CABLE SELECTION & SIZING

Power Cables:

Power cables are classified in to two types, High, Medium / Low voltage cables.

High Voltages (HV cables): 3.3, 6.6, 11,22 & 33 kV System.

Medium/Low Voltages (MV/LV cables) :415/240V System.

HV Power Cables:

Paper Insulated Lead Covered (PILC)

Poly Vinyl Chloride Insulation (PVC)

Cross Linked Polyethylene (XLPE)

The conductor shall be stranded Al. Grade H2 (or) H4 Class 2 as per IS8130 for normal application in India. The CU. conductors (Class 2) are used as per requirements.

MV/LV Power Cables:

These shall be 1100 volt grade for general purpose PVC/Heat resistant PVC cables are used.

Single core cables are used for main DC feeders.

Copper conductor cables are used for essential services.

HV VOLTAGE CABLE

HV VOLTAGE CABLE

STANDARDS USED FOR CABLES:

IS 8130:Conductors

IS 3975: Armouring

IS 692:PILC Cables

IS 1554:PVC Cables

IS 7098:XLPE Cables

IS 5831:PVC Compound Type

IS 3961:PILC Cables

Insulating Materials:

The insulating materials for cables are classified as

a) Paper,

b) Thermosetting Compounds,

c) Thermoplastic Compound,

d) Heat resistant inorganic materials such as magnesium.

Thermosetting Compounds:

Rubber based (or) synthetic rubber like compounds which are vulcanized to attain the necessary electrical & physical properties.a) Butyl Rubber,

b) Ethylene Propylene Rubber (EPR),

c) Cross Linked Polyethylene (XLPE),

d) Silicon Rubber.

EPR & XLPE have excellent heat stability,

poor flame retardance property.

EPR has very good corona resistance property compared to XLPE.

Silicon rubber has good electrical properties used in emergencies.

Thermoplastic Compound:Synthetic compounds which have definite melting temperature, Therefore at higher temperature these compounds will be soften & become semi liquid.

a) Poly Vinyl Chloride (PVC),

b) Poly ethylene,

c) Teflon/ PTFE.

PVC is highly resistant to moisture; it has high melting point temperature compared to Polyethylene

Poly Ethylene has very good melting point & poor corona

Generally In India we are using PVC, Paper, XLPE, Silicon Rubber cables.

EPR cables are not considered because it is costlier than XLPE cables.

Conductors:

Aluminium & Copper conductors are generally used.

The copper conductors shall be untinned, annealed, high conductivity copper

Classifications:

a) Circular (non compacted) Conductors: Class 1,2 & 3

b) Compacted Circular & all shaped conductors : Class 2

Class 1: Stipulates solid conductor up to 35Sq.mm

Class 2:Allows stranded conductor from lowest size of 1Sqmm.

It is recommended that all conductors should be Class 2 based on the following considerations.

Provision of stranded conductors for cable sizes up to 35mm.

Acceptance of Compacted / Stranded Conductors.

Better Flexibility for higher sized conductors.

ARMOUR:

All power cables shall be armoured type for maximum reliability. Armoured cables are recommended in the view of the following considerations.

Protect against mechanical damage.

Protection against rodent attacks.

Flame proof wiring in coal handling plant, fuel handling plant etc.

The armouring multi core cables shall be the following.

Galvanised round steel wires.

Galvanised steel strip.

For normal Application the armour cables shall comprise of single wire/strip.

Double steel wire armouring shall be specified for special application like long vertical runs of cables on bridges, submerissible cables etc..

Sizing of Power Cables:

The Power cable sizes shall be selected on the basis of

a) Current carrying capacity or Thermal ampacity,

b) Short circuit Rating

c) Permissible voltage drop.

Current Carrying Capacity or Thermal Ampacity:

The cable shall carry the full load current continuously under specified ambient temperature & other conditions of installation.

The current carrying capacity shall be worked out on the basis of

i) Base current ratings

ii) Rating factors based on site condition.

Base Current Ratings:

The base current rating shall be taken from

PILC cables : As per IS 3961 part-I

1100V grade PVC cables: As per IS 3961 part-II

3.3 & 6.6kV PVC cables : As per IS 1554 part-II

3.3 & 6.6kV XLPE cables: As per IS 7098 part-II

The base current rating for PVC & XLPE will be based on normal operating conductor temperature of 70& 90C.

But for PILC cables the normal operating conductor temperature vary for different types of cables (65-80C).

The Short circuit conductor temperature for PVC & PILC will be 160C & for XLPE cables it will be 250C.

Standard Conditions of Installations:

The current rating are based on the following conditions.

a)Thermal resistivity of soil : 150C cm/w

b) Ground Temperature : 30C

c) Ambient air temperature : 40C

d) Depth of laying : 1100 Volt grade cable : 750 mm 3.3 - 11 KV cable : 900 mmRating Factors: The base current ratings of cables shall be derated / uprated for the following conditions

a) Variations in conditions of installation compared to the standard conditions mentioned above.

b) Group running of cables.

The rating factor for variation in thermal resistivity of soil, Ambient Ground/Air Temperature, Depth of Laying, Group Derating Factor should be taken from IS : 3961 (Part I & II).

Thermal resistivity of Soil :

The thermal resistivity of soil data for the project site is usually not available.

As such, the rating factor may be taken as 1.0, unless data to contrary is available.

Ambient Ground/Air Temperature

The ambient to be taken for cable sizing shall be 5C less than the maximum ambient temperature of the Project site rounded off to next higher multiple of 5.

If no data is available, 40C shall be assumed as an average ambient temperature in air & 30C in ground for Indian Conditions.

Depth of Laying

Standard depth of laying shall be adopted in which case the rating factor will be 1.0.

To Calculate the Overall derating factor of the multi core cable in air Rating factor for the cables ambient temp laid in air (*) Rating factor for group running of cables in air

To Calculate the Overall derating factor of the multi core cable in ground

Rating factor for the cables ambient temp laid in ground(*) Thermal Resistivity of Soil(*)Rating factor for group running of cables in Ground

Short Circuit rating

The cable, if protected by circuit breaker, shall be able to withstand the fault current of the circuit for the desired fault clearing time.

The cables protected by circuit breakers shall be rated for short circuit duty.

But the cables protected by HRC fuses, Miniature Circuit breakers (MCBs) or Moulded Case Circuit Breakers (MCCBs) shall not be sized for short circuit duty.

The short circuit withstand capacity of cables shall be calculated as per followingformula:-A = Ish x (t)^1/2 KWhereA = Cross Sectional area in Sq.mmIsh = Fault current in Amps (rms)t = Fault clearing time in secondsK = Constant Amps/mm2

Ish : The maximum fault current of the system shall be taken. The breaker interrupting current rating need not be taken.

(t) : The time 't' shall be based on the maximum time taken by the breaker of the circuit to clear the fault on the operation of primary short circuit protection.

The fault clearing time is 0.16 Sec. (Breaker to Motor) & (Breaker to Trans.)

The fault clearing time is 0.5 Sec. (Breaker to breaker) The fault clearing time is 1.0 Sec. (Trans. to Breaker)

K: The value of k will be depends upon the type & conductor of the cable

The value of K for PVC Al & CU cable will be 75 & 112 Amps/mm2

The value of K for XLPE Al & CU cable will be 94 & 140 Amps/mm2

Permissible Voltage Drop:

The voltage drop in the cable shall be within permissible limits for satisfactory running & starting of the motor.

V = I (R Cos + X Sin)*L/(No of runs)V = Voltage drop I = Full load current L = Length of the Cable in Meters = Power Factor (for Motor Starting it will be =0.2) (for Motor Running it will be =0.8)R = Resistance of the Cable.X = Reactance of the Cable.

%Drop=(3^(1/2)*V*100)/(kV*1000)The voltage drop between PCC and MCC for full load current may be limited to 2%.

The voltage drop in cables between PCC/MCC to motor terminals for full load motor current may be limited to 3%.

For motor starting the voltage drop may be limited to 15%

The formulae for Voltage drop for motor starting is

V = 6*I (R Cos + X Sin)*L/(No of runs)

%Drop=(3^(1/2)*V*100)/(kV*1000)

Following code shall be used for designating the cable :

Constituent Code Letter

Aluminium conductorA

Copper conductorC

XLPE Insulation2X

PVC InsulationY

Constituent Code Letter

Metal ScreenTs

Metal sheath/Moisture BarrierTx

Non magnetic round wire armour/stringWa

Steel Strip armourF

Non Magnetic Strip ArmourFa

Double Steel Round wire ArmourWW

Double Steel strip Armour FF

PVC outer sheathY PE Outer SheathPE

Conductive coating/Conductive layerCC

Control Cables:

Control cables shall be 1.1kV grade, PVC insulated with stranded, annealed copper conductors having nominal cross sectional area of 1.5, 2.5, 4, 10, & 16mm Sq.

Conductor:

The conductor should be annealed high conductivity copper conductor with nominal cross sectional area of 1.5, 2.5, 4, 10 & 16 mm Sq.

Cores:

The control cables are available in various multi-cores such as 2, 3, 4, 5, 7, 14, 19 & 27 Cores.

Insulation, Inner & Outer Sheath:

The Insulation is based on Indian Standard IS 5831

The insulation should be Extruded PVC Compound type A. The Inner sheath should be Extruded PVC type-ST-1.The Outer sheath Should be Extruded black FRLS (Flame Retardant Low Smoke) PVC Compound type ST-1.

Armour:

The material used for armouring is galvanized round steel wires up to 13mm inner sheath diameter or Galvanized steel strips.

Screening:

The screening is done to reduce the static noise.

The screen shall consist of copper or Aluminium mylar tape of 0.04mm thickness.

It may consist of a thin tape of the same materials of 0.008mm thickness laminated to a plastic tape.

The screening tape shall be provided with round/flat tinned copper drain wire laid longitudinally in contact with screening tape.

The drain wire shall have minimum cross section of 0.5mm Sq.

Types of Cables:

In control cables most commonly used cables are twisted pair cable, Heat Resistant PVC cables, Coaxial cables & Fibre Optic cables.

Application of Control Cables:

The 2.5mm Sq. copper conductors shall be used for the following application

Control of outdoor Circuit Breaker & indoor Switchgear

Control of Control of HV & LV motors

Control of transformers viz. coolers & OLTC controls

Boiler controls like igniter, burner, soot blower, etc.

Instrument transformer leads (If higher Cross section is not needed)

Power supply to solenoids, actuators etc.

The 1.5mm Sq. copper conductors may be used for the following applications.

Wiring of field device contacts (flow level/pressure/temperature switches) to low burden circuits in annunciator relay panels, interlock relay panels local cabinets etc. if annunciator /control / interlock relays are electromagnetic type

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