Unit9.

Cables (12 marks)

 Electric power can be transmitted or distributed either by overhead system or
by underground cables. The underground cables have several advantages such
as less liable to damage through storms or lightning, low maintenance cost,
less chances of faults, smaller voltage drop and better general appearance.
 However, their major drawback is that they have greater installation cost and
introduce insulation problems at high voltages compared with the equivalent
overhead system. For this reason, underground cables are employed where it
is impracticable to use overhead lines.
 Such locations may be thickly populated areas where municipal authorities
prohibit overhead lines for reasons of safety, or around plants and substations
or where maintenance conditions do not permit the use of overhead
construction.

Underground cable:
An underground cable essentially consists of one or more conductors covered with
suitable insulation and surrounded by a protecting cover.

Requirements of a cable:
(i) The conductor used in cables should be tinned stranded copper or aluminium of
high conductivity. Stranding is done so that conductor may become flexible and carry
more current.
(ii) The conductor size should be such that the cable carries the desired load current
without overheating and causes voltage drop within permissible limits.
(iii) The cable must have proper thickness of insulation in order to give high degree of
safety and reliability at the voltage for which it is designed.
(iv)The cable must be provided with suitable mechanical protection so that it may
withstand the rough use in laying it.
(v) The materials used in the manufacture of cables should be such that there is
complete chemical and physical stability throughout.

Construction of cables:
The fig. below shows construction of 3-core cable. Its various parts are

(i) 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 Fig. 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.

(ii) Insulatian. 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.
(iii) 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 Fig.

(iv) 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.

(v) Armouring. Over the bedding, armouring is provided which consists of one or two
layers of galvanised 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.

(vi) 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.

Bedding, armouring and serving are only applied to the cables for the protection of
conductor insulation and to protect the metallic sheath from mechanical injury.

Insulating Materials for Cables

The satisfactory operation of a cable depends to a great extent upon the
characteristics of insulation used. Therefore, the proper choice of insulating material
for cables is of considerable importance. In general, the insulating materials used in
cables should have the following properties :

(i) High insulation resistance to avoid leakage current.

(ii) High dielectric strength to avoid electrical breakdown of the cable.
(iii) High mechanical strength to withstand the mechanical handling of cables.
(iv) Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture
tends to decrease the insulation resistance and hastens the breakdown of the cable.
In case the insulating material is hygroscopic, it must be enclosed in a waterproof
covering like lead sheath.
(v) Non-inflammable.
(vi) Low cost so as to make the underground system a viable proposition.
(vii) Unaffected by acids and alkalies to avoid any chemical action.

No one insulating material possesses all the above mentioned properties. Therefore,
the type of insulating material to be used depends upon the purpose for which the
cable is required and the quality of insulation to be aimed at. The principal insulating
materials used in cables are rubber, vulcanised India rubber, impregnated paper,
varnished cambric and polyvinyl chloride.

Construction of PVC Cable:

 These cables are generally used for voltages upto 11kV
 Insulation material used used is polyvinyl chloride (PVC)

Properties of PVC cable:

1. good dielectric strength
2. High insulation resistance
3. Inert to oxygen and almost inert to many acids and alkalis
4. Low current rating compared to xlpe cable
5. Smaller service life compared to xlpe cable
6. Cannot withstand extreme temperatures

Fig (i) PVC insulated

unarmoured LV multicore
cable

Fig (ii) PVC insulated armoured

LV multicore cable

Constructional features:
I. Conductor: conductors are made up of aluminium or copper. Conductors are
cables stranded to give flexibility.
 II. Insulation:PVC compound insulation is applied to the conductors by extrusion
process.
III. Innear sheath (for multi core cables): the laid up cores are surrounded by
inner sheath made up of PVC compound in case of armoured cables. Inner
sheath is also called as bedding in case of armoured cables (fig ii). In case of
unarmoured cables cores are wrapped with PVC/plastic tapes (fig. i).
IV. Armouring: depending upon the application these cables can be armoured or
unarmoured. A flat aluminium wire armour is used for single core cables
whereas galvanised round or flat steel wire armour or double steel tape is
used for multi core cables.
V. Outer sheath: outer sheath are made of black polyvinyl chloride compound
and protect armour material from corrosion.

Applications:
1. PVC unarmoured single and multicore cables are used in power stations, sub
stations, domestic connection, street lighting , building wiring etc.
2. PVC armoured single and multicore cables are useful in generating stations,
substations, distribution system, street lighting, industrial installation.

Construction of XLPE(cross-linked polyethylene) cable:

 In this type of cable the major insulation is of cross linked polyethylene.
 XLPE insulation materials provides extra ordinary electrical, thermal and
mechanical properties to cable like low dielectric loss, excellent dielectric
strength, higher continuous curent rating, high resistance to thermal aging,
larger service life.
 XLPE cables work for the working voltage of 240 V to 500 KV.

Constructional features:

I. Conductor: stranded aluminium or copper conductors.

II. Insulation: Core insulation consists of cross linked polyethylene
III. Screening: Aluminium mylar tap or annealed copper wire or tinned
copper braid is used as screen material over XLPE insulation. Screening
prevents external electromagnetic influence on cable.
IV. Sheath: PVC sheath is used .
 Applications:
 Fire Survival, Under Water Cables, Underground burial, installation on
trays and ducts.

Classification of Cables
Cables for underground service may be classified in two ways according to (i) the
type of insulating material used in their manufacture (ii) the voltage for which they
are manufactured. However, the latter method of classification is generally
preferred, according to which cables can be divided into the following groups :

(i) Low-tension (L.T.) cables — upto 1000 V

(ii) High-tension (H.T.) cables — upto 11,000 V
(iii) Super-tension (S.T.) cables — from 22 kV to 33 kV
(iv) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
(v) Extra super voltage cables — beyond 132 kV

Fig: Single core low tension cable

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
For a 3-phase service, either 3-single-core cables or three-core cable can be used
depending upon the operating voltage and load demand.

Laying of Underground Cables

The reliability of underground cable network depends to a considerable extent upon
the proper laying and attachment of fittings i.e., cable end boxes, joints, branch
connectors etc. There are three main methods of laying underground cables viz.,
direct laying, draw-in system and the solid system.
 1. Direct laying
 This method of laying underground cables is simple and cheap and is
much favoured in modern practice.
 In this method, a trench of about 1·5 metres deep and 45 cm wide is
dug. The trench is covered with a layer of fine sand (of about 10 cm
thickness) and the cable is laid over this sand bed. The sand prevents
the entry of moisture from the ground and thus protects the cable from
decay.
 After the cable has been laid in the trench, it is covered with another
layer of sand of about 10 cm thickness. The trench is then covered with
bricks and other materials in order to protect the cable from
mechanical injury.
 When more than one cable is to be laid in the same trench, a horizontal
or vertical interaxial spacing of atleast 30 cm is provided in order to
reduce the effect of mutual heating and also to ensure that a fault
occurring on one cable does not damage the adjacent cable.
 Cables to be laid in this way must have serving of bituminised paper
and hessian tape so as to provide protection against corrosion and
electrolysis.

Advantages
1. It is a simple and less costly method.
2. It gives the best conditions for dissipating the heat generated in the cables.
3. It is a clean and safe method as the cable is invisible and free from external
disturbances.
Disadvantages
1. The extension of load is possible only by a completely new excavation which
may cost as much as the original work.
2. The alterations in the cable network cannot be made easily.
3. The maintenance cost is very high.
4. Localisation of fault is difficult.
5. It cannot be used in congested areas where excavation is expensive and
inconvenient.

This method of laying cables is used in open areas where excavation can be
done conveniently and at low cost.

2. Draw in system
 • In this method, conduit or duct of glazed stone or cast iron or concrete are
laid in the ground with manholes at suitable positions along the cable route.
The cables are then pulled into position from manholes.
• Three of the ducts carry transmission cables and the fourth duct carries relay
protection connection, pilot wires. Care must be taken that where the duct
line changes direction ; depths, dips and offsets be made with a very long
radius or it will be difficult to pull a large cable between the manholes.
• The distance between the manholes should not be too long so as to simplify
the pulling in of the cables. The cables to be laid in this way need not be
armoured but must be provided with serving of hessian and jute in order to
protect them when being pulled into the ducts.

Advantages
1. Repairs, alterations or additions to the cable network can be made without
opening the ground.
2. As the cables are not armoured, therefore, joints become simpler and
maintenance cost is reduced considerably.
3. There are very less chances of fault occurrence due to strong mechanical
protection provided by the system.

Disadvantages
1. The initial cost is very high.
2. The current carrying capacity of the cables is reduced due to the close
grouping of cables and unfavourable conditions for dissipation of heat.

 This method of cable laying is suitable for congested areas where excavation is
expensive and inconvenient, for once the conduits have been laid, repairs or
alterations can be made without opening the ground.
 This method is generally used for short length cable routes such as in
workshops, road crossings where frequent digging is costlier or impossible.

3. Solid system
• In this method of laying, the cable is laid in open pipes or troughs dug out in
earth along the cable route.
• The troughing is of cast iron, stoneware, asphalt or treated wood. After the
cable is laid in position, the troughing is filled with a bituminous or asphaltic
compound and covered over.
• Cables laid in this manner are usually plain lead covered because troughing
affords good mechanical protection.

Disadvantages:
1. It is more expensive than direct laid system.
2. It requires skilled labour and favourable weather conditions.
3. Due to poor heat dissipation facilities, the current carrying capacity of the
cable is reduced.

In view of these disadvantages, this method of laying underground cables is rarely

used now-adays

Factors affecting the current carrying capacity of cables:

The current carrying capacity of an insulated conductor or cable is the maximum
current that it can continuously carry without exceeding its temperature rating. It
is also known as ampacity.

Factors affecting the current carrying capacity of the cable are as follows:

1. Type of conductor
2. Number of cores
3. Method of installation i.e. air, duct, trench, underground
4. Grouping and proximity of other conductors
5. Ambient air temperature (40◦C normal)
6. Ambient soil temperature for cables laid on ground (30◦C)
7. Type of insulation
8. Maximum permissible conductor temperature
9. Depth of laying

Selection of cable based on current carrying capacity:

• Each power cable is designed to operate under certain temperature
conditions.
• Current carrying capacity of power cable is also dependent on conductor
material (Copper / Aluminium) and insulation type.
 Thus, Copper conductor cable has greater current carrying capacity than
Aluminium.
• XLPE insulation is better than PVC, hence the current carrying capacity of XLPE
cable is more than that of PVC insulated cable.
• Operating a cable continuously beyond its rated current carrying capacity
shortens the lifespan of the cable, as the insulation becomes prone to failure.
• The current carrying capacity is also dependent on operating temperature.
Higher the temperature, lower is the current carrying capacity of the cable
and vice versa.

Selection of cable based on permissible voltage drop:

• Voltage drops occur throughout the distribution network. However, it is
necessary to regulate the voltage in order to ensure that these drops stay
within a permissible range. High voltage drops, above the permissible level,
can have many negative consequences. These result in an increase in the
system maintenance cost and a decrease in the safety and performance of the
network.
• Operating electrical equipment below its rated voltage can be dangerous as
well as reducing the expected lifetime of the equipment. When inductive
loads are operated below their rated voltage, they tend to overheat and
consume more power. Resistive loads which are operated at a too low voltage
will not be able to produce the desired output.
• Therefore size of the cable selected should be such that the voltage drop is
within permissible limits.

Q1) Determine the size of cable required to carry maximum load current of 45a. it
is also given that length of the cable is 200m And allowable voltage drop is 4% of
declared supply voltage of 230V, single phase, 50Hz supply.
Following cables are available

Size of cable Current Approx.

rating (A) ampere meter
per volt drop

No./dia in Nominal
mm area in
mm 2

19/1.12 19.35 62 1050

19/1.32 25.80 74 1475
19/1.625 38.70 97 2200

Solution:
Ampere meter required=2 X I X L
 =2 X 45 X 200 = 18000 A-m

Permissible voltage drop= 4% of 230

=0.04 X 230
=9.2 V
Considering each cable:
1)19.35 sq.mm cable
𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 = 𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
⁄𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝
18000
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 =
1050
Voltage drop=17.14 V.......................... not suitable

2) 25.80 sq.mm cable

𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 = 𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
⁄𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝
18000
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 =
1475
Voltage drop=12.2 V.......................... not suitable

3)38.70 sq.mm cable

𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 = 𝑎𝑚𝑝𝑒𝑟𝑒−𝑚𝑒𝑡𝑒𝑟
⁄𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝
18000
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑑𝑟𝑜𝑝 =
2200
Voltage drop=8.18 V.......................... suitable
38.70 sq.mm cable with current rating of 97A is selected as voltage drop is within
permissible limits.

Control cables:
 Control cables conduct low power of the order of few tens or hundreds of
Volts.
 Control cables are wired between control panels in the control room and
various equipments in the switchyard. They are necessary for measurement,
control, monitoring, signalling, communication etc.
 The control cables are generally laid on cable rocks and cable trenches.
 Cross sectional area of conductors of control cables is comparatively less.
 To avoid interference due to stray magnetic fields, control cables should be
properly laid and their sheaths should be properly earthed.

Comparison between power cables and control cables:

 The main difference between power and control cables is that control
cables are plain annealed conductors whereas the power cables are
stranded conductors of copper and aluminium.
 Control cables conduct low power of the order of few tens or hundreds of
Volts, where as power cable carry power at several hundred volts or
several kV.
 Main function of control cables is for carrying measurement, control
,monitoring, communication . power cables main function is to supply bulk
load.
 Cross sectional area of control cables is comparatively much less than
power cables.
 To avoid interference with stray magnetic fields control cables should be
properly laid and sheath should be properly earthed. No such precaution is
explicitly required in power cables.
 The number of power cable cores is small, there are single core, two core,
three core, four core (three-phase four-wire system), five core (three-
phase five-wire system) According to the grid requirements, the maximum
is usually 5 cores. The control cable transmits control signals, and the
number of cores is larger, ranging from 2 cores to 61 cores, or even more.