CABLES

Definition – Electrical cable is a set of Copper/Aluminum wires which are insulated according to voltage level & used to carry current.

Power Cables are used for transmission & distribution of electrical supply where feasibility of overhead conductors isnot there due to safety reasons especially in thickly populated area.

Cable has following advantage over overhead conductors –

  • No supply interruption due to lightning & other sever weather condition,
  • Reduction in accidents caused by breaking of conductors,
  • It does not spoil the beauty of the city as cables are laid underground.

CLASSIFICATION OF CABLES

Cables are classified according to the voltage –

  1. LT (Low Tension) cables up to 1,000V,
  2. HT (High Tension) cables up to 11,000V,
  3. ST (Super Tension) cables from 22000V to 33000V,
  4. EHT (Extra High Tension) cables from 33,000V to 66,000V,
  5. Oil filled under pressure & gas pressure cables from 66,000V to 132,000V,

CONSTRUCTION OF CBALE

A cable consists of following parts – Core, Insulation, metallic sheath, bedding/inner sheath, armouring& serving/outer sheath. Details of each part is given below –

  • Core – All cables have one central core or a number of cores of stranded Cu or Al conductors. Generally they are one, two, three or four core.
  • Insulation – Conductors are insulated with various insulating material. These insulating materials are paper, varnish, cambric & vulcanized bitumen for low voltages. Mostly, impregnated paper is used which is an excellent insulating material.
  • Metallic sheath – A metallic sheath is provided over the insulation to prevent the entry of moisture into the insulating material. The metallic sheath is usually of lead or lead alloy and in some cases aluminum.
  • Bedding –A layer of bedding comes over metallic sheath which consists of paper tape compound with fibrous material. Sometimes jute strands or hessian tape is also used. The purpose of bedding is to protect the metallic sheath from mechanical injury from armouring.
  • Armouring – Armouring is done to protect cable from mechanical injury. It consists of one or two layers of galvanized steel wires or two layers of steel tape.
  • Serving – A layer of fibrous material is again provided over & above armouring. This material to that of bedding but is called serving.

Difference between XLPE & PVC cable

XLPE cable has advantages over PVC cable in following ways –

  • XLPE cable has higher current rating,
  • It has higher short circuit rating as max temperature limit under short circuit is 2500 as against 1600 for PVC cables.
  • It has longer life as compared to conventional PVC,
  • It has higher tensile strength i.e 1.9 Kg/mm2 as against 1.2 Kg/mm2 as compared to PVC cables,
  • It has high corrosion resistance in polluted atmosphere as compared to PVC cable,
  • It has better properties of resistance to chemical & corrosive gases.
  • It has better properties to withstand vibrations & hot impacts,
  • Dielectric strength for LT XLPE cables is 40-50 as compared to 20-35 PVC cables,
  • Moisture resistance of XLPE cable is nearly 100 time than of PVC,
  • Insulation resistance of XLPE is very high as compared to PVC cables.

TYPES OF CABLES

  • According to mechanical protectionThere are two types of cables – a)Unarmoured cable & b) Armoured cable.
  1. Unarmoured cable –Unarmouredcables are thosecables which has nomechanical protection.Its flexible in nature as there is no armouring on it. On the other hand, it has other insulations like PVC etc. These cables are used for low current capacity load.
  2. Armoured cable –Armoured cables provides mechanical protection from damage. These cables are hard to bend due to armour protection. These cables are used for high current load. These cables are suitable for undergroundlaying with additional protection of bricks, sands etc.
  • According to type of materialThere are two types of cables – a) Aluminum cable & b) Copper cables.
  1. Aluminum cable –Alumnum cable is widely used in electrical system due as it is cheaper than copper cables. These cables are easy for thimbling as Aluminum is softer than copper.
  2. Copper cable – Copper cable is hard as compared to aluminum & costly also.
  • According to number of cores– There are two types of cables – a) Single core cable & b) Multi-core cable.
  1. Single core cable – Singer core cable is mostly used for HT system.
  2. Multi-core cable – It is widely used in electrical system. There are various types of multi-core cables such as 3C, 3.5C & 4C. 3C (three core) cable is used for HT system. 3.5C is used for medium voltage for high load current starting from 35sqmm cable to 400 sqmm. 4C cable is used for low current load. The higher size is 25C & lower size is 4 sqmm.
  • According to strands There are two types of cables – a) Single strand cable & b) multi-strand cable.
  1. Single strand cable–Single core cable is generally is used for low current load. Only single solid round conductor is used in it. It is also called control cables.
  2. Multi-strand cable– This cable is widely used in electrical system starting from low voltage to high voltage. Multi-strand cable provides flexibility in the cable which helps in bending & termination of cables as small – small dia conductors are grouped together & forms a multi-strand cable.
  • According to requirementThere are two types of cables – a) Power cables& b) Control cables.
  1. Power Cables – Power cables are used for higher voltage & current. For example – Voltage level from 230V,415V,11KVetc& current level – 10A,16A,25A,40A……..300A etc.

Ex.of power cable – 3.5C X 150 sqmm, Aluminum, armoured, XLPE insulated cable.

  1. Control Cables – Control cables are generally used for low or very lowcurrentsignal such as 0.05A, 0.1A, 1A,.., 1V, 5V, 24V, 36V, 50V, …230V. These control cables can be used for electrical system, fire system, LV system etc. Generally control cables are made up of Cu conductors. They are available in various cores starting from 1C, 2C, 3C, 4C, 6C………24C etc. The maximum size of control cable is 2.5 sq mm.

Ex.  – 12Cx1.5 sq mm Cu armoured PCV insulated cable.

(Note – A cable may have all features which have been mentioned above.)

SELECTION OF CABLES

Following points must remember while selecting a cable –

  • Material of cable conductor – Aluminum (Al) or Copper (Cu),
  • Number of cores – 1C,2C, 3C, 3.5C or 4C,
  • Size (dia) of conductor – 4,6,10,16,25,35,50,70,95,120,150,210,300sq mm (note –  size of cable conductor depends on current carrying capacity of Al or Cu),
  • Type of insulation – PVC or XLPE,
  • Mechanical protection – Armoured or unarmoured,
  • Laying of cable – Underground or Air (on wall or cable tray or any other support),
  • Length of cable – Consider voltage drop factor also when cable length is more.

(Note– Current carrying capacity of cable should be higher than that of actual load for longer life of cable)

Ex. – 3.5C X 300sq mm, AL, armoured, XLPE cable,

Current carrying capacity of AL cable–(1.1KV Grade, 3.5C, PVC/XLPE insulated, PVC sheathed, Armoured/Un-armoured cable)

S. No AREA OF CONDUCTOR (in mm2) APPX OVERALL DIAMETER (in mm) CURRENTRATING(in Amp)
UN-ARMOURED ARMOURED MULTI-CORE
PVC XLPE PVC XLPE PVC XLPE
1 25 23.9 22.5 25.8 24.4 76 90
2 35 26.2 24.9 28.1 27.0 92 108
3 50 29.5 27.4 31.4 29.1 100 126
4 70 33.5 31.8 35.2 33.5 135 158
5 95 37.5 34.9 38.9 36.7 165 187
6 120 41.2 38.7 42.7 40.6 185 215
7 150 43.8 41.6 46.1 43.9 210 240
8 185 47.7 43.8 49.6 47.1 235 273
9 240 56.6 53.2 58.3 55.2 275 316
10 300 62.5 59.2 64.2 61.0 305 355
11 400 69.8 66.2 71.6 68.3 335 420
12 500 77.2 73.4 79.4 74.8 350 505
(Note– Current carrying capacity &dia may slightly vary according to different manufacturers.)

 

RATING FACTOR FOR VARIATION IN GROUND TEMPERATURE –

Ground Temp 300 350 400 450
Rating Factor – XLPE 1.0 0.94 0.91 0.87
Rating Factor – PVC 1.0 0.94 0.87 0.79

 

Current carrying capacity of AL cable – (11KV Grade, 3C, XLPE insulated, PVC sheathed, Armoured cable)-

 

S. No

NOMINAL AREA OF OCNDUCTOR OVERALL DIA CURRENT RATING
IN GROUND In AIR
1 25 46 100 117
2 35 48 119 135
3 50 51 140 168
4 70 55 172 200
5 95 59 205 243
6 120 62.5 233 279
7 150 65.5 261 320
8 185 70 294 363
9 240 75.5 341 429
10 300 81 384 494
11 400 87.5 439 577
(Note– Current carrying capacity &dia may vary according to different manufacturers.)

 

Current carrying capacity of AL cable – (33KV Grade, 3C, XLPEinsulated, XLPE sheathed, Armoured cable)-

 

S. No

NOMINAL AREA OF OCNDUCTOR OVERALL DIA CURRENT RATING
IN GROUND In AIR
1 50 77.3 140 174
2 70 81.0 170 218
3 95 85.5 202 259
4 120 88.5 230 297
5 150 92.0 257 332
6 185 95.5 290 382
7 240 100.5 335 448
8 300 105.5 378 493
9 400 112.0 432 574
(Note– Current carrying capacity &dia may slightly vary according to different manufacturers.)