CABLES AND CONDUCTORS TYPES OF CABLES

Done by John Kariuki Kinuthia

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CONTENT 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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CONSTRUCTION OF CABLES

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

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PARTS OF A CABLE 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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PARTS OF A CABLE 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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PROPERTIES OF CABLE INSULATORS (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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COMPARISON BETWEEN ALUMINUM AND COOPER CONDUCTORS

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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COMPARISON BETWEEN ALUMINUM AND COOPER CONDUCTORS

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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TYPES OF CABLES RubberIt 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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TYPES OF CABLES 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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TYPES OF CABLES 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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TYPES OF CABLES 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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TYPES OF 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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TYPES OF CABLES 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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TYPES OF CABLES 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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TYPES OF CABLES 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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TYPES OF CABLES 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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TYPES OF CABLES 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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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

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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METHODS OF LAYING UNDERGROUND CABLES

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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METHODS OF LAYING UNDERGROUND CABLES

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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METHODS OF LAYING UNDERGROUND CABLES 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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METHODS OF LAYING UNDERGROUND CABLES Advantages:- of underground systems I. Better general appearanceII. Less liable to damage through storms or lightingIII. Low maintenance costIV. Less chances of faultsV. 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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TYPES OF CABLE FAULTS 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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TYPES OF CABLE FAULTS 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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TYPES OF CABLE FAULTS 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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COMPARISON BETWEEN OVERHEAD AND UNDERGROUND CABLES

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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COMPARISON BETWEEN OVERHEAD AND UNDERGROUND CABLES

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