This document discusses underground cables, including their structure, construction, classification, insulating materials, and comparison to overhead systems. It provides details on the following: - Underground cables consist of one or more conductors covered with insulating material and a protective covering, and are buried directly in the ground or installed in underground ducts. - Cable construction generally includes cores or conductors, insulation, a metallic sheath, bedding, armouring, and serving. - Cables are classified based on voltage level as low, high, super, extra high, or extra super voltage cables. - Important properties of insulating materials include high resistivity, dielectric strength, and ability to withstand high temperatures. Common materials are

1. Underground cables
Presented By:-PANKAJ KUMAR
Regd No.-1401316104
Department :-EEE
BEC,BBSR-752054
Guided by-Prof. Pabitra Mohan Dash
(HOD,EEE)

2. Contents
 Introduction of Underground cables
 Cable structure
 Construction of cables
 Classification of cables
 Properties of Insulating Material
 Insulating materials for cables
 Comparison between Underground & Overhead
System
 Advantages and disadvantages of cable
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3. Underground cables
 An underground cable consists of one or more
conductors covered with some suitable insulating
material and surrounded by a protecting cover.
 The interference from external disturbances like
storms, lightening, ice, trees etc.
 The cables may be buried directly in the ground, or
may be installed in ducts buried in the ground.
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4. CABLE STRUCTURE
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5. Construction of Cables
The general construction of a 3-conductor cable. The various parts of
cable are :
 Cores or Conductors
 Insulation
 Metallic sheath
 Bedding
 Armouring
 Serving
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6. CLSSIFICATION OF CABLES
 Low tension (L.T)  up to 1000V
 High tension (H.T)  up to 11, 000V
 Super tension (S.T)  from 22KV to 33KV
 Extra high tension (E.H.T) cables  from 33KV to
66KV
 Extra super voltage cables beyond 132KV
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7. Properties of Insulating
Material
 High resistivity.
 High dielectric strength.
 Low thermal co-efficient.
 Low water absorption.
 Low permittivity.
 Non – inflammable.
 Chemical stability.
 High mechanical strength.
 High viscosity at impregnation temperature.
 Capability to with stand high rupturing voltage.
 High tensile strength and plasticity.
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8. Insulating Materials for Cables
• 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
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9. Insulating Materials for Cables
 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.
 permit conductor temperature of 90 C and 250 C under normal and
short circuit conditions.
 suitable up to voltages of 33 KV.
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10. Insulating Materials for Cables
 Rubber
 XLPE cable
 Vulcanized India Rubber
 Impregnated Paper
 Varnished Cambric
 Polyvinyl chloride (PVC)
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11. Extra High Tension Cable
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12. Low Tension Cable
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13. 3- Core belted
Cable
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3- Core
Cables
(H-Type)
3- Core Cables
(S.L. Type)

14. Comparison between
Underground & Overhead
System
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15. Advantages & Disadvantages
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. THANKS TO ALL
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