2. Contents
Underground system introduction
Underground cables
Construction of cables
Insulating materials for cables
Classification of cables
Laying of underground cables
Types of cable faults
Comparision of underground and overhead system
Advantages and disadvantages of cables
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3. Introduction
The underground system of electrical distribution of power
in large cities in increasingly being adopted. Although it is
costly system of distribution as compared to overhead system,
it ensures the continuity of supply apart from the following
advantages :
Its maintenance is less
It has a long life
Its appearance is good
It eliminates hazards of electrocution due to breakage of
over head conductors.
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4. Underground cables
An underground cable consists of one or more
conductors covered with some suitable insulating
material and surrounded by a protecting cover. The
cable is laid underground to transmit electric power.
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5. Construction of Cables
Fig. shows 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. Various parts of cable
Cores or Conductors- A cable may have one or more than one core (conductor) depending upon the type of service for which it
is intended. For instance, the 3-conductor cable shown in Figure is used for 3-phase service. The conductors are made of tinned
copper or aluminium and are usually stranded in order to provide flexibility to the cable.
Insulation- Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the
voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or
rubber mineral compound.
Metallic sheath- In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies) in the soil and
atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in Figure.
Bedding- Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The
purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
Armouring- Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its
purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be
done in the case of some cables.
Serving- In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is
provided over the armouring. This is known as serving
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7. 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
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8. Insulating materials for cables
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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9. Classification of Cables
1. Low-tension (L.T.) cables — up to 1000 V
2. High-tension (H.T.) cables — up to 11,000 V
3. Super-tension (S.T.) cables — from 22 kV to 33 kV
4. Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
5. Extra super voltage cables — beyond 132 kV
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10. Single-core Low Tension Cable
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A cable may have one or more than
one core depending upon the type of
service for which it is intended. It may
be
(i) single-core
(ii) two-core
(iii) three-core
(iv) four-core etc.
Single core low tension cable:- The
cable has ordinary construction because
the stresses developed in the cable for low
voltages (upto 6600 V) are generally small.
11. Laying of Underground
Cables
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The reliability of underground cable network depends to a
considerable extent upon the proper laying and attachment
of fittings i.e. cable end boxes, etc. There are three methods
of laying underground cables :
direct laying
draw-in-system
solid-system
12. Types of cables faults
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, 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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14. 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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