Construction of cables Parts of a cable Properties of cable insulators Properties of conductors Types of cables Underground cables Methods of laying underground cables Types of cable faults Comparison between overhead and underground cables

1.  TYPES OF CABLES
 Done by John Kariuki Kinuthia

2.  Construction of cables
 Parts of a cable
 Properties of cable insulators
 Properties of conductors
 Types of cables
 Underground cables
 Methods of laying underground cables
 Types of cable faults
 Comparison between overhead and underground
cables
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 Cable:
 In electro-technology, cable means an insulated electrical conductor used
for transmitting electrical energy
 The purpose of a power cable is to carry electricity safely from the
power source to different loads. In order to accomplish this goal,
the cable is made up with some components or part

4. I. Cores or Conductors.
 A cable may have one or more than one core Armoring. Over the
bedding, armoring 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.
Armoring may not be done in the case of some cables. (conductor)
depending upon the type of service for which it is intended.
 The conductors are made of tinned copper or aluminum and are usually
stranded in order to provide flexibility to the cable.
II. 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
III. Metallic Sheath.
 In order to protect the cable from moisture, gases or other damaging
liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of
lead or aluminum is provided over the insulation
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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 armoring.
V. Armoring.
 Over the bedding, armoring 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. Armoring may not
be done in the case of some cables.
VI. Serving.
 In order to protect armoring from atmospheric conditions, a
layer of fibrous material (like jute) similar to bedding is
provided over the armoring. This is known as serving.

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(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.

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 Advantages of Aluminum Wiring
 Due to its lightweight nature, aluminum is fairly malleable and easy to work with.
 The lightweight nature of aluminum is beneficial when wiring is to be done over
long distances as it makes the job less rigorous.
 Aluminum also reduces corona, an electric discharge associated with high power
transmissions.
 When it comes to cost, aluminum is more affordable than copper wire. With
aluminum, you will require about half the amount you would need if copper wire
were used instead.
 Disadvantages
• If not installed properly, aluminum wiring can raise the risk of house fires. When
aluminum wire warms, it expands and when it cools, it contracts. The tightness of
the wiring decreases with each progressive warm-cool cycle experienced, creating
the phenomenon known as “cold creep." These loose connections can cause
sparking which may result in fires. Wires progressively heat up and could even
melt surrounding insulation and fixtures, triggering a fire.
• Aluminum wires require higher maintenance than copper wiring. This is partly due
to the high wear and tear rate as well as greater risk of fire. For.

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 Advantages of Copper Wiring
 Copper has one of the highest electrical conductivity rates among metals,
which allows it to be soldered with ease. It also makes it possible for
smaller conductors to be used to transmit power loads. Smaller
conductors are easier to transport and install, and they cost less, which
helps manage wiring costs. Copper doesn’t undergo the same extreme
expansion and contraction cycles as aluminum so it is a more stable
material to use.
 Due to its high ductile properties, copper can be formed into very fine
wire, making it more versatile. Copper has a high tensile strength as
well, so it can undergo extreme stress but show minimal signs of wear
and tear. This makes the wiring more durable than aluminum. Due to its
great resilience, high durability, low maintenance, and high performance,
copper wiring also adds to home value.
 Disadvantages
 Copper wire costs much more than aluminum, so when extensive wiring is
necessary, the overall costs may prove to be prohibitive. Copper is also
heavier which can add to the difficulty in wiring. More supports are
required to secure the heavier wire in place, which also adds to overall
cost.

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 Rubber
 It can be obtained from milky sap
of tropical trees or from oil
products.
 It has the dielectric strength of
30 KV/mm.
 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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 Vulcanized India Rubber
 It can be obtained from
mixing pure rubber with
mineral compounds i-e zinc
oxide, red lead and sulphur
and heated up to 150C.
 It has greater mechanical
strength, durability and
wear resistant property.
 The sulphur reacts quickly
with copper so tinned
copper conductors are used.
 It is suitable for low and
moderate voltage cables.

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 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.
 Polyvinyl chloride steel
wire armored
 (PVC SWA)

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 Polychloropene PCP
 provides good heat resistance,
 flame resistance
 resistance to oil
 sunlight and weathering
 low temperature resistance
and abrasion resistance.
 Due to its ruggedness,
neoprene is used widely in the
mining industry. Does not
deform with high temperatures
and does not contain halogens.

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 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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 Mineral-insulated copper-clad cable
 is a variety of electrical cable made from copper conductors inside a
copper sheath, insulated by inorganic magnesium oxide powder.
 The name is often abbreviated to MICC MI cable is made by placing
copper rods inside a circular copper tube and filling the intervening
spaces with dry magnesium oxide powder.
 The overall assembly is then pressed between rollers to reduce its
diameter (and increase its length). Up to seven conductors are often
found in an MI cable
 A similar product sheathed with metals other than copper is
called mineral insulated metal sheathed (MIMS) cable.

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 PIL SWA
 for power distribution in the oil, gas, petroleum and chemical industries
where underground cables are exposed to waterlogging and corrosive
liquids and vapors - cable construction including lead sheath cover
protects against penetration and degradation of electric cable insulation.
 by far the most common form of insulation between the conductors of a
cable. In earlier times oil impregnated paper was commonly used and is
still used for high voltage, say 132kV normally underground, cables.
 A typical cable is known as P I L C S W A.
 The advantages of oil impregnated paper is that oil and paper are very
good insulators, and the oil fills all small gaps which would otherwise exist
in the insulation layer

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 PRESSURE CABLES
 When the operating voltages are greater than 66 kV and up to 230 kV,
pressure cables are used. In such cables, voids are eliminated by
increasing the pressure of compound and for this reason they are
called pressure cables.
 Two types of pressure cables viz oil-filled cables and gas pressure
cables are commonly used.
 (i) Oil-filled cables :-
 In such types of cables, channels or ducts are provided in the cable for
oil circulation. The oil under pressure (it is the same oil used for
impregnation) is kept constantly supplied to the channel by means of
external reservoirs placed at suitable distances (say 500 m) along the
route of the cable.
 Oil under pressure compresses the layers of paper insulation and is
forced into any voids that may have formed between the layers.
 Oil-filled cables are of three types viz., single-core conductor channel,
single-core sheath channel and three-core filler-space channels

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 The fig alongside shows the
constructional details of a
single core sheath channel oil-
filled cable. In this type of
cable, the conductor is solid
similar to that of solid cable
and is paper insulated.
However, oil ducts are provided
in the metallic sheath as
shown.
 In the 3-core oil-filler cable
shown in Fig alongside the oil
ducts are located in the filler
spaces. These channels are
composed of perforated metal-
ribbon tubing and are at earth
potential.

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 The oil-filled cables have three principal
advantages.:-
 Formation of voids and ionization are avoided.
 Allowable temperature range and dielectric strength
are increased.
 If there is leakage, the defect in the lead sheath is at
once indicated and the possibility of earth faults is
decreased
 Disadvantages
 the high initial cost
 complicated system of laying

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 Gas pressure cable
 The construction of the cable is
of triangular shape and thickness
of lead sheath is 75% that of solid
cable. The triangular section
reduces the weight and gives low
thermal resistance but the main
reason for triangular shape is that
the lead sheath acts as a pressure
membrane. The sheath is
protected by a thin metal tape.
 The cable is laid in a gas-tight
steel pipe. The pipe is filled with
dry nitrogen gas at 12 to 15
atmospheres. The gas pressure
produces radial compression and
closes the voids that may have
formed between the layers of
paper insulation.
 Advantages:-
 Such cables can carry more load
current and operate at higher
voltages than a normal cable.
 Moreover, maintenance cost is
small and the nitrogen gas helps
in quenching any flame.
 Disadvantage:-
 the overall cost is very high

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 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
 Before laying cable under the ground, its route should be surveyed
& selected. The position of water mains or drains etc. Should be
ascertained. moisture of soil should not enter the core of cable.
 Properties Of Under Ground Cables
I. it must possess high insulation resistance.
II. it should not be costly.
III. it should be sufficiently flexible.
IV. it should not be bulky.
V. it should be able to withstand heat produced due to flow of
current.
VI. it should not be capable of being damaged while laying in the
ground.

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 1. Direct Laying
 In direct laying method, the
cables with steel tape or wire
armoring are laid directly as
they afford excellent protection
from mechanical injury. This
method of the laying
underground cables is simple
and cheap and in much use. In
this method of laying, a trench
about 1.5 m deep and 45cm
wide is dug through out the
route of the cable. The trench is
covered with a layer of fine
sand and the cable is laid over
this sand bed. The purpose of
sand is to prevent the entry of
moisture from the ground and
thus protects the cable from
decay

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 Advantages:-
 Repairs, alterations or additions to the cable network can be
made without opening the ground.
 As the cables are not armored, therefore, joints become
simpler and maintenance cost is reduced considerably.
 There are very less chances of fault occurrence due to
strong mechanical protection provided by the system.
 Disadvantages:-
 The initial cost is very high.
 The current carrying capacity of the cables is reduced due to
the close grouping of cables and unfavorable conditions for
dissipation of heat.
 This method is generally used for short length cable routes
such as in workshops, road crossings where frequent digging
is costlier or impossible.

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 2. Draw in system
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.
In this method, a line of
conduits or ducts are of the
glazed stoneware cement or
concrete.
After laying conduits or
ducts, the cables are put into
the position from man-holes
or brick pits spaced at regular
intervals.
The Figure below shows
section through four way
underground duct line. Three
of the ducts carry transmission
cables and the fourth duct
carries relay protection
connection, pilot wires

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Advantages of Draw In System
 Repairs, alterations or additions to the cable network can be
made without opening the ground.
 As the cables are not armored, therefore, joints become
simpler and maintenance cost is reduced considerably.
 There are very less chances of fault occurrence due to
strong mechanical protection provided by the system.
 Disadvantages of Draw In System
 The initial cost is very high.
 The current carrying capacity of the cables is reduced due to
the close grouping of cables and unfavorable conditions for
dissipation of heat.
 This method is generally used for short length cable routes
such as in workshops, road crossings where frequent digging
is costlier or impossible.

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Solid system
 In this system the cable is laid in
open pipes or troughs dug out in
earth along the cable route. The
toughing is of cast iron or
treated wood Toughing is filled
with a bituminous after cables is
laid.
 Advantages
 It provides good mechanical
strength.
 Disadvantages
 It has poor heat dissipation
conditions.
 It requires skilled labor and
favorable weather conditions.
 It is very much expensive
system.

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 Advantages:- of underground systems
I. Better general appearance
II. Less liable to damage through storms or lighting
III. Low maintenance cost
IV. Less chances of faults
V. Small voltage drops
 Disadvantages:- of underground systems
I. 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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 1. Open Circuit Fault
 When there is a break in the conductor of a cable, it is called
open circuit fault.
 Procedure Of Testing Open Circuit Fault
I. 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.
II. The resistance between each conductor and earth is measured
by a megger and it will indicate zero resistance in the circuit
of the conductor that is not broken.
III. However, if the conductor is broken, the megger will indicate
infinite resistance in its circuit

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 2. Short Circuit Fault
 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.
 Procedure For Testing Short Circuit Fault
I. For this purpose the two terminals of the megger are
connected to any two conductors.
II. If the megger gives zero reading, it indicates short
circuit fault between these conductors.
III. The same steps is repeated for other conductors
taking two a time.

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 3. Earth Fault
 When the conductor of a cable comes in contact with
earth, it is called earth fault or ground fault.
 Procedure Of Finding An Earth Fault
I. To identify this fault, one terminal of the megger is
connected to the conductor and the other terminal
connected to earth.
II. 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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Particular Overhead Underground system
Public
safety
It is less safe It is more safe
Initial cost It is less expensive it is more expensive
Faults Faults occur frequently Very rare chances of
faults
Appearance It is more flexible as new
conductors can be laid along
existing conductors
It is not flexible as new
conductors are to be
laid in new channels
Location of
fault
Fault point can be easily
located
Fault point cannot be
easily located
Repair Can be easily repaired Cannot be easily
repaired
Working
voltage
It can work up to 400kV It can only work up to
66kV due to insulation
difficulty

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Lightning More chances of being
subjected to lightning
Very little chances of being
subjected to lightning
Supply
interruption
More chances of supply
interruption
Little chances of accidents
Interference
with
communication
systems
It interferes with
communication systems
No interference with
communication systems
Insulation cost Less – the overhead conductors
are bare supported on steel
towers through insulators
More insulation cost- under
ground cables are provided
with various wrappings of
high grade tape, lead sheath
is also provided
Erection cost Much less comparatively Erection cost of high voltage
cable is quite high
Uses This is used for long distance
transmission
The large charging current on
high voltage limits the use of
long distance transmission