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CABLES 11.1

CHAPTER-11

CABLES

SNO.

1. TYPE OF CABLES & RATING

2. SELECTION OF CABLE

3. GUIDELINE FOR UNDERGROUND CABLE LAYING

4. JOINTING AND TERMINATION OF CABLES

5. COMPARISON BETWEEN OVERHEAD LINE AND CABLE

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CABLES 11.2

TYPE OF CABLES AND RATINGS

A cable means a length of insulated single conductor (solid or stranded) or of two ormore such conductors, each ro!ided with its insulation which are laid together. Such

insulated conductor or conductors ma" or ma" not be ro!ided with an o!erallmechanical rotecti!e co!ering.

#n ower s"stem , cables fall into two categories$

1) Power cables 2) Conrol cables.

%ower cables are designed to transmit electric ower and come in a wide range of !oltageratings. &hese cables are called ' Low &ension Cables uto 1 !olt grade, *igh&ension cables abo!e 1 !olt grade and uto ++ !olt grade and 'Etra *igh&ension Cables abo!e ++ !olt grade. -eending on the number of cores, these cables

are single core, twin core, three core, threeandhalfcore, and fourcore. %ower cables!ar" in design according to the ser!ice the" are intended for and according to theirthermal and electrical characteristics.

Control cables are designed for oeration in alternating current circuits with !oltages uto/ !olt or in direct current circuits with !oltages uto 1 !olt. &hese cables ser!e asthe means for inter connection of all the rotecti!e, indicating, metering and remote 0automatic control de!ices installed throughout ower stations and !arious substations.

&he t"e of a articular cable to be used at a articular location is determined b" themechanical considerations and the !oltage at which it is reuired to oerate. suall" theoerating !oltage decides the t"e of insulation and cables are laced in !ariouscategories deending uon the !oltage for which the" are designed.

A cable ma" ha!e the following arts.

1. Con!"cor

#. Ins"la$on

%. S&$el!$n' (or 1) an! #)

*. Inner + S&ea& ,Coon + coer$n')

/. Aro"r$n'

0. O"er + S&ea&

Conductor and insulation are the essential arts of a cable. Armouring is otional as erreuirement. Shielding (or screening) is generall" used in hightension cables (Abo!e134 and uto ++34)

Based uon current carr"ing caacit" and !oltage le!el, t"e of conductors andinsulations are selected. Change in the t"e of conductor, insulation, inner sheath andarmouring ma5es the t"e of cable different. *ere is some brief descrition of different

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CABLES 11.+

t"es of cables using !arious t"es of conductors, insulations, inner sheaths, armouringetc.

1. TYPE OF CONDCTOR SED 2

Coer and aluminium are used for ma5ing conductors of the cable. Coer isin!ariabl" used in fleible cables. Aluminium (Al) is +./ times lighter thancoer (Cu), its resistance is 1.6/ times that of coer (Cu) for same length andcrosssection of conductor at 27C. 8wing to abundance of Al, its low cost andlighter weight, the ma9orit" of ower cables are at resent manufactured with Alconductors seciall" for higher ratings.

Conductor ma" be solid or stranded.

#. N3BER OF CORES SED2

A cable conductor ma" be single core or multicore as er the reuirement. Acore comrises the conductor which ma" consists of one or more wires strandedtogether and also the conductor insulation. &he stranding is necessar" to securefleibilit".

+. TYPE OF INSLATION SED$

8erating !oltage of the cable is the main factor for selecting insulating materialfor its conductor. #n general an insulating material to be used in a cable must ha!e thefollowing main roerties$

1. *igh dielectric strength2. *igh insulation resistance+. :ood mechanical roerties i.e elasticit" and tenacit";. #mmune to attac5s b" acids and al5alis/.

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CABLES 11.;

-ielectric Constant > +.6

Since aer is h"groscoic, it is essential to rotect such cables from theossible ingress of moisture. @or, it is enclosed in a moisture roof sheathof lead (mostl") or aluminium (in secial cases)

b) 4LCANISED INDIA RBBER ,4.I.R)

&hese are not used in large sies but are used in comarati!el" smallwiring such as lighting and ower installations.

-ielectric strength of 4.#. > 12 34?mm-ielectric constant of 4.#. > 2./

%aer cables are suerior than 4.#. cables.

c) 4ARNISHED CA3BRIC 2

&his is a cotton cloth imregnated and coated with !arnish which is a urelinseed oil with etroleum bitumen. &he cambric is laed on theconductor in the form of a tae. As this insulating material is h"groscoic,it has lead sheath o!er it.

-ielectric strength > ; 34?mm-ielectric constant > 2./ +.D

!) POLY4INYL CHLORIDE ,P.4.C) 2

#t has relaced most of abo!e insulating materials. =ow a da"s %4C isbeing used etensi!el". #t is s"nthetic material obtained from acet"lene.%4C is inert to o"gen, oone and to man" acids and al5alis. #t is flameretarding itself. En!ironmentall" it is referable to rubber.

e) SILICON-RBBER 2

#t gi!es greater thermal stabilit" o!er the great range of 7C to 2/7C,because of larger sie and structure of silicon molecules. #ts resistance tooils, to oone, and to water are fairl" good but to strong acid and al5alis, itis oor.

-ielectric strength of silicon rubber > 12+34?mm-ielectric constant of silicon rubber > +.6.

f) CROSS LIN5ED POLYETHYLENE PO6R CABLES2

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Abo!e +.+34, for high tension cables FL%E insulation is selectedbecause of imortant roerties and ad!antages of FL%E o!er %4C asunder$

a) *igher continuous current carr"ing caacit"b) *igher short circuit current caacit"c) *igher emergenc" load carr"ing caacit"d) Lower dielectric losse) Lower charging currentsf) Lower weight and smaller bending radii ma5ing la"ing and

installation easier.

FL%E cables normall" use circular stranded Aluminium wires forconductor, FL%E insulation alied o!er conductor b" etrusion rocess,shielding?screening o!er conductor as well as FL%E insulation, inner

sheath (common co!ering) of wraed lastic taes or etruded %4C,armouring of flat steel wires (Stris) or round steel wires and finall" toughouter sheat of %4C comound etruded o!er the armouring.

8&E S*EA&* A

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a) Etruded %4Cb) Etruded n!ulcanised rubberc) Graing of %4C taes.

Lead and aluminium ae used as sheath for imregnated aer insulated

cables uto 6.634. Aluminium is used uto 11 34 such cables.&) TYPE OF AR3ORING SED2

-eending uon the t"e of alication used, cables ma" be armoured orunarmoured since all the insulating materials used in the construction ofcables, are mechanicall" wea5, Some form of rotection againstmechanical in9ur" is reuired. &his urose is sol!ed b" means ofarmouring.

Armouring is alied o!er the core insulation in case of single core cables

and o!er the inner sheath in case of multi core cables. Ghere thecalculated diameter below armouring does not eceed 1+mm, the armourshall consist of gal!anied round steel wire. &he armoure of cables ha!ingcalculated diameter below armouring greater than 1+mm shall consist ofeither gal!anied round steel wire or gal!anied steel stris. #n case ofsingle core cables the armouring shall be of nonmagnetic material if cableis used in a.c s"stem. #f the single core cables are to be used in d.c s"stem,the cable can be armoured with steel wires.

$) TYPE OF OTER SHEATH SED2

All cables whether armoured or not, ha!e an eternal co!ering or outersheath against corrosion moisture and damage. A tough outer sheath ofsuitable grade of %4C comound is etruded o!er the armouring in case ofarmoured cables or inner sheath in case of unarmoured cables. &his isalwa"s blac5 in colour for best resistance to outdoor eosure.

&he beha!iour of electric cables in resence of fire has been a matter ofconcern. Although %4C itself is flame retarding , it does roduce highl"toic and corrosi!e fumes in the e!ent of fire.

@ume toicit" is imortant as it causes serious conseuences for humansafet", suffocation, intoication and anic due to reduced !isibilit"articularl" in closed and crowded laces.

#n !iew of abo!e suitable %4C @LA

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a) Low?

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CABLES 11.D

GG $ Steel double round wire armour

@@ $ Steel double flat stri armour

I $ %4C outer sheath, when at last lace in t"e designation it

stands for sheath.E7A3PLES2

1. AYFY % core 8 /9 s:. s 11994 'ra!e2

#t shall be read as Aluminium Conductor, %4C insulated, flat steel striarmoured, %4C o!erall sheathed, + core cable, conductor sie / s.mmstranded section, 114 grade.

#. YY %; Core 8 1./ S:. re 1199 4ol 'ra!e2

#t shall be read as circular solid coer conductor, 1./ s.mm, %4Cinsulated, %4C sheathed unarmoured +H core cable, 114 grade.

CRRENT RATINGS2

Based on #S$ +61 %art## and %art4, the curent ratings for %4C insulatedlight dut" cables are based on normal conditions of installation describedbelow$

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CABLES 11.

that all the heat generated during the short circuit is absorbed b" theconductor and there is no eternal loss.

&hese ratings are sulied b" the manufacturer on the basis of followingassumtions$

a) Conductor tem. rior to short circuit > H7C @or %4C insulatedcables. (6/7C @or aer insulated cables ) .

b) 167C

c) -uration of short circuit > one second.

@or an" other duration 't second, !alue gi!en b" the manufacturer is to be di!ided b" Mt.

GENERAL RATINGS OF CABLES SED IN FERTILISER CO3PLE7.

1. PO6ER CABLES2

Single core, 2core, +core, +1?2core and ;core armoured or unarmoured, @LS orFL%E, %4C insulated cables of the sies 1./ s.mm, 2./ s.mm, ; s.mm, 6 s.mm, 1s.mm, 16 s.mm, 2/ s.mm, +/ s.mm, / s.mm, H s.mm, / s.mm, 12 s.mm,1/ s.mm, 1D/ s.mm 2; s.mm, + s.mm and ; s.mm. 8ut of abo!e, FL%E *.&cables (+.+34 0 1134 grade) ha!e been used in four sies i.e 1/ s.mm, 1D/ s.mm2; s.mm and + s.mm. *owe!er manufacturing range for 1.134 single core owercables is easil" a!ailable in the mar5et from 1./ s.mm to 1 s.mm.

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CABLES 11.1

1. SYSTE3 4OLTAGE < FRE=ENCY.

a) %hase to neutral and hase to hase s"stem !oltage and freuenc".b)

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CABLES 11.11

6) EN4IRON3ENTAL CONDITIONS

a) =ormal temerature and humidit", wet or dr" location etc.b) Altitude

c) Secial conditions li5e chemicals, oil, !ibrations. Bums, harmfulorganism etc.

H) 4OLTAGE - DROP

4oltage dro at full load should not eceed the statutor" limits.

D) STANDARDISATION

A cable with standard t"e and sie of conductor should be referred.&his will enable in easier rocurement of cable and the accessories.

) FTRE PLANS

#ncrease in load current due to future lans and eansion rogrammesshould be ta5en into account while deciding the conductor sie.

1.) 3ATCHING

&he cable to be selected should match the associated euiments li5etransformer and switch gear in erformance characteristics.

11) STATTORY RESTRICTIONS2

A 5nowledge of statutor" restrictions for limited alication of coer as aconductor material is necessar".

12) OTHER SPECIAL CONDITIONS> IF ANY2

N @or eamle, length of cable in single length, t"e of drum wooden ormetallic etc.

-eending uon the alication of the cable, all or few of abo!e ointswill come into icture while selecting a reuired and most suitable cable.

Ghile floating an enuir" or lacing a urchase order, the followingtechnical information regarding %4C insulated cable should be furnishedto the sulier$

1. 4oltage grade.

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CABLES 11.12

2. Conductor sie and where alicable sie of reduced neutral conductor.+. Conductor material Aluminium or coer.;. =o of cores./. &"e of inner sheath and material$

a) Etruded rubber or %4C .

b) Graing of %4C taes.6. Armouring $a) Ghether armoured or unarmoured cable.b) @or multi core armoured cables, t"e of armouring material should be

secified !i. :al!anised round steel wire or flat steel stri.c) @or single core cables, non magnetic armouring material (generall" K*;

grade aluminium wires) should be secified.

H) &"e of outer sheath (%4C or @LS %4C) .

D) eference to #ndian Standard .

) 8ther secial reuirement, if an" e.g single length of cable in an indi!idual drum,t"e of drum wooden or steel etc.

?OINTING AND TER3INATION OF CABLES

Stes ta5en for selection of 9ointing 5it.

1. Select the 9ointing 5it according to the sie of the cable and t"e of 9oint i.estraight 9oint or end termination.

2. Ghether the end termination is indoor or outdoor t"e.

+. &"e of cable and t"e of 9ointing 5it to be chosen.

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1. Semi conducting self bonding tae (FL%E cables)11. %4C &ae12. #nstruction s5eet1+. %4C Boot for (*& cables out door termination onl")1;. Self bond #nsulating tae (for outdoor cable end termination)

1/. Self Amalgamating silicon tae16. #nsulators1H. Criming t"e lugs1D. ="lon string for FL%E cables1. euired uantit" of esin com. @or cast iron 5its.

CO3PARISION BET6EEN O4ERHEAD LINES AND CABLE

nderground cables are used where 8!erhead lines are not ossible as in large citiesdesite the fact that in their case, cost er 3G is much more as comared to 8!er head

transmission lines. Another ad!antage of o!er head s"stem for distributors is that taingcan be made at an" time without an" disturbance which is of great imortance in raidl"de!eloing areas. *owe!er, nderground cables are more ad!antageous for feederswhich are not li5el" to be disturbed for taing uroses because, being less liable todamage through storms or lighting or e!en damage, the" offer a safer guarantee ofcontinuit" of sul". But this ad!antage ma" be offset b" the cost of trending andeensi!e 9ointing necessar" in case of reairs.

*owe!er, cables score o!er o!erhead lines in case where !oltage regulation is moreimortant, because due to !er" small sacing of their conductors the" ha!e a !er" lowinductance and hence low inducti!e dros.

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. 8!er head line maintenance is more and initial cost is low which as undergroundline maintenance is nearl" ero but initial cost is more.

GIDE LINE FOR NDERGROND CABLES.

1. Sur!e" the route and ma5e a roer lan on drawing so that roer route will bedecided.

2.