cables selection

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Power cables

IntroductionOverhead distribution or transmission becomes difficult in densely populated areas like cities

and towns. In such areas it is necessary to use cables laid below the ground surface, these are known

as underground cables. All cables are fundamentally similar in that they contain conductors for

carrying current, insulation for surrounding the conductor, and some form of covering to provide

mechanical and possible corrosion protection to ensure that the insulator may continue to operate is

satisfactorily throughout the life of the cable once the cable has been installed. The primary function

of cable is to carry energy reliably between source and utilization and source equipment. In carrying

this energy, there are heat losses generate in the cable that must be dissipated. The ability to dissipate

these losses depends on how the cables are insulated, and this affects their ratings. The selection of

conductor size requires consideration of the load current to be carried and the load cycle, emergency

overloading requirements and duration, fault clearing time and interrupting capacity of the cable overcurrent protection or source capacity, voltage drop, and ambient temperatures for particular

installation conditions. Cable may be installed in raceway, in cable trays, underground in duct or

direct buried, in cable bus, or as open runs of cable.

In general, Cables are classified according to their uses into the following categories

1! "ower Cables

o #ingle core

o $ouble cores %often &ine '(eutral).

o Triple cores %often *!phase).

o +our cores %often *!phase '(eutral).

o +ive cores %often *!phase, (eutral 'round).

2! Control ' measurement cables %multi!core with the same color).

3! Telephone cables %in pairs).

Electrical power could be transmitted via two methods

-verhead lines.

nderground cables.

' we can compare them as a following

Underground cableOverhead lineeature

/igher.

$ifficult.

sed in congested industrial areas '

urban areas.

p to ** k0

1&"2 up to 345 k0.

&ower.

2asy.

sed in open country for

long distance.

p to highest voltage.

Cost

!aintenance

Use

"ated voltage

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!etallic sheath!aterials used for cable sheathing are lead layer, aluminum layer or copper wire.

%he main function of it is to provide&

o ood method of cable earthing.

o ood path to the fault current.

.edding *illing+It consists of plastics materials which provide&

o 9ound cross!section!area of cables to take largest volume in least space area.

o #eparate the metallic sheath from the armored layer to prevent galvanic corrosion.

/rmoredIt is a protective noncorrosion la'er which provides&

o Carrying earth fault current.

o ives some protection against mechanical damage both during installation ' in

service.

E-ternal 0acetIt is a protective >acket which saves the cable from the surrounding solid.

It could be made of

o "0C material.

o ?ute impregnated with bitumen.

a'ing of cables%here are different methods for laying depended on cable type, cable importance ' soil

surrounding cable. #uch as

Cable supported using brackets above ground level.

Cables laid directly in dug in trenches below ground level %direct burial), the depth of trench

may be vary between 6 to 6.@m depends on voltage level. p to 66 k0 h5.@m

BB k0 h5.7@m

6*4 k0 h 6.55m

445 k0 h6.@m

The bottom of the trench is leveled freed from stones had sharp edge of rock, a layer 65cm

thick of clean sand is laid at the bottom of the trench .After laying The cable it is covered once again

with a 65cm thick layer of sand.


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)election of cable si4e%o achieve ma:imum economy in first cost and subsequent appertain of cables, an important

aspect is the selection of the optimum size of conductor. )everal factors are involved which are&

o Continuous current carrying capacity.

o Ability to carry short circuit current.

o Cost of losses.

o 0oltage drop.

%hese factors are based on certain standard conditions such as

o Type of conductor material copper or aluminumo Core construction single or multi!core.

o Insulation material "0C, 1&"2 E

o 8ethod of installation in air or in ground.

o #urrounding ambient temperature.

o #oil thermal resistively.

o $epth of laying.

o 8ethod of cable grouping.

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Classification of cables according to voltageCables can be classified according to rated voltage as follows

1! &ow voltage cables 2! 8edium voltage cables 3! /igh voltage cables

ow voltage cables3556755 8 9756:75 *)tranded Copper Conductor+

;escription&

)oft annealed solid or stranded copper conductors insulted with "0C compound rated

75F C or =@F C./pplication

or indoor fi:ed installations in dry locations, laid in conduits, as well as in steel

support brackets.

#ingle core cable with stranded and "0C Insulated

3556755 8 9756:75 *le-ible Copper+

;escription&

)oft annealed Copper fine wires bunched in sub ! units or stranded bunched together

groups into a main unit, which forms the fle:ible conductor. Insulated with soft "0C 75 FCor =@ FC Compound.

/pplication

or indoor fi:ed installations in dry locations, where particular fle:ibility is required

for electrical panelsG connection or for electrical apparatus they can be laid in groups around

steak sheets.

#ingle Core Cable and "0C Insulated

3556755 *le-ible Copper+

;escription)oft annealed Copper fine wires bunched together in sub ! units or stranded bunched

groups into a main unit, which forms the fle:ible conductor. These conductors are insulated

with "0C compound rated 75 FC and sheathed with "0C compound layer.

/pplication&

or indoor movable installation in dry location connecting to source power portable

electrical appliances operating under unfavorable conditions, such as portable lamps, fans,

refrigerators, washing machines, vacuum cleaners, T0 ' house hold heating and ventilating

apparatus.

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8ulticore Cable with "0C Insulated and "0C #heathed

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8ulticore Cables, with #tranded Copper Conductors, "0C Insulated,

#teel acket of

"0C compound.

/pplication&

or outdoor installations in damp wet locations where mechanical damages aree:pected to occur.

8ulticore Cables, with #tranded Aluminum Conductors, "0C Insulated, #teel

acket of "0C compound.

/pplication

or outdoor and indoor installations in damp and wet locations. They are normally

used for power distribution in urban networks, in industrial plants, as well as in Thermo '

/ydro power stations.

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8ulticore Cables, with #tranded, Copper Conductors, 1&"2 Insulatedand "0C #heathed

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or outdoor installations in damp wet locations where mechanical damages are

e:pected to occur.

8ulticore Cables, with #tranded Aluminum Conductors, 1&"2 Insulated, #teel Tape

Armored and "0C #heathed

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!edium voltage cables

B615 *12+ *!ulticore with )tranded Copper Conductors, >PE Insulated, )teel #ire

/rmored and PC )heathed)

;escription&

)tranded circular compacted Copper conductor, semi conducting layer as conductor

screen, 1&"2 insulated, semi conducting layer as non metallic insulation screen, Copper tape

or wire as metallic insulation screen, three cores assembled together with non!/ygroscopic

"olypropylene fillers, wrapped with polyester tape and "0C sheathed.

/pplication&

%hese cables are generally suitable for direct burial or for installation on trays or

ducts.

8ulticore Copper Conductors 1&"2 Insulated and "0C #heathed

=615 *12+ *!ulticore with )tranded /luminum Conductor, >PE Insulated, )teel #ire

/rmored and PC )heathed+;escription&

)tranded circular compacted Aluminum conductor, semi conducting layer as

conductor screen, 1&"2 insulated, semi conducting layer as non metallic insulation screen,

Copper tape or wire as metallic insulation screen, three cores assembled together with non

hygroscopic "olypropylene fillers, wrapped with polyester tape and "0C sheathed.

/pplication&


ducts.

8ulticore Aluminum Conductors, 1&"2 Insulated and "0C #heathed

=615 *12+ *!ulticore Copper or /luminum Conductors, >PE Insulated, )teel %ape/rmored and PC )heathed+

4=


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;escription&

)tranded circular compacted Copper or Aluminum conductor, semi conducting layer

as conductor screen, 1&"2 insulated, semi conducting layer as non metallic insulation

screen, Copper tape or wire as metallic insulation screen, three cores assembled together with

non hygroscopic polypropylene fillers, wrapped with polyester tape, covered with a layer of

"0C compound as a bedding, steel tape armored and "0C sheathed.

/pplication&


ducts.

8ulticore Copper or Aluminum Conductors, 1&"2 Insulated, #teel Tape

Armored and "0C #heathed.

=615 *12+ *!ulticore Copper or /luminum Conductors, >PE Insulated, )teel #ire

/rmored and PC )heathed+

;escription&

)tranded circular compacted Copper or Aluminum conductor, semi conducting layer

as conductor screen, 1&"2 insulated, semi conducting layer as non metallic insulation

screen, Copper tape or wire as metallic insulation screen, three cores assembled together with

non hygroscopic "olypropylene fillers, wrapped with polyester tape, covered with a layer of

"0C compound as a bedding, steel wire armored and "0C sheathed.

/pplication


ducts.

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8ulticore Copper or Aluminum Conductors, 1&"2 Insulated, #teel


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#ingle Core Copper Conductor, 1&"2 Insulated, &ead #creenedand /$"2 #heathing

;esign of underground cables under stud')everal factors are considered in selecting cable si4e, such as&

o Continuous current carrying capacity.

o Ability to carry short!circuit current.

o Cost of losses.

Continuous current carr'ing capacit'&

Current carrying capacity or Ampacity of cable is defined as the ma:imum allowable

current, which may flow through cable conductor without damaging the conductor or its insulation.

3*46

)%

FFFFF

A

=

=

)(factorratingOerall

factorratingOerall

currentrated!ableAmpacit"!able

#here

6F 9ating factor of soil temperature.

4F 9ating factor of soil thermal resistivity.*F grouping factor.3F 9ating factor of buried depth.

(ote&or cables installed in air there is only one rating factor % F ) which is called Gthe air

temperature rating factorG

8ost cable manufacturers issue catalogues which contain ratings for the cables which they

supply. ?owever these ratings are based on certain standard conditions such as&

o Type of conductor material, copper or aluminum.

o Core construction, single!core or multi!core depends on the rated current in cable.

o Insulation material, "0C, 1&"2 E

o 8ethod of installation, in air or in ground %direct buried or in duct).

o

#urrounding ambient temperature.o #oil thermal resistivity.

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o $epth of laying.

o 8ethod of cable grouping.

All of these previous factors should be taken into consideration in order to get the current

carrying capacity of the cable under actual operating conditions.

)hortcircuit current

%he most important factor for the cable selection is the capability of cable to carry the short

circuit current. The permissible rise in temperature during period of short circuit is 6@5FC in "0C

cable and 4@5FC in 1&"2 cables.

The electrodynamics load induced due to short circuit reduces the time that cable can be withstood.

In addition the symmetrical short circuit increases the armored and the outer sheath losses of the

cable.

Cost of losses

One important factor for cable selection is the cost of losses, which means that the cost of

power which dissipated in the cable core resistance.Cable manufacturers issue catalogues which contain resistance, inductance ' capacitance for the

cables which they supply.

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cables selection

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