The document discusses the construction of underground cables. It describes the various parts of an underground cable including conductors, insulation, metallic sheath, bedding, armoring, and serving. It explains the purpose and materials used for each part. Common insulating materials like rubber, impregnated paper, PVC, and XLPE are also explained. Underground cables have advantages like better appearance, lower maintenance costs and reduced faults compared to overhead cables, but higher installation costs.

1. Electrical Power System-1 (2150908)
Active Learning Assignment
on
Construction of Underground Cable
Prepared By:
Patel RajalKumar H.
(160123109013)
Guided By :
Prof. Piyush Pandya
Electrical Department
Batch-B3 1Gandhinagar Institute Of Technology

2. contents
• Introduction
• Parts of Cable
• Advantages And Disadvantages
• Conclusion
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3. Introduction
 This requires the better appearance, rugged construction, greater
service reliability and increased safety.
 An underground cable essentially consists of one or more conductors
covered with suitable insulation and surrounded by a protecting
cover.
 The interference from external disturbances like storms, lightening,
ice, trees etc.
 should be reduced to achieve trouble free service. The cables may be
buried directly in the ground, or may be installed in ducts buried in
the ground.
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4. Construction of cable
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5. Parts of Cable
1. Conductors
2. Insulation
3. Metallic sheath
4. Bedding
5. Armoring
6. Serving
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6. Introduction of each Parts
1. Conductors :-
 Conductors used for cables are generally made up of tinned copper or
aluminium.
 To Provide the sufficient flexibility conductors are used in stranded form.
 Cable may consists of one, two, three or four conductors depending
upon the service required.
2. Insulation:-
 As the cable is laid underground conductor has to be provided with
sufficient thickness of insulation.
 The type and thickness of insulation depends upon the voltage
level.
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7.  Insulating materials should provide the following
properties:-
1) High resistivity.
2) High dielectric strength.
3) Low thermal co-efficient.
4) Low water absorption.
5) Low permittivity.
6) Non – inflammable.
7) Chemical stability.
8) High mechanical strength.
9) High viscosity at impregnation temperature.
10)Capability to with stand high rupturing voltage.
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8. 1) Rubber.
►It can be obtained from milky sap of tropical trees or from oil
products.
►It has the dielectric strength of 30 KV/mm.
►Insulation resistivity of 10 exp 17 ohm.cm.
►Relative permittivity varying between 2 and 3.
►They readily absorbs moisture, soft and liable to damage due
to rough handling and ages when exposed to light.
►Maximum safe temperature is very low about 38 C.
Insulating Materials for Cables
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9. 2) Vulcanized India Rubber
►It can be obtained from mixing pure rubber with mineral
compounds i-e zinc oxide, red lead and sculpture and heated
upto 150 C.
►It has greater mechanical strength, durability and wear
resistant property.
►The sculpture reacts quickly with copper so tinned copper
conductors are used.
►It is suitable for low and moderate voltage cables.
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10. 3) Impregnated Paper
►This material has superseded the rubber, consists of
chemically pulped paper impregnated with napthenic and
paraffinic materials.
►It has low cost, low capacitance, high dielectric strength
and high insulation resistance.
►The only disadvantage is the paper is hygroscopic, for this
reason paper insulation is always provided protective
covering.
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11. 4) Varnished Cambric
►This is simply the cotton cloth impregnated and coated with
varnish.
►As the varnish cambric is also hygroscopic so need some
protection.
►Its dielectric strength is about 4KV / mm and permittivity is 2.5
to 3.8.
5) Polyvinyl chloride (PVC)
►This material has good dielectric strength, high insulation
resistance and high melting temperatures.
►These have not so good mechanical properties as those of rubber.
►It is inert to oxygen and almost inert to many alkalis and acids.
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12. 6) XLPE Cables (Cross Linked Poly-ethene)
►This material has temperature range beyond 250 – 300 C
►This material gives good insulating properties
►It is light in weight, small overall dimensions, low
dielectric constant and high mechanical strength, low water
absorption.
►These cables permit conductor temperature of 90 C and
250 C under normal and short circuit conditions.
►These cables are suitable up to voltages of 33 KV.
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13. 3. Metallic Sheath:-
 As the cable is placed under ground, soil may present, moisture ,
gases and some other liquids
 Therefore to protect the cable metallic sheath made up of lead or
aluminium is provided over the insulation.
4. Bedding:-
 To protect the metallic sheath from corrosion and some mechanical
injury, bedding is provided.
 It is made up of some fibrous material such as jute or hessian tape.
5. Armouring:-
 Armouring is used to protect the cable from mechanical injury
while handling.
 It consists of one or two layers of galvanised steel wire or steel
tape.
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14. 6. Serving:-
 Serving is provided to protect the armoring from atmospheric
conditions.
 It is made up of some fibrous material like jute.
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15. Advantages & Disadvantages of Under
Ground cable
Advantages
– Better general appearance
– Less liable to damage through storms or lighting
– Low maintenance cost
– Less chances of faults
– Small voltage drops
Disadvantages
– The major drawback is that they have greater installation cost and
introduce insulation problems at high voltages compared with
equivalent overhead system.
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16. Conclusion
• After learning this topic i understand the construction of
underground cable.
• In this cable there are construction wise different types of
cables like two core or more than two core, gas filled cable, oil
filled cable.
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