Electrical Cable MCQS With Full Explanations Part-1

1. If the effect of the earth is taken into account, then the capacitance of line to ground

A. Remains unchanged
B. Decreases
C. Increases
D. Becomes infinite

Answer: C. Increases

Explanation:

Effect of Earth on Capacitance of Transmission Line:
In Transmission Line, two types of capacitance exist:

• The first one is the capacitance between the phase conductors and the second one is shunt capacitance between the conductor and the earth.
• The capacitance between the phase conductors is not only dependent on conductor spacing, the radius of conductor, and height of conductor from the ground rather it is also influenced by the earth.

As we know that the capacitance of a single-phase transmission line is given as

c=πε0/ln⁡(D/r)

Where D = Separation between the go and Return conductor
r = Radius of conductor

From the above value of capacitance, we observe that the capacitance is inversely proportional to the distance (D) hence capacitance of the conductor has increased due to the effect of Earth.

2. Grading of cable is preferred in order to achieve

A. uniform stress

B. reduction in the quantity of insulation

C. reduction in the quality of insulation

D. A & B Only 

Answer: D. A & B Only 

Explanation:

Grading:

• Grading is defined as the process of equalizing the stress in the dielectric of the cable.
• It is used to achieve uniform stress and reduction in the quantity of insulation
• Generally, the electrical stress is maximum at the surface of the conductor or the innermost part of the conductor while it is minimum at the outermost sheath of the conductor.
• If the stress is equal to all the dielectric of the conductor, then the thickness of the conductor is reduced.
• But if the stress is maximum at any of the dielectrics then it increases the thickness of the cable due to which the cost of the cable also increases.

There are two methods of grading the cable

• Capacitance Grading
• Inter-sheath Grading

3. Cables used for underground services from 33 kV to 60 kV are

A. extra high-tension cables

B. high tension cables

C. super tension cables

D. extra super voltage cables

Answer: A. extra high-tension cables

Explanation:

Classification of underground cables on the basis of voltage level is given below

Type of Cable VS Voltage level (kV)

• Low tension (L.T.) Cable-----0 – 1 kV
• High tension (H.T.) Cable-----1 – 11 kV
• Super tension (S.T.) Cable-----11 – 33 kV
• Extra high-tension (E.H.T.) Cable----33 – 66 kV
• Extra super-tension(E.S.T.) Cable-----66 kV and above

4. Dielectric strength of rubber is around

A. 3 kV/mm

B. 10 kV/mm

C. 30 kV/mm

D. 300 kV/mm

Answer: C. 30 kV/mm

Expalanation:

Dielectric Strength:

• It reflects the electric strength of insulating materials at various power frequencies.
• It is the voltage per unit thickness at which a material will conduct electricity.

Material VS Dielectric Strength (kV/mm)

• Air--3
• Oil--5-20
• Rubber--30-40
• Mica--18
• Alumina--13.4
• Diamond--2000

5. Sheaths are used in underground cables to

A. Provide proper insulation

B. Provide mechanical strength

C. Protect the cable form moisture

D. None of the above

Answer: C. Protect the cable form moisture

Explanation:

Lead 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 as shown in the figure. It has minimum dielectric stress in a cable.

The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.

Bedding: 

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. It 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 injuries while laying it or handling it.

6. Low voltage cable has operating voltage up to

A. 1 kV

B. 11 kV

C. 22 kV

D. 66 kV

Answer: A. 1 kV

Explanation:

The classification of cables based on their operating voltages is as given below.

• Low-tension (L.T.) cables — up to 1 kV
• High-tension (H.T.) cables — up to 11 kV
• Super-tension (S.T.) cables — from 22 kV to 33 kV
• Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
• Extra super voltage cables — beyond 132 kV

7. The cable used for high voltage applications is

A. Vulcanized India Rubber (VIR) cables

B. Polythene insulated cable

C. Elastomer insulated cable

D. Gas-filled cable 

Answer: D. Gas-filled cable 

Explanation:

The following types of cables are generally used for 3-phase service:

1. Belted cables - up to 11 kV

2. Screened cables - from 22 kV to 66 kV

3. Pressure cables - beyond 66 kV

Pressure cables are of two types:

Oil-filled cables

Gas-filled pressure cables

8. Which of the following protects the cable from moisture, gases, and other damaging liquids such as acids or alkalis?

A. Armoring

B. Lead sheath

C. Bedding

D. None of the above

Answer: B. Lead sheath

Explanation:

Lead 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 aluminium is provided over the insulation as shown in the figure. It has minimum dielectric stress in a cable.

The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.

Bedding: 

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. It 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 injuries while laying it or handling it.

9. Screened cables can be used for voltages

A. up to 11 kV

B. 11 kV to 33 kV

C. 33 kV to 66 kV

D. 66 kV to 132 kV

Answer: C. 33 kV to 66 kV

Explanation:

• In practice, underground cables are generally required to deliver 3-phase power. For the purpose, either three-core cable or three single-core cables may be used.
• For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons.
• However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used.

The following types of cables are generally used for 3-phase service:

• Belted cables — up to 11 kV
• Screened cables — from 22 kV to 66 kV
• Pressure cables — beyond 66 kV.

Screened cables: 

These cables are meant for use up to 33 kV, but in particular cases, their use may be extended to operating voltages up to 66 kV. Two principal types of screened cables are H type cables and S.L. type cables.

10. As the cable length increases, there is

A. less insulation resistance and more leakage current

B. more insulation resistance and less leakage current

C. less insulation resistance and less leakage current

D. more insulation resistance and more leakage current

Answer: A. less insulation resistance and more leakage current

Explanation:

Insulation resistance:

• Insulation resistance is defined as the ratio of applied direct voltage across insulation to the corresponding current flowing through it
• The current flow through insulation is called leakage current
• Leakage resistance or insulation is the opposition that a substance offers to the flow of leakage current

 

Insulation resistance of the cable is given by

The resistance of insulation is inversely proportional to its length.

Application:

• As the insulation resistance is inversely proportional to the cable length, with the increase in cable length, the insulation resistance decreases.
• As the insulation resistance decreases, leakage current will increase.

11. A standard conductor cable is expressed as 3/0.029. The number of strands in the cable is

A. 6

B. 9

C. 3

D. 7

Answer: C. 3

Explanation:

Representation of ACSR strand Conductor:

A standard conductor is represented as A/S/D for ACSR conductor.

Where,

A is the number of aluminum strands.

S is the number of steel strands.

D is the diameter of each strand.

Example: If the ACSR conductor having 7 steel strands surrounded by 25 aluminum conductors with a diameter of 0.05 mm will be specified as

25/7/0.05.

Application:

Given: 

Cable is expressed as 3/0.029 

This implies the cable consists of 3 strands of 0.029 mm.

12. Which of the following protects a cable against mechanical injury?

A. Bedding

B. Sheath

C. Core

D. Armoring

Answer: D. Armoring

Explanation:

Lead 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 aluminium is provided over the insulation as shown in figure.

It has a minimum dielectric stress in a cable.

 

Bedding: 

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape.

It is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

 

Armouring: 

Over the bedding, armouring is provided which consist of one or two layers of galvanized steel wire or steel tape.

Its purpose is to protect the cable from mechanical injuries while laying it or handling it.

13. HT cables are generally used up to

A. 11 kV

B. 33 kV

C. 66 kV

D. 132 kV

Answer: A. 11 kV

Explanation:

• Low tension line--Up to 1 kV
• High tension line--Up to 11 kV
• Super tension line---22 kV to 33 kV
• Extra high tension line--33 kV to 66 kV
• Extra super voltage cable---Beyond 132 kV

14. Which of the following is NOT a property of good insulating material used for cables?

A. Non-inflammable

B. High dielectric strength

C. High insulation resistance

D. High permittivity

Answer: D. High permittivity

Explanation:

The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. In general, the insulating materials used in cables should have the following properties.

• High insulation resistance to avoid leakage current
• High dielectric strength to avoid electrical breakdown of the cable
• High mechanical strength to withstand the mechanical handling of cables
• 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 a lead sheath
• Non-Inflammable
• Low relative permittivity
• Low cost so as to make the underground system a viable proposition
• Unaffected by acids and alkalies to avoid any chemical action

The various insulating materials used in the manufacturing of cables are rubber, VIR, paper, Polyvinyl Chloride (PVC), Varnished Cambric, Polyethylene, Gutta-Percha, Silk, Cotton, enamel etc.

15. _________conductors are most suitable for indoor and outdoor wires and cables.

A. Aluminum

B. Hard drawn copper

C. Annealed copper

D. Silver

Answer: C. Annealed copper

Explanation:

Annealed Copper:

• Due of superior conductivity, annealed copper became the international standard to which all other electrical conductors are compared.
• In 1913, the International Electrotechnical Commission defined the conductivity of commercially pure copper in its International Annealed Copper Standard.
• Copper is used as the electrical conductor in many categories of electrical wiring.
• Copper wire is used in power generation, power transmission, power distribution, telecommunications, electronics circuitry, and countless types of electrical equipment.
• Copper and its alloys are also used to make electrical contacts. Electrical wiring in buildings is the most important market for the copper industry.
• Roughly half of all copper mined is used to manufacture electrical wire and cable conductors.

Aluminum:-

• An aluminum wire have a 1.5 times larger cross-section to pass the same current as a copper wire, but two times lighter.
• Weight is one of the most important parameters for high-voltage power lines that transmit power over long distances.
• Therefore, only aluminum wires are used in main overhead power lines.

Hard drawn copper:-

• Hard-drawn copper has high electrical conductivity and great tensile strength.
• Use in transmission lines.

Silver:-

• Because of best thermal and electrical conductivity among all metals, it is best for electrical applications.
• Its cost is high and less abundance on Earth so have limited applications.

16. The material commonly used for insulation in high voltage cables is

A. Teflon

B. impregnated paper

C. rubber

D. PVC

Answer: B. impregnated paper

Explanation:

Generally, insulation of cables is required to have high dielectric strength, to provide the necessary withstand against normal operating and impulse voltages.

There are mainly two types of cable insulating materials.

Impregnated Paper

Polymer.

• Impregnated paper is the most popular insulating material, especially for the land installations in the H.V and EHV lines.
• Polymer insulation is preferably used in new installations like DC and the submarine system.
• Around 83% of HV cables are paper insulated.
• Solid paper insulated cables are suitable for application up to the MV range. At higher voltage, there is some operation problem with them due to high dielectric losses and pressure of voids.
• In oil-filled cables at high voltage, the formation of voids is prevented by maintaining a flow of oil under pressure.
• The impregnation of paper improves the dielectric properties of paper insulation.
• Polymer insulation is immune to thermal degradation and must not be prone to defects such as voids, protrusions, or contaminants.
• For extra-high voltage (400 kV and higher) new types of types mode up of PPLP (Poly propylite laminated paper) are used.
• PPLP has the advantage of combining the technical traditional technology used for impregnated paper cables with the excellent electrical and dielectric properties of synthetic insulators.

17. Which of the following property is not desirable for the insulating material used in electrical cables?

A. Non-inflammability

B. Low resistivity

C. High flexibility

D. High dielectric strength

Answer: B. Low resistivity

Expalantion:

The insulating material used in cable should have the following properties:

• High insulation resistance i.e. high resistivity.
• High dielectric strength
• Low permittivity
• High mechanical strength
• Non-hygroscopic i.e., it should not absorb moisture from air or soil
• Non-inflammable
• High flexibility
• Unaffected by acids and alkalis
• The electrical and chemical properties of the material should not be affected by the temperature.
• Low cost

Insulating materials are used in general in two distinct ways,

(1) To support and insulate components of an electrical network from each other and from ground, and

(2) To function as the dielectric of a capacitor.

For the first use, it is generally desirable to have the capacitance of the support as small as possible, consistent with acceptable mechanical, chemical, and heat-resisting properties. A low value of permittivity is thus desirable. For the second use, it is desirable to have a high value of permittivity, so that the capacitor is able to be physically as small as possible.

18. In design of underground cables the Mode of installation determines

A. Rating of cable to be used

B. Type of cable to be used

C. Size of cable to be used

D. The temperature of cable to be used

Answer: B. Type of cable to be used

Expalantion:

Underground cables:

An underground cable consists of one or more conductors covered with some suitable insulator and surrounded by a protecting cover.

The construction structure of an underground cable is shown below.

In designing underground cables, the mode of the installation determines the type of cable or wire used in the system.

There are many types of cables used in the underground system, some of them are

• Extruded Dielectric type
• High-pressure pipe type
• Self-contained fluid filled cables
• Gas isolated cables

19. ______ Specifies the safe voltage that the insulation of a cable can withstand.

A. Minimum operating voltage

B. Nominal operating voltage

C. Voltage grading of cables

D. The specific voltage of cables

Answer: C. Voltage grading of cables

Explantion:

Concept of Voltage Rating:

• The voltage rating of a cable refers to the maximum voltage to which it may be connected and have running through it.
• Voltage rating specifies the safe voltage that the insulation of a cable can withstand.

Explanation:

Voltage Rating of cable gives the capacity of insulation for that cable withstand. Voltage grading is nothing but the voltage rating for which insulation of cable can withstand. so the answer is voltage grading of cable specifies the safe voltage that the insulation of a cable can withstand.

20. The thickness of the insulation layer on the conductor in cables is dependent upon

A. Reactive power

B. Power factor

C. Voltage

D. Current

Answer: C. Voltage

Explanation:

• The thickness of the insulation layer on the conductor in cables dependent upon voltage.
• In a conductor, the voltage gradient is developed along the length of the conductor which decreases radially outwards from the conductor. If no insulation is being used than air behaves as dielectric to absorb that voltage gradient and may take a few meters to completely absorb the voltage gradient. For example overhead bare conductors, that's why they have separated apart with a certain distance. In distance between the bare conductor increases with the increase in voltage.
• But in the case of underground cables insulation covers the bare conductor. The material used in insulation has greater dielectric strength and absorbs all the voltage gradient just within a few mm or cm distance. As the voltage increases the thickness of insulation increases.
• Thus insulation absorbs the voltage gradient. Also, insulation is made of such material so that it can dissipate the heat generated in the conductor inside. As the thickness of the conductor depends on current, so proportional thickness of the conductor allows optimal heating in a conductor, and heat generated dissipates through the insulation although this is very small.
• Thus insulation is independent of current and depends only on voltage.

21. _______ is a material that is used to make cable sheaths.

A. Aluminium

B. Copper

C. Lead

D. Cast iron

Answer: C. Lead

Explanation:

Lead 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 as shown in the figure. It has minimum dielectric stress in a cable.

The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.

Bedding: 

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. It 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 injuries while laying it or handling it.

22. Which of the above constitute specifications that need be considered while selecting wires?

A. Minimum size of cable 

B. Voltage drop

C. Current carrying capacity

D. Type of insulation used 

E. All of above 

Answer: E. All of above

Expalantion:

Cable selection parameters:

Voltage of Cable

• Low-tension (LT) cables — upto 1000 V
• High-tension (HT) cables — upto 11,000 V
• Super-tension (ST) cables — from 22 kV to 33 kV
• Extra high-tension (EHT) cables — from 33 kV to 66 kV
• Extra Super tension cables — beyond 132 kV

Current Carrying Capacity

• The current carrying capacity of a cable is called Ampacity.
• Ampacity is defined as the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration and is the rms electric current which a device or conductor can continuously carry while remaining within its temperature rating.

Short Circuit Values

• The “short-circuit current rating” is the maximum short-circuit current that a component can withstand.
• Failure to provide adequate protection may result in component destruction under short circuit conditions.
• Short circuits and their effects must be considered in selecting cables.
• These cables should have a short circuit rating which is the highest temperature the cable can withstand during an electrical short circuit lasting up to about half a second.

Type of Conductor & size of cable

• Type of Conductor Material Copper or Aluminum and size of cable are main criteria for selection of Cable

No of Core

• No of core selection is depends upon power system.
• For Single Phase Power Supply, we can use 2 core cable for three phase supply we can use 3.5 core or 4 core cable for HV supply. We may be use single core cable.

Voltage drop

• It is a primary concern when installing lengths of cables is voltage drop.
• The amount of voltage lost between the originating power supply and the device being powered can be significant.
• All cables have resistance, and when current flows in them, this results in a volt drop.

Type of Insulation

• Type of Cable Insulation Material like, PVC, XLPE, Rubber
• PVC Cable is cheaper than XLPE Cable

Method of Installation

• If we lay cable in ground armor cable is required. But if we lay cable in cable tray, we may be used an armor cable to reduce cost of cable.
• If we lay cable on cable tray, then shielded cable is required.
• Mutual heating effect due to cable group laying is also considered while selecting a cable. When multiple cables are in close proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. Therefore, cable de rating is necessary consideration for multiple cables in close proximity.

Shielded Cable or unshielded Cable

• The choice of a shielded cable or non-shielded cable depends upon some criteria.
• An area such as a production/factory floor where heavy equipment being used is a prime example of a place where we might consider a shielded cable.
• Grounding can also be a concern in some installations. If shielded cable is used to connect equipment from two different circuits, a ground loop can occur, causing noise on a network line. If the ground voltage difference is great enough, it may even cause damage.
• Terminations of the shielded cable must also be made with care to provide for a smooth dielectric transition from the shielded condition to the unshielded condition.
• The substantial space required if shielded cables were used. Shielded cables require a significant amount of space at each end of the cable for installation of the stress cone kit. Also, the minimum bending radius for shielded cables is twelve times cable outside diameter, whereas the minimum bending radius for unshielded cables is only eight times outside diameter (and even less with extra-flexible appliance connection cables used in controllers).
• The two factors, high cost and large space requirements, preclude use of shielded cable in switchgear.

Economics

• It is also an important factor for selecting the type of cable.
• It is to be kept in mind that the cost of cable should not be such large that it causes loss and another cable may fetch the same results in low cost and loss.

Environmental Conditions

• Cable operates at its best when it is installed in its optimum environmental conditions.
• For example, Elastomeric Cable is applied in trailing, coal cutter, wind mill, panel wiring, battery cable and such other areas. XLPE cables work good in areas where moisture content is good. Thus, proper cable should be selected so that the system becomes more efficient.

23. the inner sheath grading in the cable are used to

A. Minimize the stress

B. Avoid the requirement of good insulation

C. Provided proper stress distribution

D. None of the above

Answer: C. Provided proper stress distribution

Explanation:

Grading of a cable is nothing but the process of achieving uniform electrostatic stress in the dielectric of cable. This is achieved by making potential gradient equal throughout the dielectric layer. It can be done in two ways - (i) Capacitance grading and (ii) Inter-sheath grading.

Capacitance Grading:

• Capacitance grading is done by employing various layers of different dielectrics having different permittivity’s between the core and the sheath
• Hence the dielectric insulation provided is no longer homogeneous, but composite
• The various layers are arranged so that the permittivity decreases from the surface of the conductor to the sheath of a cable i.e. the permittivity of dielectric is inversely proportional to the distance from the center

Inter-sheath Grading:

• In this method, instead of using various dielectrics and having a composite dielectric, we use a homogeneous dielectric material
• However, in order to distribute the stress properly, we use extra metallic sheaths between the conductor and the main sheath
• These intermediate sheaths are called ‘inter-sheaths’; These inter-sheaths are then held at adequate voltage levels
• This method improves voltage distribution in the dielectric of the cable and consequently uniform potential gradient is obtained.

24. The colour of neutral three core flexible cable is

A. Blue

B. Brown

C. Red

D. Black

Answer: A. Blue

Explanation:

Colour of different types of Wire or Cable:

Type VS Colour code

• Single-phase line--Red/Brown
• Single-phase neutral--Black/Blue
• Ground wire---Green
• Three-phase line 1---Red
• Three-phase line 2---Yellow
• Three-phase line 3---Blue
• Three-phase neutral---Black
• Three-phase protective ground or earth--Green (or) Green - Yellow
• Neutral wire (3-core flexible cable)--Blue

 

25. Which of the following is a valid grade for which electric cables are commercially manufactured?

A. 230/400V

B. 100/200V

C. 270/300V

D. 50/100V

Answer: A. 230/400V

Explanation:

Electric cables are commercially manufactured for the following grades

• 230/400 V  (domestic)
• 1.9/3.3 kV  (earthed system)
• 3.8/6.6 kV  (earthed system)
• 6.35/11 kV  (earthed system)
• 12.7/22 kV  (earthed system)
• 19/33 kV  (earthed system)
• 3.3/3.3 kV  (Unearthed system)
• 6.6/6.6 kV  (Unearthed system)
• 11/11 kV (Unearthed system)

26. Which of the following is/are the type of derating factor of cable?

A. Temperature derating factor

B. Conductor grouping factor

C. Burial depth derating factor

D. All of the above

Answer: D. All of the above

Explanation:

• The derating factor depends on ambient temperature and how we are laying the cable i.e. in Air, Duct, Buried. Normally the current carrying capacity of a particular cable is calculated at 40°C, but if we lay the cable in the air then it includes the temperature around it, if it increases more the 40°C then the conductor gets heated up by increasing the resistance in it thus the current carrying capacity of a given cable decreases, so we will derate the cable.

Types of derating factor:

• Soil thermal resistivity
• Conductor grouping factor(cable distribution in trays horizontal -verticals)
• Temperature derating factor
• Burial depth derating factor

• Soil thermal resistivity: Basically the maximum temperature in the surrounding. 40°C is the standard one.
• Conductor grouping: Due to the arrangement of multiple conductors there is a  development of an EM field that opposes the flow and thus we need to consider this factor.
• Temperature derating Factor: In this, the arrangement of cables in a conduit is of prime importance. It should be in such a way that all cables have minimum space to dissipate heat into the surroundings. The cables can touch each other but anyone cable should not be fully surrounded by other cables.

Burial Depth: 

Depends on the length below ground where the conduit is laid.

27. The allowable temperature for oil-filled and gas-pressure cables is

A. 75°C

B. 85°C

C. 95°C 

D. 65°C 

Answer: B. 85°C

Explanation:

Oil-filled cables:

• In this type of cable, a channel is formed at the center of the core by standing the conductor wires around a hollow cylindrical steel spiral tape.
• This channel formed at the center of the core is filled with thin oil by means of the oil reservoir and feeding tanks along its length and maintained at a pressure, not below atmospheric one at any point along the cable.
• This oil is the same as light mineral oil of very low viscosity as used for initial impregnation.
• The system is so designed that when the oil gets expanded on the account of increase in temperature of the cable, the extra oil collects in the external reservoir, which sends it back during contraction an account full in temperature during light load conditions.
• The pressure within the cable must not go below the atmospheric and a lower limit of about 21700 N/m2 gauge is set.
• The breakdown inception stress for oil-filled cables is 30 – 40 kV/mm.
• Maximum stresses up to 13 kV/mm are acceptable for 275 kV and 400 kV oil-filled cables
• The allowable temperature for oil-filled and gas-pressure cables is 85°C.

28. Why are the ternary lead cables used near the railway track?

A. Because they have high tensile strength

B. Have a low coefficient of thermal expansion

C. Have low specific gravity

D. Can withstand shocks and vibrations

Answer: C. Have low specific gravity

Explanation:

• In electrical signaling and power cables, lead has been used as a sheathing material, the choice of selecting lead is made because of its cheapness, corrosion resistance, and ease of fabrication.
• However the low strength of the lead resulted in a number of failures, mostly due to fatigue, and consequently lead alloys began to be used, those containing up to 3% of tin being the popular choice, but subsequently, other binary and ternary lead alloys were allowed.
• Lead and tin are both soft materials. A small addition of tin is used to strengthen lead. larger additions of 15% - 25% are used to make a protective coating on the steel by dipping in the molten alloy.

Ternary alloys:

• When the lead is mixed with the other two alloys resulting in the mixture of three different materials forming a single material called ternary lead alloys.
• Ternary alloys consist of lead, tin, and antimony, with generally 10% - 20% antimony, and 2% - 10% tin.
• This alloy is suitable for making sheaths and cables near the railway track due to having low specific gravity. 

29. Which of the following is NOT the advantage of Unshielded Twisted Pair (UTP) in the transmission media?

A. Least expensive

B. Easy to install

C. Bulky

D. High-speed capacity

Answer: C. Bulky

Explanation:

Twisted Pair Cable

This cable is the most commonly used and is cheaper than the others. It is lightweight, cheap, can be installed easily,

Unshielded Twisted Pair Cable

It consists of two conductors usually copper, each with its own color plastic insulator. Identification is the reason behind colored plastic insulation. The advantages of Unshielded Twisted Pair Cable are given below.

• Installation is easy
• Flexible
• Cheap
• It has a high-speed capacity,
• 100-meter limit
• Higher grades of UTP are used in LAN technologies like Ethernet
• Shielded Twisted Pair Cable

This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by a metal casing. The advantages of Shielded Twisted Pair Cable are given below.

• Easy to install
• Performance is adequate
• Can be used for Analog or Digital transmission
• Increases the signaling rate
• Higher capacity than unshielded twisted pair
• Eliminates crosstalk
• Bulky

30. Which of the following materials is a synthetic fiber?

A. Jute
B. Paper
C. Cotton
D. Rayon

Answer: D. Rayon


Explanation:

• Synthetic fibers are man-made fibers, most of them are prepared from raw material petroleum called petrochemicals.
• All fabrics are obtained from fibers, while fibers are obtained from artificial or man-made sources.
• They consist of a small unit or a polymer which is made from many repeating units known as monomers.
• They include nylon, acrylics, polyurethane, and polypropylene. Millions of tons of these fibers are produced all over the world each year.
• The modern textile industry is unthinkable today without synthetic fibers. Man-made fibers like silk have always been greatly valued for its gloss and fineness. Man-made fibers are smooth.
• They can be distinguished by looking at a cross-section. Some of the synthetic fibers are rayon, nylon, polyester, etc. This fiber finds its applications in household articles like ropes, buckets, furniture, etc.

Advantages  & Disadvantages

• They are very durable and do not wrinkle easily.
• They are elastic and can be easily stretched out.
• They are soft and hence used in clothing material.
• They are strong and can sustain the heavy load.
• They are cheaper as compared to natural fibers.
• Most synthetic fibers do not absorb moisture.
• Synthetic fiber can be affected if washed using hot water.
• They are non-biodegradable and offer poor insulation.

31. In a cable voltage stress is maximum

A. At the surface of the insulator
B. At the surface of the armoring
C. At the surface of the sheath
D. At the surface of the conductor

Answer: D. At the surface of the conductor

Unequal stress distribution:

• The electrostatic or voltage stress in a single core cable has a maximum value (gmax) at the conductor surface and goes on decreasing as we move towards the sheath.
• The maximum voltage that can be safely applied to a cable depends upon gmax i.e., electrostatic stress at the conductor surface.
• For safe working of a cable having homogeneous dielectric, the strength of dielectric must be more than gmax.
• If a dielectric of high strength is used for a cable, it is useful only near the conductor where stress is maximum. But as we move away from the conductor, the electrostatic stress decreases, so the dielectric will be unnecessarily over strong.

Disadvantages:
The unequal stress distribution in a cable is undesirable for two reasons. Firstly, insulation of greater thickness is required which increases the cable size. Secondly, it may lead to a breakdown of insulation.

Methods to achieve uniform stress distribution:

• The process of achieving uniform electrostatic stress in the dielectric of cables is known as the grading of cables.
• In order to overcome the above disadvantages, it is necessary to have a uniform stress distribution in cables. This can be achieved by distributing the stress in such a way that its value is increased in the outer layers of the dielectric. This is known as the grading of cables.

The following are the two main methods of grading of cables:

• Capacitance grading
• Inter-sheath grading

Results of grading of cables:

• Uniform stress distribution
• Reduction in the quantity of insulation
• Increase in quality of insulation