnetic material. In three-core cable, common armour is provided for all the three cores and the

material of armour may be Galvanised steel.

1. Belt

Insulating wrapping over the insulation of all the three cores.

1. Belted cable

A Multi-cored cable in which part of the insulation is on each conductor individually and remainder is in the form of overall belt.

1. Serving of an armoured or metal sheathed cable

A layer of fibrous material (permeated with water proof compound) on external surface along with layers of water proof compound serving is the outermost non metallic water proof covering over the armour. Serving may be water-proof, corrosion proof, mechanical abrasion proof etc.

1. Braiding of a cable

A plated protective cover generally of fibrous material.

1. Filler

An insulating material such as treated hemp, synthetic rubber, thermo plastic material etc., which is used for filling up the space inside the sheaths / shield to fill-up the gaps.

1. Jacket/Sheath/Covering

It is provided over the insulation for mechanical protection, to avoid moisture entry, & provides a return path of fault current in cables with metallic sheath.

Metallic Sheath : Lead or Aluminium.

Non-metallic Sheath : Rubber or Plastic.

1. Insulation

Part of the cable which provides insulation over individual conductor and or over a group of insulated conductors.

1. Self contained oil-filled cable

The cable filled with oil (in the central duct) and does not need any external pressure system.

1. Low pressure cable

Pressure is approximately 1 to 2 Atm.

1. Medium pressure cable

Pressure is approximately 5 Atm. 2.1.19 High pressure cable

Pressure is approximately 13 Atm.

2.1.20 Gas-pressure cable

A pressure cable in which the pressure medium employed is an inert gas.

1. Impregnated gas-pressure cable

A mass-impregnated paper-insulated cable with a mechanically reinforced lead sheath in which the space inside the lead sheath is brought up to high pressure by the introduction of inert gas in contact with the dielectric.

1. Dry gas pressure cable

An un-impregnated paper insulated cable with a mechanically reinforced lead sheath in which the space inside the lead sheath is brought up to high pressure by the introduction of inert gas in contact with the dielectric. Abbreviations for Synthetic Insulation's

XLPE. - Cross-linked polyethylene.

PVC. - Polyvinyl chloride.

E.P.R. - Ethylene propylene rubber.

PP. - Polypropylene.

SF6 - Sulphur Hexa-fluoride (gas).

GIC. - Gas insulated cable (SF6)

CGIC. - Compressed Gas Insulated Cable.

GIT. - Gas insulated transmission line.

2.2. TYPES OF POWER CABLES

Cables are classified according to their insulation into following main types :

2.2.1 Paper insulated cable (P.I.C) or Paper insulated lead covered cable

(P.I.L.C). These cables are not covered in the module as they are outdated.

2.2.2 Tropodur type PVC cable.

2.2.3 Oil filled paper insulated cable.

2.2.4 High pressure oil filled cable.

2.2.5 Compressed gas insulated cable.

2.2.6 Vulcanised rubber cable.

2.2.7 XLPE insulated cable (Cross-Linked-Polyethylene Insulated).

The cables are either single conductor cable or 3 conductor cable or 4 conductor

cable. Accordingly they are called single core cable, triple core cable or 4 core

cable. Etc.

2.3 DIFFERENCE BETWEEN POWER CABLES & CONTROL CABLES

Power cables carry the main load current at medium / high / low voltage of main

circuit. Power cables have higher normal current rating and therefore larger

cross-section of conductors. Control cables (Pilot cables) are used for auxiliary

low voltage a.c. or d.c. circuits for protection, communication, control, signalling

systems. They have several PVC insulated cores of smaller conductor cross-

section. Control cables are provided with screens. Control cables have lower

current rating.

Power cables are designed to carry specified power current at specified voltage.

Typical voltage ratings are 415V, 3.3 kV, 6.6 kV, 11 kV. Their current rating is

associated with temperature rise limit. They may have 1 core or 3 core or 4

cores. Control cables are used in protection circuits, auxiliary circuits, control

circuits and at low voltages such as 220 V, 110 V, control cables may have

several cores. The colour of the insulation of each core is different. Control

cables are not suitable for carrying heavy current.

Jointing

6.1 GENERAL

The emphasis should be laid on quality and selection of proper cable

accessories, proper jointing techniques and skill and workmanship of the working

personnel. The quality of joint should be such that it does not add any resistance

to the circuit. The materials and techniques employed should give adequate

mechanical and electrical protection to the joints under all service conditions. The

joint should further be resistant to corrosion and other chemical effects.

Before studying the technique of making good cable jointing, it is necessary to

examine the requirement of a good joint. A good joint shall not introduce any

weakness in the cable system and must be so constructed that :

- The conductivity of the joined conductor is not less than that of an equivalent

length of enjoined conductor, and the conductivity of the enclosing metallic

sheath and armouring is maintained unimpaired.

- The insulation of cable is maintained and at the joint is not less effective than

that of the cable cores.

- The joint is properly enclosed to prevent mechanical damage or the ingress of

moisture.

- The joint will withstand the mechanical stresses imposed by a short-circuit and

the thermal effects of normal or fault currents.

Physical inspection of the joints where the possible helps us to ascertain the

healthiness of the Joints. Physical indications like crack, leakage of oil bitumen

compound of noise of partial discharge give unhealthy symptoms of the joints. In

such cases, Joint may fail in near future and hence preventive action is invited.

6.2 BASIC TYPES OF JOINTS

The basic type of cable joints are :

Straight though joint

This joint is used to connect two cables lengths together.

Tee / branch joint

This is normally used for jointing a service cable to the main distribution cable in

city distribution network.

Termination or sealing end

This is generally used to connect a cable to switchgear terminal in switchboards

and distribution pillars transformers boxes, motor terminal boxes and to overhead

lines.

Types of Cable Jointing Accessories

The basic types of cable jointing accessories available in the country, at present

are :

a) Joint sleeves with insulating and bitumen based filling compound suitable for

jointing cables of voltages up to and including 33 kV.

b) Cast resin based cable accessories suitable for jointing cables of voltages up

to and including 11 kV.

Note:- Other accessories; such as heat shrinkable tubes, taped joints and slip on

(pre-moulded) joints are also in use.

Sleeve Type Joint

This joint comprises:

a) Dressing of cable ends and conductor joints,

b) Replacing factory made insulation by manual wrapping of tapes or application

of pre-formed insulating sleeves,

c) Plumbing metallic sleeve or wiping gland to the lead sheath of the cable to

prevent moisture from entering the joint,

d) Filling the metallic sleeve with molten bitumen compound or insulating

compound, and

e) Fixing a cast iron or any other protective shell around the joint filling the same

once again with molten bitumen compound.

Cast Resin Joint

It comprises:

a) Dressing of cable ends and conductor joints;

b) Wiping dry - The core insulation should be wiped dry and all parts, which are

to be embedded in casting resin should be roughened and cleared with relevant /

degreasing agents,

c) Fixing two halves of mould around the cable joints or ends and sticking them

together and sealing to form a leak proof cast mould,

d) Pouring pre-mixed cast resin and hardener into the mould,

e) Allowing sufficient time for setting casting resin, and

f) Removing plastics mould. In case of buried joint, the plastics mould may be left

intact.

6.3 GENERAL INSTALLATION GUIDELINES

It would be best to follow strictly the instructions furnished by the suppliers of

cable and joint boxes. However, the recommendations are given for general

guidance.

Joint Position

During the preliminary stages of laying the cable, consideration should be given

to proper location of the joint position so that when the cable is actually laid the

joints are made in the most suitable places. There should be sufficient overlap of

cables to allow for the removal of cable ends which may have been damaged.

This point is extremely important as otherwise it may result in a short piece of the

cable having to be included. The joint should be near pipe end or at the bend.

Joint Pits

Whenever practicable, joint pit should be of sufficient dimensions so as to allow

jointers to work with as much freedom of movement and comfort as possible. For

this purpose, the depth of the pit should be at least 0.3 m below the cables to be

jointed. The sides of the pit should be draped with small tarpaulin sheets to

prevent loose earth from falling on the joint during the course of making. If the

ground has been made up by tipping, or if running sand is met with, the pit

should be well shored up with timber so as to prevent collapse. The floor of the

joint pit should be well consolidated. The two lengths of cable meeting at a joint

are laid with an overlap of at least half the length of joint box when pulling in. This

enables the jointer to adjust the position of his joint slightly to allow for any

obstructions that may be encountered.

When two or more cables are laid together, the joints are arranged to be

staggered so as to reduce the excess width of trench and also be isolate the

joints from each other and reduce the possibility of one joint failure affecting the

other joints.

Sump Holes

When jointing cable in water-logged ground or under monsoon conditions, a

sump hole should be excavated at one end of the joint hole in such a position so

that the accumulating water can be pumped or baled out without causing

interference to the jointing operation.

Tents

As far as possible a tent should be used where jointing work is being carried out

in the open.

Measurement of Insulation Resistance

Before jointing is commenced, it is advisable that the insulation resistance of both

sections of the cable to be jointed be checked by insulation resistance testing

instruments like megger.

6.4 PRECAUTION TO BE TAKEN ON LIVE CABLES IN SERVICE

When a cable which is in service is cut for making a joint, normally it is first

isolated, discharged, tested and earthed, before proceeding further, although

work on live conductors is permissible under certain conditions. The test lamp is

one of the apparatus used to determine whether the cable is 'alive' or 'dead'. The

jointer should wear rubber boots or gloves or stand on a rubber mat. When

rubber mats are used in wet holes, pieces of board should be placed under the

mat and water should not be allowed to creep over the edges of the mat.

Before cutting the cable prior to making a straight joint, the most convenient core

to be cut (the neutral should always be cut last) should be selected, and

separated from the others by means of a wooden wedge. A small piece of rubber

insertion should then be placed between the core to be cut and the remaining

cores and cut through the selected core. The cut ends should be separated and

tested to ascertain if either or both are live or dead. Irrespective of being live or

dead, both ends should be taped as a measure of safety. The same procedure

should be followed with the remaining cores. It is advisable to step the positions

of the cuts, so that it is impossible for the hacksaw, knife or any other tools which

may be in use to cause a short circuit by coming into contact with two or more of

the cut ends simultaneously.

Before making conductor joints, the lead sheath and armour should be wrapped

with insulating material - an old bicycle inner tube is useful for this purpose. The

precaution will prevent a 'short' to earth if a tool slips between the live conductor

and the sheath or armour neutral conductor should be joined first.

In the case of baring of the conductors for 'tee' and 'service' joints where

conductors are not cut, the jointer should be instructed to remove sufficient

armouring and lead sheathing for making a joint in the most suitable position.

The cores of the cable to be tee-jointed should be spread and suitable tapping

positions selected on the main cable. The most convenient core to be teed

should be selected and separated from others by means of a wooden wedge (as

far as possible, the neutral should be selected first). A rubber insulation should

be inserted between this core and the rest and prepare the core for tee-jointing.

The paper insulation of the main cable core should be cut for a suitable length

and the jointing work completed. Rubber insertion between this core and the rest

should not be removed till this exposed core is completely taped. This process

should be repeated for other cores taking care that only one core is handled at a

time.

All jointing accessories and materials such as solders, plumbing metal, lead

sleeves, ferrules and bitumen compound should be in accordance with the

relevant Indian Standards, wherever such standards exists.

6.5 STRAIGHT THROUGH JOINTS

For PVC Cables Up to 11 kV

These joints are preferably done using cold setting casting resins, primarily

because of thermoplastic nature of insulation and sheath. Cast iron boxes with

bitumen based filing compounds can be used with PVC cables with certain

precautions.

Typical drawings for cast resin and bitumen compound filled straight through

joints are shown in enclosed Figure A.

For 1.1 kV grade, moulds conforming to relevant Indian standard should be used.

For voltages above this neither moulds nor casting resins are standardised.

Hence cable supplier may be consulted for his advice on selection.

While jointing control cables having large number of cores jointing of proper

cores should be ensure. Core identification should be properly studied for this

purpose.

For XLPE Cables up to 11 kV

For XLPE cables up to 3.3 kV (un-screened), these joints are done by using cold

setting casting resins in suitable moulds.

For cables above 3.3 kV and up to 11 kV (screened) self amalgamating tapes

(both insulating and semi-conducting) are used for providing stress relieving

mechanism to these joints. Cold setting resins are used for further protection

against water and corrosion.

All conductors are to be jointed by crimping / compression / welding methods.

Note - Jointing by soldering may be resorted to provide the temperature of

conductor under short circuit conditions is not likely to exceed 160 C.

A typical drawing of straight through joint for HT (high tension) screened type

XLPE cable is shown in Figure B.

Tee or Branch Joints

These joints should be restricted to 1.1 KV grade cables. Tee joints on HT cables

up to and including 11 kV may be done only in exceptional cases.

These joints are made either using cast resin kits or C.1. Boxes with cast resin

kits for PVC and XLPE cables.

6.6 END TERMINATION'S OR SEALING ENDS

For PVC Cables

For PVC cables up to 11 kV cast resin end termination's are recommended both

for indoor and outdoor connections.

Indoor termination's in dry and non-corrosive atmosphere for 1.1 kV grade can

either be done by means of brass glands or by simple dressing.

For corrosive and aggressive atmospheres such as those prevailing in chemical,

fertiliser, cement, paper mills etc., cast resin termination's should be adopted.

For XLPE Cables.

Up to 3.3 kV (unscreened) cables, cast resin termination's are recommended for

indoor and outdoor termination's.

Indoor termination's up to 3.3 kV (unscreened) in dry and non-corrosive

atmosphere can be done by means of brass glands only.

For indoor termination's up to 3.3 kV (unscreened) in corrosive atmosphere, cast

resin termination's may be adopted.

For XLPE cables above 3.3 kV and up to 11 kV (un-screened), self-

amalgamating tapes (both insulating and semi-conducting) are used for providing

stress-relieving mechanism in the joints. Performed stress-relieving cloths or

tubes are also used for stress relieving mechanism in place of self-amalgamating

tapes.

All conductors of XLPE termination's are to be terminated either by crimping /

compression or by welding methods.

A typical drawing of 6.6 kV (UE), indoor type cable termination on XLPE cables is

given in Figure C1.

6.7 ALUMINIUM CONDUCTOR CONNECTIONS

There are number of methods of jointing aluminium conductors.

Four standard methods which are most commonly used are :

a) Fluxless friction solder method

b) Soft soldering method using organic fluxes

c) Welding method ; and

d) Crimped or compressed connection

Fluxless Friction Solder Method - In this method each strand of the conductor is

carefully cleared and scraped with scraper tongs to remove oxide film. Then all

the strands are tinned by rubbing a special friction solder stick over the heated

strands. This is known as metallising. Aluminium conductor thus prepared may

be soldered on to copper cable lugs, ferrule, terminal studs using 60 percent

solder. No flux is used in any of the operation. This method is not recommended

for jointing conductors in XLPE cables.

Soldering Method Using Organic Flux

The individual strands should be separated and cleaned thoroughly by a

scrapper and the impregnation compound and oil if any, should be removed. If

necessary the strands can be stepped. The conductors should then be preheated

by basting with solder, the temperature of which should be maintained at 316 C

or as recommended by manufacturers. The excess solder should then be wiped

off quickly and aluminium solder flux should be applied to the conductor by a stiff

brush on all sides of conductor.

The conductor should then be basted several times with soiled. If necessary the

flux should be applied again and the conductor basted with solder till a bright

shining appearance is obtained.

The copper ferrule, which is generally of a weak-back type, should be tinned and

fitted on to the conductor and closed firmly but not completely.

The ferrule should then be basted with solder and the gap should be filled in with

the solder. The ferrule then be closed firmly and basted with solder, till the solder

solidifies. The excess solder should be wiped off and the joint allowed to cool.

During jointing operation copious fumes are given off when the flux is heated.

These fumes contain small quantities of fluorine and it is, therefore, advisable to

avoid inhaling them as far as possible. It is also recommended that proper

ventilating be maintained at the place of jointing.

Organic fluxes tend to char and are rendered ineffective when exposed to

temperature in excess of 300 C. Emphasis should, therefore, be laid on the

need to control pot temperature.

Welding Method - Welding method gives the best possible results. Welded

conductor joints have lesser resistance and equal or better mechanical strength

than the conductor itself. Welding, therefore, should be given preference for all

larger cross sections. For smaller cross section welding may not always be

feasible or economical. In this method the end of the stranded conductor are first

welded to the cable lug, terminal stud or to each other, in open or closed mould

using aluminium welding rods or strands taken from conductor. After cooling

welded connections are filed smoothened and cleaned.

Crimped or Compressed Connections - In this method conductor and lug ferrules

are pressed together firmly by means of tools and dies to form a joint. The

methods normally used are indent compression, hexagonal compression or

circular compression. Tools and accessories should meet the requirement of

relevant Indian Standards where available.

6.8 EARTHING AND BONDING

The metal sheath, metal screen (if any) and armour of any cable should be

efficiently earthed at both ends.

In case of single-core cables of larger sizes, the armour, lead sheath metal

screen, if any, is bonded at times only at one point. Intention is drawn in this case

to the presence of standing voltages along armour or lead sheath and to the

considerable increase in such voltages when cables carry fault currents. These

voltages must be taken into count when considering safety and outer sheath

insulation requirement.

All metal pipes or conduits in which the cables have been installed would be

efficiently bonded and earthed.

Where cables not having metallic sheath are used, embedding additional earth

electrodes and connecting the same with steel armour cable becomes

necessary.

Earthing and bonding should be done in accordance with IS:3043-1966*.

6.9 HEAT SHRINKABLE CABLE TERMINATION

1. Introduction

Since the late nineteen sixties, the electricity supply industry has installed over five million heat shrinkable cable termination's and straight through joints throughout the world for distribution voltage up to 33 kV.

The long-term performance of these numerous installations in some of the most

demanding conditions led to widespread acknowledgement of the reliability and

ease of application of the heat shrinkable cable accessories.

6.9.2 Cable Accessory Requirements

A cable accessory has to perform a number of basic functions. These may be

analysed as follows :

Stress Relief

The design of a termination or joint must provide for the control of the electrical

stresses in the insulation used. This is essential to ensure a long service life and

prevent damaging partial discharges.

Insulation

Joints must re-insulate the cables with high quality dielectric grade insulation and

termination's require protective insulation able to withstand the rigors of sunlight,

humidity, pollution, and surface electrical effects with tracking.

Environmental Seals

The accessory must be capable of fully sealing the cable against the ingress of

moisture. The failure mechanism of plastic insulated cables through water treeing

is now well understood.

Various technologies have attempted to satisfy the above functions using

different approaches as below.

- field engineered designs such as tape wrapped designs, resin filled designs.

- factory engineered designs such as pre-moulded and cold shrink.

The field engineered designs accommodate variations in cable dimensions found

on site but are subject to the variables of assembly in the field and are extremely

sensitive to installer's skills. These accessories have limited shelf life, especially

in tropical climates. Factory engineered designs remove site variables of

assembly, but are different to seal from environment and may require strict

tolerance on cable dimensions which is difficult in cables available in India.

6.9.3 Heat Shrink Technology - A Modern Technology

Heat shrink technology has succeeded in combining the versatility of field

engineered design and the reliability of factory controlled manufacture to meet

the above functions. Heat shrink systems are highly tolerant of variations in cable

dimensions, types and cable stripping dimensions and are manufactured under

closely controlled factory conditions.

A good heat shrinkable cable necessarily requires combination of the following

advanced technologies :

a) Cross-linked Polymers.

Certain plastics, or polymers, acquire enhanced properties when subjected to

high-energy electron-beam radiation's. During this process, their chain-like

molecules bond together "cross-link" at random points. When exposed to high

temperatures, the cross-linked material will become elastic, but will no longer

melt. At low temperatures, it remains flexible, yet resistant to both chemical and

physical abuse. Cross-linked polymers play a central role in many cable

accessories.

b) Elastic Memory

If a plastic is formed into a particular shape before being cross-linked, it will

return to, or "remember" that shape after cross-linking when heated, no matter

how it is expanded or deformed.

c) Adhesives

The adhesives which are incorporated into many heat shrinkable products fall

into two basic categories; hot melt thermoplastics and thermo sets. When

heated, a thermoplastic will melt and flow, providing a durable, watertight seal.

The material can be re-heated, many times without undergoing a chemical

change. A thermo set, however, cures chemically when its two components are

mixed. Once cured, it is infusible and insoluble, and thus well suited to high

temperature environments.

d) High Performance Polymers

A good heat shrinkable cable accessories should include special synthesised

polymers which should offer exceptional mechanical strength as well as

protection against harsh environmental conditions. These rugged materials

should be characterised by their light weight, high heat deflection temperatures

and extraordinary versatility.

6.9.4 Design Basis

It is interesting to look at how heat shrinkable termination's and joints meet the

basic requirements of cable accessories discussed above.

6.9.5 Termination's

Stress control is provided in high voltage cable termination's for one primary

purpose; that is to control the stress which exists at a point where the screen or

shield is terminated. If no form of stress control were applied, discharges could

occur and the life of the termination would be limited depending on the stress at

the end of the screen and the discharge resistance of the primary dielectric but

would typically not exceed one year. Some form of stress control is therefore

required at the termination of all screened power cables which have been

designed to operate at 5kV or higher voltage to eliminate discharge activity

during operation.

To relieve these stresses, a heat shrinkable tubing with correct electrical

characteristics is applied from the end of the screen over the cable dielectric for

an appropriate distance. This layer is coupled capacitively to the conductor. This

reduces the electrical field strength along the end of the cable. In other words,

the voltage drop per centimetre is well below the value of the breakdown in air.

It is essential that in heat shrinkable stress control tubing provided has stable

impedance over the typical operating temperature range of a cable in service.

Further, impedance of the material also should remain constant when aged to

100 Degree. C for more than 10,000 hours.

The outer surface of the termination is protected by a non-tracking and erosion

resistant material. It is formulated and tested to be stable under the extremes of

climate and resistance to the effects of the UV radiation in sunlight.

Condensation, rain or pollution will cause leakage currents to flow on the surface

of the termination's. It is essential that materials used do not track. Tests of

varying degrees of severity are available to demonstrate the effectiveness of an

outdoor material. These include the ASTM-D-2303, Inclined Plan Tracking and

Erosion test and Atlas Weather-O-Meter test.

Pre-coating the moulded sheds and gloves with heat activated adhesive / sealant

ensures positive seals between the cable and the termination.

In short: A cable termination must provide:

(a) A stress relieving function.

(b) An outer non-tracking, erosion and weather resistant surface and

(c) Be fully sealed.

1. Stress Relieving Function:

Consider a concentric cylinder electrode configuration as might be expected in a continuously shielded (screened) cable. The variation in the electric field is in the radial direction. The flux lines are orthogonal to the electrode system and the flux lines and lines of equipotential are closer in the region of the conductor. Thus the stress is a function of the geometry of the cable and in practice the insulation thickness is sufficient to maintain the stress at acceptable levels for the dielectric. When a cable is terminated the shield is removed for such a distance that the

electrical breakdown along the interface of the insulation between the conductor

and the shield cannot occur. This causes discontinuity of the linear field

distribution along the axis of the cable. The flux lines make their way to the

nearest earth point and hence the lines of equipotential become 'crowded' near

the end of the shield. This produces a high electrical stress at the end of the

shield, several times the maximum stress within the cable. More ever the stress

is occurring along the insulation / air interface and within the surrounding air

which

will have a lower breakdown strength than the cable insulation. Discharges will

occur and eventually the cable will breakdown.

Raychem termination's employ heat shrinkable tubing's with correct electrical

properties to provide a stress relieving function. The tubing is shrunk onto the

insulation of the cable and the cable screen and is coupled capacitively to the

conductor, and increases in potential as the distance from the screen end (earth)

increases. Clearly an incorrect choice of the tubing's electrical properties will

result in inferior performance. If the impedance of the tubing is too high then the

tubing in the limit acts as an insulator and the potential distribution would be

similar to that of a non stress rekueved system. If the impedance is too low then

the tubing in the limit approaches a conductor and this merely transfers the high

stress point to the high voltage end of the stress grading.

The parameters which give an ideal potential distribution along the Raychem

stress control tubing are:

(I) Volume resistivity > 10 ohms cm

(ii) Permittivity > 20

(iii) An ac impedance at 50 Hz between -1 x 10 8 ohms cm.

In service the electrical properties of the stress control material should remain

constant in spite of the differences in stress which will exist within the sleeve and

the differences in temperature experienced in the termination due both to the

heating effect within the conductor and the temperature of the environment.

The stress control material is applied to the outer surface of the cable dielectric

and is to some extent thermally insulated from the conductor. Impedance of the

stress control tubing should remain constant as a function of both temperature

and time.

A cable system, complete with its joints and termination's is expected to operate

without malfunction for many years. For high voltage cables with polymeric

dielectrics such as cross linked polyethylene (XLPE) or cross line diethylene

propylene elastomer (EPR) this is usually the case, provided that the installation

is discharge free and remains so. If discharges are present, failure can occur

rapidly. It is difficult to predict with any accuracy, the life of cable systems from

laboratory tests, but it is now generally accepted that load cycling is one of the

better

6.9.6 Joint

Heat shrinkable jointing system incorporates cable preparation and installation

techniques for joints identical to those for heat shrinkable termination's. Further,

same basic design is also shared by heat shrinkable joints for different types of

cables, etc., thus setting new standards of efficiency and simplicity.

Heat shrinkable stress control materials prevent any stress concentrations at the

screen termination or the connector in the middle of the joint. A stress relieving,

oil resistant mastic placed around the connector makes both a seal and a smooth

high voltage electrode.

A plastic cable, which may have water in the conductor stands, can thus be

sealed internally as well as externally.

Heat shrinkable insulation tubes remove most of the site variables from jointing

while still providing wide dimensional flexibility.

One sleeve that includes both insulation and an outer void free conductor stands,

can thus be sealed internally as well as externally.

Heat shrinkable insulation tubes remove most of the site variables from jointing

while still providing wide dimensional flexibility.

One sleeve that includes both insulation and an outer void free conductive layer

(co - extruded dual wall tubing) completes the key part of the joint. Various outer

cases may be provided to reinstate the armour and earth continuity of the original

cable.

The entire design is sealed with a robust outer sleeve coated with a heat

activated sealant.

6.9.7 CABLE PREPARATION

Cable with metal tape shield

Table 1

Max. *L *L *L b K

System Indoor Indoor Outdoor

voltage Straight Crossed

Connection Connection

(kV) (mm) (mm) (mm) (mm)

7.2 250 450 450 130 according

12 300 450 650 130 to depth

17.5 350 500 650 130 cable lug

24 450 550 800 180 barrel hole

36 600 800 800 230 + 5 mm

* L = min. Length required

The actual length will be determined by the overall geometry of the equipment.

1. Cut the cable to the required length and remove the over sheath, inner sheath

and armour as per the dimensions shown. (Figure - 1).

Leave enough length to set the cores into the final position. Clean and decrease

the end of the oversheath for about 100 mm.

Note : - The minimum termination length (L) is given in Table 1.

2. Spread the armour wires and insert the support ring. (Figure - 2).

3. Bind the armour wires using the tinned copper binding wire. The armour wires

should make firm contact with the support ring. (Figure - 3).

4. Place the tinned copper braid (smaller one) over each core and solder tack it

to the copper tape screen. (Figure - 4).

5. Bring down the tinned copper braids of individual cores on to the armour wires

/ strips. Place the larger tinned copper braid over the tinned copper braid of

individual cores and bind them at three places with tinned copper wires provided

in the kit. Solder tack the braids on to the armour wires / strips.

Insert the jublee clips over the armour wire and the earth braid.

Tighten the clips till the armour wires make firm contact with the supporting.

(Figure - 5).

6. Apply adequate layers of PVC tape to cover the armour, earth braids, armour

clamps and all sharp edges. (Figure - 6).

7. Place a temporary wire binder around the cores at a distance b from the crutch

of the cable, as shown in the figure. Tear off the tape shield against the wire

binder.

Remove the metal tape shield according to dimension b (see Table 1).

Thoroughly remove the core screen to 20 mm above the metal tape shield cut.

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

material.

Smooth out any irregularities. Clean and degrease the insulation using solvent

provided in the kit. (Figure - 7).

8. Remove the release paper and slide the breakout over the cores.

Pull the breakout as far down the crutch as possible.

Shrink the breakout inter place starting at the centre.

Work first towards the lower end and then shrink the turrets onto the cores.

The numbers in the drawing indicate the shrinking sequence. (Figure - 8).

9. Remove the wire binder from the end of the metal tape shield.

Remove the release paper and wrap the void filling strip (yellow) for 5 mm onto

the metal tape shield continuing over the core screen and 10 mm onto the

insulation.

Stretch the strip to half of its original width to achieve a fine, thin edge onto the

installation.

Smooth out the insulation surface for 50 mm, 140 mm and 260 mm with a thin

film of silicon grease. For 11 kV, 22 kV and 33 kV respectively. (Figure - 9).

10. Place the stress control tubing (black) over the cores and position them 50

mm below the end of the semi-conducting layer cutback.

Shrink down the tubing starting at the bottom and working upwards. (Figure - 10).

11. Remove the release paper from the red tubing. Place the tubing over the

cores with the cores with the sealant coated end downwards.

Push the tubing over the breakout turrets as far as possible and shrink it down

starting at the crutch and working upwards. (Figure - 11).

12. Cut back the insulation according to

K= depth of cable lug barrel hole + 5 mm. Install the cable lugs, by crimping or

any other equivalent methods.

Clean and degrease the insulation and the lugs. (Figure - 12).

13. Remove the release paper and wrap the sealant tape (red) around the barrel

of the cable lug with a small overlap and slight tension. (Figure - 13).

14. Remove the release paper from the sealing boots. Position the sealing boots

so that they cover the cores and connectors equally and shrink them into place.

(Figure - 14).

15. Shrink the skirts into place on the two outer cores at the positions shown in

the drawing.

Set all three cores in their final position so that the minimum core to core

clearance c is maintained.

Shrink the skirt into place on the middle core at positions which maintain the

clearance d shown in the drawing. (Figure - 15).

6.9.8 ADVANTAGES OF HEAT SHRINKABLE SYSTEMS

The advantages of heat shrinkable systems may be briefly summarised as

follows :

- Flexibility of field engineered systems.

- Factory engineered components eliminate installation variability.

- No shelf life or health risks.

- Do not require excessive skill, installation techniques are quickly learned.

- Not dependent on cable dimensions.

- Systems approach uses similar components for termination's and joints.

- Available for paper, polymeric and all cable designs.

- Proven field service.

- Meet specifications requirements of both IEC and IEEE system.

- Accessories available up to 72 kV system voltages.

Location of parts in a heat shrinkable straight through join in shown in Figure C2.

Summary

The following shall be checked / ensured.

a) Identification number tags of the cable for the equipment to which the supply is

fed are provided correctly at both ends of the cable.

b) The tag size is not less than 2 mm thick and 20 mm wide and of enough length

to contain all required details.

c) Cable termination is done with proper crimping lug and use of antioxidant

paste.

d) For cable glands of flameproof design, identification mark on the gland

preferably embossing symbol as per IS should be available, and the required

CMRS certification is verified.

e) The ferruling on all termination's conforms to wiring drawings.

f) Tightness of all termination's. (Confirm the bimetallic washer, if required, is

correctly installed.)

g) Earthing connections and earth continuity are in order.

h) Wherever lugs are used for termination, size of lug matches with cable core

and material of lug is suitable for application.

i) Proper mechanical protection for the cable is available.

j) Pipes, if provided, are sealed at both ends.

k) Bending radius is as per BIS standard.

l) Before back filling cable trench, the straight through joints of High Tension

cables are tested for leakage current.

m) Cable termination's are done as per the manufacturer's instructions.

n) Insulation resistance values between phases and phase to ground (after

termination) are in order.

o) Cable near the termination are supported to relieve the strain on the terminals.

7.0 TESTING OF CABLE INSTALLATION

7.1 TESTING AND ELECTRICAL MEASUREMENTS OF CABLE

INSTALLATIONS (IS 1255 - 1983).

Insulation Resistance Test on Newly Installed Cables Before Jointing -

All new cables should be tested for insulation resistance before jointing. After

satisfactory results are obtained cable jointing and termination work should

commence. The test is meant to reveal gross insulation fault(s). A fairly low

insulation resistance reading compared to the values obtained at factory should

not be a cause of worry since the insulation resistance varies greatly with

parameters such as length and temperature. This is particularly more

pronounced in the case of PVC cables. The voltage rating of the insulation

resistance tester for cables of different voltage grades should be chosen from the

following table :

Voltage Grade of Cable Voltage Rating of IR Tester

1.1 kV 500 V

3.3 kV 1000 V

6.6 kV 1000 V

11 kV 1000 V

22 kV 2.5 kV (see Note)

33 kV 2.5 kV (see Note)

Note - For long feeders, motorised insulation resistance tester should be used.

More accurate insulation resistance values can be measured only by a portable

resistance measuring bridge.

7.2 TEST RESULT OF COMPLETED CABLE-INSTALLATION

The test of completed installation may be measured and entered into record book

for comparison purpose during service life of cable installation and during fault

location.

7.2.1 Insulation Resistance

Insulation resistance is measured by a suitable bridge. In non-screened cables,

the insulation resistance of each core is measured against all the other cores and

armour / metal sheath connected to earth. With screened construction the

insulation resistance of each core is measured against all the other cores and the

metal screen connected to earth.

7.2.2 Conductor Resistance (DC)

The resistance of conductor is measured by a suitable bridge. For this purpose

conductors at other end are looped together with connecting bond of at least

same effective electrical cross-section as conductor. The contact resistance is

kept to a minimum by proper clamped or bolted connections. With properly

installed and jointed cables, values thus measured and corrected to 200 C, are in

general agreement with values given in test certificates.

The measured loop resistance is converted to ohms per km per conductor as:

R

Rt = ------

2L

Where

R = measured loop resistance in ohms at temperature, t0 C;

Rt = measured resistance per conductor at t0 C in ohms; and

L = length of cable (not the loop) in km.

The ambient temperature at the time of measurement to be recorded and the

conductor resistance to be corrected to 200 C by the following formula :

Rt

R20 = ----------------- ohm / km at 200 C

(I + ) (t - 20)

Where

R20 = conductor dc resistance at 200 C

t = ambient temperature during measurement in 0C, and

= temperature coefficient of resistance

(3.93 x 10_ ohms / 0C for aluminium).

7.2.3 Capacitance

For unscreened cables, capacitance is measured for one conductor against

others and metal sheath / armour connected to earth. Case of screened cable it

is measured between conductor and screen. Capacitance bridge is used for this

purpose. This measurement may carried in case of cables above 11 kV;

alternatively values given in the certificate are considered sufficient.

7.2.4 High Voltage Test

Cables after jointing and terminating are subjected to high voltage test. The

recommended values of test voltages are as under. The leakage current shall

also be measured and ordered for future reference. TABLE 6 DC TEST VOLTAGES AFTER INSTALLATION (BEFORE COMMISSIONING) RATED VOLTAGE TEST VOLTAGE BETWEEN DURATION OF CABLE

Any Conductor and Conductor to

Metallic Sheath / Conductor (For

Screen / Armour Un-screened Cables)

Uo / U

kV kV kV Minutes

(1) (2) (3) (4)

0.65 / 1.1 3 3

1.9 / 3.3 5 9

3.3 / 3.3 9 9

1. / 6.6 10.5 18 2. 6.6 / 6.6 18 18 5

6.35 / 11 18 30 11 / 11 30 30

12.7 / 22 37.5 -

19 / 33 60 -

Generally DC test should be preferred as test equipment required is compact,

easily portable and power requirements are low.

The cable cores must be discharged on completion of DC high voltage test and

cable should be kept earthed until it is put into service.

DC test voltage for old cables is 1.5 times rated voltage or depending on the age

of cables, repair work or nature of jointing work carried out, etc. In any case, the

test voltage should not be less than the rated voltage. Test voltage in these

cases should be determined by the engineer-in-charge of the work.

It may be noted that frequent high voltage tests on cable installations should not

be carried out. This test should be carried only when essential. During the high

voltage test, all other electrical equipment related to the cable installation, such

as switches, instrument transformers, bus bars, etc., must be earthed and

adequate clearance should be maintained from the other equipment and

framework to prevent flash over.

In each test, the metallic sheath / screen / armour should be connected, to earth.

7.3 CABLE INSTALLATION PLAN

On completion of laying, terminating and jointing of the cables, a plan should be

prepared, which should contain the following details of the installation.

a) Type of cables, cross-section area, rated voltage. Details of construction,

cable number and drum number;

b) Year and month of laying;

c) Actual length between joint-to-joint or ends;

d) Location of cables and joints in relation to certain fixed reference points, for

example, buildings, hydrant, boundary stones, etc.;

e) Name of the jointer who carried the jointing work;

f) Date of making joint; and

g) Results of original electrical measurements and testing on cable installation.

All subsequent changes in the cable plan should also be entered.

General Construction

3.1 CONDUCTORS

The conductors of power cables are normally made from electrical purity

aluminium, and those of control cables are of annealed high conductivity copper.

However, copper conductor power cable can also be supplied. All conductors

conform to IS:8130-1984.

A point to be noted here is that for conductors for fixed installation (Class 1 and

Class 2), IS:8130 - 1984 specifies only the minimum number of wires and the

maximum d.c. resistance of the conductor at 200 C for a particular cross-section;

the diameter of the wire is not specified.

Normally, aluminium conductors up to size 10 sq. mm. are solid circular in cross

section, and sizes above 10 sq. mm. are stranded. In case of single core and

twin core cables up to 50 sq. mm., they are circular in cross section while for 3

core and 4 core cables, conductors of cross section 25 sq. mm. and above are

normally sector shaped.

11 kV PVC insulated cables are designed with cores having round compacted

stranded conductors.

3.2 INSULATION

The conductors are insulated with the high quality PVC base compound. Cables

with Heat Resisting Insulation are also available for maximum operating

conductor temperature of 850 C for 1.1 kV grade cables.

The insulation and outer sheath compounds shall be conforming to IS:5831 -

1984 as per the requirement of IS:1554 (Part 1 and 2) of 1988.

3.3 CORE IDENTIFICATION

1. Colour Scheme :

Cores are identified by colour scheme of PVC insulation. The following colour scheme is normally adopted : 1 core - Red, Black, Yellow, Blue or Natural (non-pigmented)

2 core - Red and Black

3 core - Red, Yellow, and Blue

4 core - Red, Yellow, Blue and Black (also 3-1/2 core) (reduced neutral core is

black)

5 core - Red, Yellow, Blue, Black and Grey

For cables having more than 5 cores :

Two adjacent cores (counting and directional) in each layer are coloured blue

and yellow respectively and the remaining cores are grey.

Alternatively cores with number printing can be offered. For 11 kV PVC/XLPC

cables, cores shall be identified by means of number printing tape.

1. Inner Sheath :

For all cables having two or more cores, a common covering (inner sheath) is applied over the laid up cores either by extruded sheath of un-vulcanised rubber/PVC compound or wrapping of plastic or proofed tapes. Single core cables do not have inner sheath.

1. Armouring :

For multi-core cables, armouring is applied over the inner sheath. In case of cables where fictitious diameter over the inner sheath does not exceed 13 mm., the armour consists of galvanised round steel wires; above this size, normally the armour is of galvanised formed steel wires.

Armouring of PVC mining cables consists of galvanised round / formed steel

wires, but wherever necessary, a few tinned copper wires / strips are also

included to meet the resistance requirements of armouring for mining cables as

specified in IS:1554 (Part 1 & 2) 1988.

For single core armoured cables, non magnetic armouring is provided.

1. Outer Sheath :

Outer sheath of PVC is extruded over the armouring. In case of multi-core unarmoured cables, over the inner sheath, whereas, in case of unarmoured single-core cables, it is extruded over the insulation. This is always black in colour for best resistance to outdoor exposure. Any other colour can be available on request. The manufacturer's name and trademark along with the year / year code of

manufacture are embossed on the outer sheath; additionally in the case of LT

cables, the word 'ELECTRIC' and in the case of HT cables, the voltage grade is

also embossed. In case of LT cables with Heat Resisting Insulation, the word

"HR 85" is also embossed. In the case of mining cables, the word MINING is

added in the embossing script. The embossing script repeats in such a way that

every meter of the cable be are the same script.

3.4 TYPE DESIGNATION

Type designated of PVC Cables is based on the following alphabetical

nomenclature.

A - Aluminium conductors - when first letter of type designation. When type

designation does not contain 'A' in the beginning, then the cable is with copper

conductors.

Y - When at first or second place in type designation, it stands for PVC insulation.

CE - Individual core screening.

W - Round steel wire armouring.

F - Formed steel wire armouring.

Gb - Steel tape counter helix.

Y - When last in type designation, it stands for PVC outer sheath.

WW - Steel double round wire armour.

FF - Steel double formed wire armour.

3.5 CONDUCTOR TYPES

re - Circular solid conductor.

rm - Circular, stranded conductor (non - compacted)

rm/v - Circular, stranded compacted conductor.

sm - Sector shaped, stranded conductor.

Number of cores, conductor cross section, voltage grade are written in the usual

manner.

Example :

AYFY 3 x 400 s. m. 650/1100 V

Aluminium conductor, PVC insulated, formed steel wire armoured, PVC overall

sheathed 3 -core cable, conductor size 400 sq. mm stranded sector shaped

650/1100 V grade.

YY 37 x 1.5 re 650/1100 V

Copper conductor, PVC insulated and PVC sheathed (un-armoured) 37 core

cable, conductor size 1.5 sq. mm. solid round 650/1100 V grade.

3.6 FIELD OF APPLICATION

3.6.1 Control Cables 650/1100V as per IS:1554 (Part-1) - 1988

Type and Construction

Type YY

Un-armoured 1.5 and 2.5 sq. mm. copper conductors up to 61 cores.

Application

These cables are suitable for control purposes or measuring circuits, in

generating stations, sub-stations, industrial installations etc., as well as for

railway signalling. They can be installed indoors or outdoors, in air or in cable

ducts.

Type YWY/YFY

Armoured 1.5 and 2.5 sq. mm. copper conductors up to 61 cores.

Application

These cables are suitable for control purposes or measuring circuits in

generating stations, sub-stations, industrial installations etc., as well as for

railway signalling. On account of armouring, the cables can withstand rough

installations and operation conditions and tensile stresses. These can be laid in

water or buried direct in the ground, even on steep slopes. They can also be

installed indoors or outdoors, in air or in cable ducts.

Power Cables 650/1100V As Per IS:1554 (Part - 1) - 1988

Type AYY

Unarmoured Single-core with Aluminium conductors up to 1000 sq. mm.

Application

Light weight and small permissible bending radii make single core Cables very

easy to install. These cables are therefore particularly useful in power stations

and sub-stations where a comparatively large number of cables in short lengths

are to be used.

3.6.2 Power Cables 650/1100 V As Per IS:1554 (Part - 1) - 1988

Type AYY

Unarmoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These power cables are suitable for use in generating stations, sub-stations,

house service connections, street lighting, industrial installations, building wiring

etc. They can be installed indoors or outdoors, in air or in cable ducts.

Type AYWY/AYFY

Armoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These power cables are suitable for use in generating stations, sub-stations,

distribution systems, house service connections, street lighting, industrial

installation etc. On account of the armouring, the cables can withstand rough

installation and operating conditions and tensile stresses, and can be laid in

water or buried direct in the ground, even on steep slopes. They can also be

installed indoors and outdoors, in air or in cable ducts.

3.6.3 Power Cables 1.9/3.3 kV* to 6.35/11 kV. As Per IS:1554 (Part - 2) - 1988

Type AYFY 3.8/6.6 kV (E)

Armoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These cables can be used both indoors and outdoors, and can be directly laid in

the ground or under water. These are suitable for power stations, switching

stations, industrial plants and as feeders in electricity supply undertakings.

Type AYCEFY 11 kV (E)

Armoured Screened Multi-core cables with Aluminium conductors up to 500 sq.

mm.*

Application

11 kV cables are suitable for all types of industrial applications and particularly

for hydro and thermal power stations and sub-stations. They are ideally suited for

chemical and fertiliser industries on account of their better corrosion resistance or

in heavy industries where severe load fluctuations occur and for systems where

there are frequent over voltages.

Cables of 6.35 / 11 kV grade (Earthed system) can be used on 6.6 / 6.6 kV

(Unearthed system) also.

In a distribution system, if the feeding Transformer neutral is not grounded,

during single phase to ground Fault condition, Transformer neutral will shift and

phase to ground voltages may reach equal to phase to phase voltages. Hence in

unearthed system, insulation level of cables will be higher compared to earthed

system.

3.6.4 Mining Cables 650/1100 V As Per IS: 1554 (Part - 1) - 1988 And 1.9/3.3

kV; 3.8/6.6 kV(E) And 6.35/11 kV As Per IS:1554 (Part - 2) - 1988.

Type YFFGbY

Armoured Multi-core cables, with Copper conductors up to 500 sq. mm.

Application

Mining cables are light in weight, and hence easier to handle and install. It is

provided with an armour consisting of double layer of formed galvanised steel

wire which affords complete protection from falling rocks, wagon tipping or any

other mechanical impact, and makes the cable strong enough to withstand

tensile for many times its own weight especially when used as a shaft cable.

Besides, the incorporation of a counter - helix over the top armour layer prevents

bird caging. In addition, to ensure an adequate path for earth fault currents, the

armour resistance is so designed that it never exceeds that of the main

conductor by more than 33%. All the above features make mining cables ideally

suited for use in all types of mines.(During laying of Power cables, if the cables

are not handled as per the laying practices armour of the cable may buidge. This

building of the armour will look like cage of bird and hence the name of bird

caging"

3.6.5 Flame Retardant Low Smoke PVC Cables (FRLS)

Until recently, flame retardance was never a major consideration when choosing

a cable. However, in the recent past, the growing awareness of hazards due to

fire incidence in power plants, high rise buildings, cable galleries etc., has

resulted in the development of FRLS Cables.

As compared to normal PVC cables, TROPODURE (FRLS) cables offer

improved performance characteristics in respect of the following:

1. Flame Retardance

2. Low Smoke Emission

3. Low Acid Gas Emission

In order to meet the above, special material is used for the outer sheath. The

design, construction and testing of (FRLS) cables is as per IS:1554 Part-1 and 2.

Over and above the standard tests, FRLS cables and its components are

subjected to a series of special tests to fulfill the need of the improved fire

performance characteristics.

1. Physical Testing

2. Smoke Density Measurement Tests as per ASTM D-2843

3. Oxygen Index Tests as per ASTM D-2863

4. HCL Liberation as per IEC-754

1. Smoke Generation Test:

Optical method is most practised to measure the amount of smoke generated

while burning the sample. A fixed dimension sample of material is put to flame of

specified intensity for 4 minutes and the percentage obstruction on an optical

path is measured at 15 seconds interval. A graphical plot of such measurements

evaluates the average smoke density. This area integration method is only the

quantitative indication of smoke generation by the PVC compound.

2. Oxygen & Temperature Index Test:

Oxygen index is the measurement of oxygen content at which the vertically held

sample when ignited ceases to burn off its own within 3 minutes. This is a relative

measure of combustion resistance of materials in the context of atmospheric

oxygen.

Temperature index is the temperature at which the vertically held sample ceases

to burn off on its own within 3 minutes at an Oxygen concentration of 21% i.e.

atmospheric air.

3. HCL Gas Emission Test:

All PVC Compounds when decomposed due to fire emit gaseous fumes which

are basically corrosive in nature. This is entirely due to the ingredients used in

the formulations. These fumes, if containing higher amounts of corrosive gas i.e.

Hydrochloric Acid Gas, may damage various instruments and equipment's in the

vin-city of fire. More and more stress is being given these days to control this

content in reasonable limits. A sample of cable compound is subjected to above

test and the decomposed gases are collected in alkaline solutions. Such alkaline

medium is then analysed to estimate the HCL evolved.

Apart from these tests on materials used on cables, following tests on finished

cables are carried out :

1. Test on electrical cables under the conditions as per IEC 332-1

2. Flammability testing - Chimney test for Class F3 as per Swedish Standards SS

424 1475

3. Flame tests as per IEEE Standard 383, 1974.

Applications

FRLS cables are ideal for use in high rise buildings, data processing centres,

hospitals, theaters, hotels, schools, warehouses, industrial complexes, power

stations, underground railways, oil platforms and areas where safety of personnel

or protection of equipment is necessary.

3.6.6 XLPE Cables (Cross Linked Polyethylene)

3.6.6.1 Properties and Advantages :

The excellent thermal properties of XLPE cables permit maximum continuous

conductor operating temperature of 900 C and short circuit temperature of 2500

C. Moreover, it has very low dielectric loss which does not vary much over the

entire operating temperature range.

These characteristics, along with the low dielectric constant make XLPE cables

particularly suitable for high voltage applications. Given below are additional

outstanding features.

High Continuous Current Rating :

Its ability to withstand higher operating temperature of 900 C enables much

higher current ratings than those of PVC or PILC cables.

High Short Circuit Rating :

Maximum allowable conductor temperature during short circuit of 2500 C is

considerably higher than for PVC or PILC cables resulting in greater short circuit

withstand capacity.

High Emergency Load Capacity :

XLPE cables can be operated even at 1300 C during emergency, therefore in

systems where cables are installed in parallel, failure of one of two cables will not

bring down the system capacity because the remaining cables can carry the

additional load even for longer duration until repairs / replacements are carried

out.

Low Dielectric Losses :

XLPE cables have low dielectric loss angle. The dielectric losses are

quadratically dependent on the voltage. Moreover, these losses occur

continuously in every charged cable whether it carries load or not. Hence use of

XLPE cable at higher voltages would result in considerable saving in costs.

Charging Currents :

The charging currents are considerably lower permitting close setting of

protection relays.

Easy Laying and Installation :

Low weight and small bending radii make laying and installation of cable very

easy. The cable requires less supports due to low weight.

High Safety :

High safety against mechanical damage and vibrations.

3.6.6.2 Applications :

Because of the excellent mechanical and electrical properties XLPE cables are

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

ideally suited for chemical and fertiliser industries where cables are exposed to

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

corrosion or in heavy industries where severe load fluctuations occur and for

systems where there are frequent over voltages. Cables can also be used at

higher ambient temperature on account of their higher operating temperature.

There excellent installation properties permit the cable to be used even under

most difficult cable routing conditions and also in cramped conditions e.g. City

distribution network. Single core cables due to their excellent installation

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

advantage.

3.6.6.3 Construction :

XLPE cables are manufactured and tested in accordance with IS : 7098 (Part II) -

1985. Its salient constructional features are as under :

Conductor :

The conductors made from electrical purity aluminium wires, are stranded

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

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

1984.

Cables with copper conductor can also be offered.

Insulation :

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

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

linked by continuous vulcanisation process.

Shielding :

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

and insulation shielding. Both conductor and insulation shielding consists of

extruded semi conducting compound.

Additionally, insulation is provided with semi-conducting tape and non-magnetic

metallic tape screen over the extruded insulation.

Conductor shielding XLPE insulation and insulation shielding are all extruded in

one operation by a special process. This process ensures perfect bonding of

inner and outer shielding with insulation.

Inner Sheath

(Common Covering) :

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

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

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

sheath can also be provided.

Armouring :

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

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

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

armour consisting of hard drawn flat or round aluminium wires.

Outer Sheath :

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

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

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

metallic part of insulation screening in case of unarmoured single core cables.

This is always black in colour for best resistance to outdoor exposure. The outer

sheath is embossed with the voltage grade and the year of manufacture. The

embossing repeats every 300/350 mm along the length of the cable.

XLPE cables are manufactured under advanced manufacturing and testing

facilities. The cables are type tested and routine tested in accordance with IS

7098 (Part II) - 1985.

The following tests are carried out as on every length of cable manufactured :

a) Conductor resistance test.

b) Partial discharge test.

c) High voltage test.

d) Insulation resistance.

e) Bending test.

f) Heating cycle test.

g) Dielectric power factor test.

h) Impulse withstand test.

3.6.6.4 Test Voltages :

The following test voltage is applied between conductor and screen / armour (IS

1255 - 1983) :

_________________________________________________________

Voltage rating of Cables Test voltage

3.8 / 6.6 kV (E) 12 kV (rms) for 5 minutes

6.35 / 11 kV (E) 17 kV (rms) for 5 minutes

11 / 11 kV (UE) 28 kV (rms) for 5 minutes

12.7 / 22 kV (E) 32 kV (rms) for 5 minutes

19 / 33 kV (E) 48 kV (rms) for 5 minutes

_________________________________________________________

A) Conductor Resistance Measurement :

This is generally taken on whole cable drum. During test, current flow is

proportional to electrical resistance of conductor, heat loss in cable in turn

depends on this resistance. Resistance of the conductor is of utmost importance

from system design point of view, which is measured during this test.

In order to achieve consistency in quality, in addition to above tests, rigorous

quality control measures are effected at every stage of production. Accordingly,

every batch of raw materials and in process cables are tested to check for their

physical and electrical properties.

To carry out testing and quality control programme, the manufacturing plant has

physical, chemical and electrical laboratories which are recognised by the

Department of Science & Technology, Government of India. Our R & D facility is

recognised as an in-house R & D unit by the Department of Science & Industrial

Research, Ministry of Science & Technology, Government of India.

Besides standard types, these laboratories also possess special equipment for

precision testing and quality control particularly for XLPE cable such as Insertion

Tensile Testing Machine, Melt Index Tester, Differential Scanning Calorimeter,

Infra-red spectrograph, Oscillating Disc Rheo meter, Micro tomes, Blown File

Extruder.

Partial Discharge Detection in Finished Cables :

This is a modern PD detection equipment where the discharge magnitude can be

directly read on a calibrated meter. The detector is associated with 50 kV noise-

free transformer, discharge-free transformer, discharge-free capacitor and

suitable matching units. Besides, specially designed corona free terminators are

used for testing purposes.

Special Features of Partial Discharge Detector Laboratory

The presence of partial discharge of even minute magnitude is not desirable in

XPLE cables and hence a highly sensitive detector has been employed with the

help of which discharges as low as 5 pc/cm can be detected. The noise from

supply source is prevented by providing isolating transformer and low pass filters.

The complete equipment is housed in a specially shielded room to prevent

external noise that would otherwise interfere with the measurements.

B) Partial Discharge Test :

This is the test for insulation and is carried out for the partial discharge occurring

in screened electric cables due to voids which remain unnoticed in the normal

high voltage tests and could be harmful to the life of insulation. Test is carried out

as per the relevant standards. Partial discharge observed should be with

permissible limits at 1.5 time the test voltage. Not only the insulation but the

semiconductor layer also should have homogeneous coating surrounding,

otherwise small voids and deep impression in that may create larger partial

discharges. Similarly, if the metallic foil over semi-conducting layer is not tight

enough then it gives use to higher level of discharges.

C) High Voltage Test :

The insulation material in cable is used to isolate the conductors from one

another and from ground and also provides mechanical strength. The insulation

has to withstand the voltage imposed on it in service. This is evaluated by

applying higher voltage stress to the insulation for a short duration. The cable

has to withstand the applied voltage without breakdown for specified period.

Thus high voltage test confirms the specified voltage rating of cable.

D) Insulation Resistance :

Any insulating material in cable should naturally have maximum resistance in

order to establish its dielectric properties. This insulation resistance is measured

between the phases or phase and ground. Decrease in the insulation resistance

indicates impurity and imperfection in cable insulation. This test evaluates quality

of insulation.

E) Bending Test :

This test condition simulate bending stresses for the cables which are always

there during handling and installation of cables. It is carried out as per the

relevant standard. After the test the cable sample should be thoroughly examined

for physical damage or cracks on the sheath. Immediately after this again partial

discharge test is to be carried out. During manufacturing if overlapping of metallic

screen is not tight and smooth, then in bending test it may open out. Partial

discharge test after this of course establishes the satisfactory performance of

cable during bending operation, without any physical damage.

F) Heating Cycle Test :

In actual service cables undergo cyclic heating and cooling resulting in expansion

and contraction which may cause either mechanical distortion or degradation of

screen which may lead to failure of cable by initiation of high dielectric loss of

higher partial discharges. Hence after this test, the sample is to be subjected to

dielectric factor test and partial discharge test. As it is subjected to cyclic heating

and cooling with specified temperature limit, the performance of cable under

actual service conditions is tested. Measuring of partial discharge level and

dielectric power factor after the test tells us about mechanical displacement of

metallic screen and homogeneity of insulation.

G) Dielectric Power Factor Test :

Dielectric power factor of any insulating material should be as small as possible

in order to have less heating of dielectric. If the cable insulation contains

impurities and voids then this value may be higher. This test should be done as a

function of voltage. Then the sample is heated by passing current up to a desired

value and again this tan delta measurement should be done. These should be

within limits which ensures more purified insulating material.

H) Impulse with Stand Test :

Because of the nearby lightning strokes, the cable insulating material in H.V.

cables may be subjected to transient over voltages. So for insulation design and

manufacturing process of cable, the withstand ability with transient over voltages

is established by this test. When the specified impulse voltage is applied no

breakdown of insulation should occur and the dielectric material must be able to

withstand transient over voltages ensuring reliability of cable insulation.

COMPARISON OF XLPE CABLES WITH OTHER TYPES OF CABLES PROPERTIES PVC POLYETHYLENE PAPER XLPE

1. Normal operation 70 70 65-80 90

Temperature 0C

2. Permitted overload - - - 130

Temperature 0C

3. Short circuit 130 120 160 250

Temperature 0C

4. Chemical resistance Very Good Fair - Very Good

5. Moisture resistance Very Good Very Good Poor Very Good

6. Thermal resistivity 600 350 600 350 0C CM / W

7. Fire resistance Excellent Poor Poor Fair

* These temperatures are applicable to cables designed with suitable over

sheaths etc. GENERAL CONSTRUCTION OF CABLES

3.1 CONDUCTORS

The conductors of power cables are normally made from electrical purity

aluminium, and those of control cables are of annealed high conductivity copper.

However, copper conductor power cable can also be supplied. All conductors

conform to IS:8130-1984.

A point to be noted here is that for conductors for fixed installation (Class 1 and

Class 2), IS:8130 - 1984 specifies only the minimum number of wires and the

maximum d.c. resistance of the conductor at 200 C for a particular cross-section;

the diameter of the wire is not specified.

Normally, aluminium conductors up to size 10 sq. mm. are solid circular in cross

section, and sizes above 10 sq. mm. are stranded. In case of single core and

twin core cables up to 50 sq. mm., they are circular in cross section while for 3

core and 4 core cables, conductors of cross section 25 sq. mm. and above are

normally sector shaped.

11 kV PVC insulated cables are designed with cores having round compacted

stranded conductors.

3.2 INSULATION

The conductors are insulated with the high quality PVC base compound. Cables

with Heat Resisting Insulation are also available for maximum operating

conductor temperature of 850 C for 1.1 kV grade cables.

The insulation and outer sheath compounds shall be conforming to IS:5831 -

1984 as per the requirement of IS:1554 (Part 1 and 2) of 1988.

3.3 CORE IDENTIFICATION

1. Colour Scheme :

Cores are identified by colour scheme of PVC insulation. The following colour scheme is normally adopted : 1 core - Red, Black, Yellow, Blue or Natural (non-pigmented)

2 core - Red and Black

3 core - Red, Yellow, and Blue

4 core - Red, Yellow, Blue and Black (also 3-1/2 core) (reduced neutral core is

black)

5 core - Red, Yellow, Blue, Black and Grey

For cables having more than 5 cores :

Two adjacent cores (counting and directional) in each layer are coloured blue

and yellow respectively and the remaining cores are grey.

Alternatively cores with number printing can be offered. For 11 kV PVC/XLPC

cables, cores shall be identified by means of number printing tape.

1. Inner Sheath :

For all cables having two or more cores, a common covering (inner sheath) is applied over the laid up cores either by extruded sheath of un-vulcanised rubber/PVC compound or wrapping of plastic or proofed tapes. Single core cables do not have inner sheath.

1. Armouring :

For multi-core cables, armouring is applied over the inner sheath. In case of cables where fictitious diameter over the inner sheath does not exceed 13 mm., the armour consists of

galvanised round steel wires; above this size, normally the armour is of galvanised formed steel wires. Armouring of PVC mining cables consists of galvanised round / formed steel

wires, but wherever necessary, a few tinned copper wires / strips are also

included to meet the resistance requirements of armouring for mining cables as

specified in IS:1554 (Part 1 & 2) 1988.

For single core armoured cables, non magnetic armouring is provided.

1. Outer Sheath :

Outer sheath of PVC is extruded over the armouring. In case of multi-core unarmoured cables, over the inner sheath, whereas, in case of unarmoured single-core cables, it is extruded over the insulation. This is always black in colour for best resistance to outdoor exposure. Any other colour can be available on request. The manufacturer's name and trademark along with the year / year code of

manufacture are embossed on the outer sheath; additionally in the case of LT

cables, the word 'ELECTRIC' and in the case of HT cables, the voltage grade is

also embossed. In case of LT cables with Heat Resisting Insulation, the word

"HR 85" is also embossed. In the case of mining cables, the word MINING is

added in the embossing script. The embossing script repeats in such a way that

every meter of the cable be are the same script.

3.4 TYPE DESIGNATION

Type designated of PVC Cables is based on the following alphabetical

nomenclature.

A - Aluminium conductors - when first letter of type designation. When type

designation does not contain 'A' in the beginning, then the cable is with copper

conductors.

Y - When at first or second place in type designation, it stands for PVC insulation.

CE - Individual core screening.

W - Round steel wire armouring.

F - Formed steel wire armouring.

Gb - Steel tape counter helix.

Y - When last in type designation, it stands for PVC outer sheath.

WW - Steel double round wire armour.

FF - Steel double formed wire armour.

3.5 CONDUCTOR TYPES

re - Circular solid conductor.

rm - Circular, stranded conductor (non - compacted)

rm/v - Circular, stranded compacted conductor.

sm - Sector shaped, stranded conductor.

Number of cores, conductor cross section, voltage grade are written in the usual

manner.

Example :

AYFY 3 x 400 s. m. 650/1100 V

Aluminium conductor, PVC insulated, formed steel wire armoured, PVC overall

sheathed 3 -core cable, conductor size 400 sq. mm stranded sector shaped

650/1100 V grade.

YY 37 x 1.5 re 650/1100 V

Copper conductor, PVC insulated and PVC sheathed (un-armoured) 37 core

cable, conductor size 1.5 sq. mm. solid round 650/1100 V grade.

3.6 FIELD OF APPLICATION

3.6.1 Control Cables 650/1100V as per IS:1554 (Part-1) - 1988

Type and Construction

Type YY

Un-armoured 1.5 and 2.5 sq. mm. copper conductors up to 61 cores.

Application

These cables are suitable for control purposes or measuring circuits, in

generating stations, sub-stations, industrial installations etc., as well as for

railway signalling. They can be installed indoors or outdoors, in air or in cable

ducts.

Type YWY/YFY

Armoured 1.5 and 2.5 sq. mm. copper conductors up to 61 cores.

Application

These cables are suitable for control purposes or measuring circuits in

generating stations, sub-stations, industrial installations etc., as well as for

railway signalling. On account of armouring, the cables can withstand rough

installations and operation conditions and tensile stresses. These can be laid in

water or buried direct in the ground, even on steep slopes. They can also be

installed indoors or outdoors, in air or in cable ducts.

Power Cables 650/1100V As Per IS:1554 (Part - 1) - 1988

Type AYY

Unarmoured Single-core with Aluminium conductors up to 1000 sq. mm.

Application

Light weight and small permissible bending radii make single core Cables very

easy to install. These cables are therefore particularly useful in power stations

and sub-stations where a comparatively large number of cables in short lengths

are to be used.

3.6.2 Power Cables 650/1100 V As Per IS:1554 (Part - 1) - 1988

Type AYY

Unarmoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These power cables are suitable for use in generating stations, sub-stations,

house service connections, street lighting, industrial installations, building wiring

etc. They can be installed indoors or outdoors, in air or in cable ducts.

Type AYWY/AYFY

Armoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These power cables are suitable for use in generating stations, sub-stations,

distribution systems, house service connections, street lighting, industrial

installation etc. On account of the armouring, the cables can withstand rough

installation and operating conditions and tensile stresses, and can be laid in

water or buried direct in the ground, even on steep slopes. They can also be

installed indoors and outdoors, in air or in cable ducts.

3.6.3 Power Cables 1.9/3.3 kV* to 6.35/11 kV. As Per IS:1554 (Part - 2) - 1988

Type AYFY 3.8/6.6 kV (E)

Armoured Multi-core cables with Aluminium conductors up to 500 sq. mm.

Application

These cables can be used both indoors and outdoors, and can be directly laid in

the ground or under water. These are suitable for power stations, switching

stations, industrial plants and as feeders in electricity supply undertakings.

Type AYCEFY 11 kV (E)

Armoured Screened Multi-core cables with Aluminium conductors up to 500 sq.

mm.*

Application

11 kV cables are suitable for all types of industrial applications and particularly

for hydro and thermal power stations and sub-stations. They are ideally suited for

chemical and fertiliser industries on account of their better corrosion resistance or

in heavy industries where severe load fluctuations occur and for systems where

there are frequent over voltages.

Cables of 6.35 / 11 kV grade (Earthed system) can be used on 6.6 / 6.6 kV

(Unearthed system) also.

In a distribution system, if the feeding Transformer neutral is not grounded,

during single phase to ground Fault condition, Transformer neutral will shift and

phase to ground voltages may reach equal to phase to phase voltages. Hence in

unearthed system, insulation level of cables will be higher compared to earthed

system.

3.6.4 Mining Cables 650/1100 V As Per IS: 1554 (Part - 1) - 1988 And 1.9/3.3

kV; 3.8/6.6 kV(E) And 6.35/11 kV As Per IS:1554 (Part - 2) - 1988.

Type YFFGbY

Armoured Multi-core cables, with Copper conductors up to 500 sq. mm.

Application

Mining cables are light in weight, and hence easier to handle and install. It is

provided with an armour consisting of double layer of formed galvanised steel

wire which affords complete protection from falling rocks, wagon tipping or any

other mechanical impact, and makes the cable strong enough to withstand

tensile for many times its own weight especially when used as a shaft cable.

Besides, the incorporation of a counter - helix over the top armour layer prevents

bird caging. In addition, to ensure an adequate path for earth fault currents, the

armour resistance is so designed that it never exceeds that of the main

conductor by more than 33%. All the above features make mining cables ideally

suited for use in all types of mines.(During laying of Power cables, if the cables

are not handled as per the laying practices armour of the cable may buidge. This

building of the armour will look like cage of bird and hence the name of bird

caging"

3.6.5 Flame Retardant Low Smoke PVC Cables (FRLS)

Until recently, flame retardance was never a major consideration when choosing

a cable. However, in the recent past, the growing awareness of hazards due to

fire incidence in power plants, high rise buildings, cable galleries etc., has

resulted in the development of FRLS Cables.

As compared to normal PVC cables, TROPODURE (FRLS) cables offer

improved performance characteristics in respect of the following:

1. Flame Retardance

2. Low Smoke Emission

3. Low Acid Gas Emission

In order to meet the above, special material is used for the outer sheath. The

design, construction and testing of (FRLS) cables is as per IS:1554 Part-1 and 2.

Over and above the standard tests, FRLS cables and its components are

subjected to a series of special tests to fulfill the need of the improved fire

performance characteristics.

1. Physical Testing

2. Smoke Density Measurement Tests as per ASTM D-2843

3. Oxygen Index Tests as per ASTM D-2863

4. HCL Liberation as per IEC-754

1. Smoke Generation Test:

Optical method is most practised to measure the amount of smoke generated

while burning the sample. A fixed dimension sample of material is put to flame of

specified intensity for 4 minutes and the percentage obstruction on an optical

path is measured at 15 seconds interval. A graphical plot of such measurements

evaluates the average smoke density. This area integration method is only the

quantitative indication of smoke generation by the PVC compound.

2. Oxygen & Temperature Index Test:

Oxygen index is the measurement of oxygen content at which the vertically held

sample when ignited ceases to burn off its own within 3 minutes. This is a relative

measure of combustion resistance of materials in the context of atmospheric

oxygen.

Temperature index is the temperature at which the vertically held sample ceases

to burn off on its own within 3 minutes at an Oxygen concentration of 21% i.e.

atmospheric air.

3. HCL Gas Emission Test:

All PVC Compounds when decomposed due to fire emit gaseous fumes which

are basically corrosive in nature. This is entirely due to the ingredients used in

the formulations. These fumes, if containing higher amounts of corrosive gas i.e.

Hydrochloric Acid Gas, may damage various instruments and equipment's in the

vin-city of fire. More and more stress is being given these days to control this

content in reasonable limits. A sample of cable compound is subjected to above

test and the decomposed gases are collected in alkaline solutions. Such alkaline

medium is then analysed to estimate the HCL evolved.

Apart from these tests on materials used on cables, following tests on finished

cables are carried out :

1. Test on electrical cables under the conditions as per IEC 332-1

2. Flammability testing - Chimney test for Class F3 as per Swedish Standards SS

424 1475

3. Flame tests as per IEEE Standard 383, 1974.

Applications

FRLS cables are ideal for use in high rise buildings, data processing centres,

hospitals, theaters, hotels, schools, warehouses, industrial complexes, power

stations, underground railways, oil platforms and areas where safety of personnel

or protection of equipment is necessary.

3.6.6 XLPE Cables (Cross Linked Polyethylene)

3.6.6.1 Properties and Advantages :

The excellent thermal properties of XLPE cables permit maximum continuous

conductor operating temperature of 900 C and short circuit temperature of 2500

C. Moreover, it has very low dielectric loss which does not vary much over the

entire operating temperature range.

These characteristics, along with the low dielectric constant make XLPE cables

particularly suitable for high voltage applications. Given below are additional

outstanding features.

High Continuous Current Rating :

Its ability to withstand higher operating temperature of 900 C enables much

higher current ratings than those of PVC or PILC cables.

High Short Circuit Rating :

Maximum allowable conductor temperature during short circuit of 2500 C is

considerably higher than for PVC or PILC cables resulting in greater short circuit

withstand capacity.

High Emergency Load Capacity :

XLPE cables can be operated even at 1300 C during emergency, therefore in

systems where cables are installed in parallel, failure of one of two cables will not

bring down the system capacity because the remaining cables can carry the

additional load even for longer duration until repairs / replacements are carried

out.

Low Dielectric Losses :

XLPE cables have low dielectric loss angle. The dielectric losses are

quadratically dependent on the voltage. Moreover, these losses occur

continuously in every charged cable whether it carries load or not. Hence use of

XLPE cable at higher voltages would result in considerable saving in costs.

Charging Currents :

The charging currents are considerably lower permitting close setting of

protection relays.

Easy Laying and Installation :

Low weight and small bending radii make laying and installation of cable very

easy. The cable requires less supports due to low weight.

High Safety :

High safety against mechanical damage and vibrations.

3.6.6.2 Applications :

Because of the excellent mechanical and electrical properties XLPE cables are

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

ideally suited for chemical and fertiliser industries where cables are exposed to

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

corrosion or in heavy industries where severe load fluctuations occur and for

systems where there are frequent over voltages. Cables can also be used at

higher ambient temperature on account of their higher operating temperature.

There excellent installation properties permit the cable to be used even under

most difficult cable routing conditions and also in cramped conditions e.g. City

distribution network. Single core cables due to their excellent installation

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

advantage.

3.6.6.3 Construction :

XLPE cables are manufactured and tested in accordance with IS : 7098 (Part II) -

1985. Its salient constructional features are as under :

Conductor :

The conductors made from electrical purity aluminium wires, are stranded

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

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

1984.

Cables with copper conductor can also be offered.

Insulation :

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

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

linked by continuous vulcanisation process.

Shielding :

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

and insulation shielding. Both conductor and insulation shielding consists of

extruded semi conducting compound.

Additionally, insulation is provided with semi-conducting tape and non-magnetic

metallic tape screen over the extruded insulation.

Conductor shielding XLPE insulation and insulation shielding are all extruded in

one operation by a special process. This process ensures perfect bonding of

inner and outer shielding with insulation.

Inner Sheath

(Common Covering) :

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

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

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

sheath can also be provided.

Armouring :

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

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

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

armour consisting of hard drawn flat or round aluminium wires.

Outer Sheath :

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

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

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

metallic part of insulation screening in case of unarmoured single core cables.

This is always black in colour for best resistance to outdoor exposure. The outer

sheath is embossed with the voltage grade and the year of manufacture. The

embossing repeats every 300/350 mm along the length of the cable.

XLPE cables are manufactured under advanced manufacturing and testing

facilities. The cables are type tested and routine tested in accordance with IS

7098 (Part II) - 1985.

The following tests are carried out as on every length of cable manufactured :

a) Conductor resistance test.

b) Partial discharge test.

c) High voltage test.

d) Insulation resistance.

e) Bending test.

f) Heating cycle test.

g) Dielectric power factor test.

h) Impulse withstand test.

3.6.6.4 Test Voltages :

The following test voltage is applied between conductor and screen / armour (IS

1255 - 1983) :

_________________________________________________________

Voltage rating of Cables Test voltage

3.8 / 6.6 kV (E) 12 kV (rms) for 5 minutes

6.35 / 11 kV (E) 17 kV (rms) for 5 minutes

11 / 11 kV (UE) 28 kV (rms) for 5 minutes

12.7 / 22 kV (E) 32 kV (rms) for 5 minutes

19 / 33 kV (E) 48 kV (rms) for 5 minutes

_________________________________________________________

A) Conductor Resistance Measurement :

This is generally taken on whole cable drum. During test, current flow is

proportional to electrical resistance of conductor, heat loss in cable in turn

depends on this resistance. Resistance of the conductor is of utmost importance

from system design point of view, which is measured during this test.

In order to achieve consistency in quality, in addition to above tests, rigorous

quality control measures are effected at every stage of production. Accordingly,

every batch of raw materials and in process cables are tested to check for their

physical and electrical properties.

To carry out testing and quality control programme, the manufacturing plant has

physical, chemical and electrical laboratories which are recognised by the

Department of Science & Technology, Government of India. Our R & D facility is

recognised as an in-house R & D unit by the Department of Science & Industrial

Research, Ministry of Science & Technology, Government of India.

Besides standard types, these laboratories also possess special equipment for

precision testing and quality control particularly for XLPE cable such as Insertion

Tensile Testing Machine, Melt Index Tester, Differential Scanning Calorimeter,

Infra-red spectrograph, Oscillating Disc Rheo meter, Micro tomes, Blown File

Extruder.

Partial Discharge Detection in Finished Cables :

This is a modern PD detection equipment where the discharge magnitude can be

directly read on a calibrated meter. The detector is associated with 50 kV noise-

free transformer, discharge-free transformer, discharge-free capacitor and

suitable matching units. Besides, specially designed corona free terminators are

used for testing purposes.

Special Features of Partial Discharge Detector Laboratory

The presence of partial discharge of even minute magnitude is not desirable in

XPLE cables and hence a highly sensitive detector has been employed with the

help of which discharges as low as 5 pc/cm can be detected. The noise from

supply source is prevented by providing isolating transformer and low pass filters.

The complete equipment is housed in a specially shielded room to prevent

external noise that would otherwise interfere with the measurements.

B) Partial Discharge Test :

This is the test for insulation and is carried out for the partial discharge occurring

in screened electric cables due to voids which remain unnoticed in the normal

high voltage tests and could be harmful to the life of insulation. Test is carried out

as per the relevant standards. Partial discharge observed should be with

permissible limits at 1.5 time the test voltage. Not only the insulation but the

semiconductor layer also should have homogeneous coating surrounding,

otherwise small voids and deep impression in that may create larger partial

discharges. Similarly, if the metallic foil over semi-conducting layer is not tight

enough then it gives use to higher level of discharges.

C) High Voltage Test :

The insulation material in cable is used to isolate the conductors from one

another and from ground and also provides mechanical strength. The insulation

has to withstand the voltage imposed on it in service. This is evaluated by

applying higher voltage stress to the insulation for a short duration. The cable

has to withstand the applied voltage without breakdown for specified period.

Thus high voltage test confirms the specified voltage rating of cable.

D) Insulation Resistance :

Any insulating material in cable should naturally have maximum resistance in

order to establish its dielectric properties. This insulation resistance is measured

between the phases or phase and ground. Decrease in the insulation resistance

indicates impurity and imperfection in cable insulation. This test evaluates quality

of insulation.

E) Bending Test :

This test condition simulate bending stresses for the cables which are always

there during handling and installation of cables. It is carried out as per the

relevant standard. After the test the cable sample should be thoroughly examined

for physical damage or cracks on the sheath. Immediately after this again partial

discharge test is to be carried out. During manufacturing if overlapping of metallic

screen is not tight and smooth, then in bending test it may open out. Partial

discharge test after this of course establishes the satisfactory performance of

cable during bending operation, without any physical damage.

F) Heating Cycle Test :

In actual service cables undergo cyclic heating and cooling resulting in expansion

and contraction which may cause either mechanical distortion or degradation of

screen which may lead to failure of cable by initiation of high dielectric loss of

higher partial discharges. Hence after this test, the sample is to be subjected to

dielectric factor test and partial discharge test. As it is subjected to cyclic heating

and cooling with specified temperature limit, the performance of cable under

actual service conditions is tested. Measuring of partial discharge level and

dielectric power factor after the test tells us about mechanical displacement of

metallic screen and homogeneity of insulation.

G) Dielectric Power Factor Test :

Dielectric power factor of any insulating material should be as small as possible

in order to have less heating of dielectric. If the cable insulation contains

impurities and voids then this value may be higher. This test should be done as a

function of voltage. Then the sample is heated by passing current up to a desired

value and again this tan delta measurement should be done. These should be

within limits which ensures more purified insulating material.

H) Impulse with Stand Test :

Because of the nearby lightning strokes, the cable insulating material in H.V.

cables may be subjected to transient over voltages. So for insulation design and

manufacturing process of cable, the withstand ability with transient over voltages

is established by this test. When the specified impulse voltage is applied no

breakdown of insulation should occur and the dielectric material must be able to

withstand transient over voltages ensuring reliability of cable insulation.

COMPARISON OF XLPE CABLES WITH OTHER TYPES OF CABLES PROPERTIES PVC POLYETHYLENE PAPER XLPE

1. Normal operation 70 70 65-80 90

Temperature 0C

2. Permitted overload - - - 130

Temperature 0C

3. Short circuit 130 120 160 250

Temperature 0C

4. Chemical resistance Very Good Fair - Very Good

5. Moisture resistance Very Good Very Good Poor Very Good

6. Thermal resistivity 600 350 600 350 0C CM / W

7. Fire resistance Excellent Poor Poor Fair

* These temperatures are applicable to cables designed with suitable over

sheaths etc.

Failures / Causes

Overloading causes the temperature rise of insulation above normal values. The

dielectric strength of insulation reduces inversely with increased temperature.

The life of insulation reduces rapidly when temperature rises above safe limits.

Normal load is specified in terms of rated current which is maximum continuous

r.m.s. current. These rating are different for buried cables and cables in ducts,

cables in open air.

Overloading factor is specified in terms of the ratio.

O.L.F = Actual r.m.s. load current____

Rated continuos r.m.s. current

The permissible Over-load factor is determined on the basis of the permissible

maximum temperature of insulation.

Maximum allowable continuous conductor temperature 0C.

PVC cables 700C.

XLPE cable 900C.

PILC cable 65 - 700C.

Cables should not be operated at maximum allowable loading except during

emergency.

8.1 CAUSES OF FAILURES OF POWER CABLES.

1. Temperature dependent causes and Thermal Degradation. Thermal

degradation of insulation due to increase in temperature, sudden increase in

temperature, unequal temperature rises related with construction and

surrounding of cable and loading conditions.

The insulation gets ionised, voids are formed, partial discharges of current

through insulation.

2. Dielectric Instability. Increase in dielectric losses is caused by increase in

service voltage or increase in temperature (within permissible limit as per IE

rules). Dielectric instability causes increased dielectric losses and temperature

rise.

3. Void formation - In insulation due to alternate heating and cooling of cables

with load cycles.

4. Corrosion of outher sheath : due to electrolytic currents.

5. Fatigue failure : Of sheaths due to temperature cycle.

6. Excessive voltage stress : Due to over voltage.

7. Moisture in insulation.

8. Mechanical Damage while digging, accidents, etc.

Summary

Cable failures are practically mainly due to :

1) Excessive overloads and temperature rise.

2) Alternate loading / unloading.

3) Over voltage.

4) Faulty laying.

The insulation should not rupture and leakage currents should not increase

above permissible limits.