Underground cables have several advantages over overhead cables including better appearance, reduced vulnerability to damage from storms and lightning, lower maintenance costs, and fewer faults. However, underground cables also have higher installation costs and can experience greater insulation problems at high voltages. Underground cables consist of one or more conductors surrounded by insulation and a protective covering. They are either directly buried in the ground or installed in underground ducts for protection from external disturbances. Common faults that can occur include open circuits, short circuits, and earth faults, which can be difficult to locate and repair due to the cables being underground.

1. Underground Cables
Submitted to:- Submitted by:-
Seema Rizvi Rahul Bishnoi
(Assistant professor) 13EAIEE078
Department of Electrical Engg. Electrical Engg.
ARYA INSTITUTE OF ENGINEERING & TECHNOLOGY
KUKAS,JAIPUR
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2. Introduction
 Since the loads having the trends towards growing
density. 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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3. CABLE STRUCTURE
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4. 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. 4

5. Construction of Cables
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6. 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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7. 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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8. Extra High Tension Cable
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9. Low Tension Cable
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10. 3-core belted Cable
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11. 3- Core Cables (H-Type)
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12. 3- Core Cables (S.L. Type)
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13. 3- Core Cables (Gas
pressure)
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14. 3- Core Cables (Oil filled)
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15. Gas Pressure Cables
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16. Grading of Cables
• Since the stresses are maximum at surface of the
conductor or inner most part of the dielectric.
• The stress goes on decreasing as outer most layer is
reached.
• Since the process of achieving the uniform electrostatic
stresses on the dielectric of cables is known as Grading
of cables
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17. Cables are generally laid in the ground or in ducts in the
underground distribution system. For this reason, there
are little chances of faults in underground cables.
However, if a fault does occur it is difficult to locate
and repair the fault because conductors are not visible.
Nevertheless, the following are the faults most likely to
occur in underground cables
1) open circuit fault
2) short circuit fault
3)earth fault
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18.  When there is a break in the conductor of a cable, it is called
open circuit fault.
 The open circuit fault can be checked by megger. For this
purpose, the three conductors of the 3-core cable at the far end
are shorted and earthed.
 When two conductors of a multi-core cable come in
electrical contact with each other due to insulation
failure, it is called a short circuit fault.
 Again, we can seek the help of a megger to check this
fault.
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19. EARTH FAULTS
 When the conductor of a cable comes in contact with
earth, it is called earth fault or ground fault.
 To identify this fault, one terminal of the megger is
connected to the conductor and the other terminal
connected to earth.
 If the megger indicates zero reading, it means the
conductor is earthed. The same procedure is repeated
for other conductors of the cable.
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20. THANKS
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