1. The bedding on a cable consists of
A. Hessian cloth
B. Jute
C. Any of the above
D. None of the above
Answer: C. Any of the above
Explanation:
- Bedding is provided for the protection of metallic sheath against corrosion and mechanical injury, a layer of “bedding” consisting of paper tape compounded with a fibrous material like jute or Hessian tape is used. This protects the metallic sheath from corrosion and mechanical injury due to armoring.
- The compound used should be such that it does not react with the armoring material and lead sheath but at the same time it should be adhesive enough so that it sticks on, both to the lead sheath and armoring. Bedding is used in paper-insulated lead-covered cables but not in polyvinyl chloride (PVC) cables.
2. The insulating material for a cable should have
A. Low cost
B. High dielectric strength
C. High mechanical strength
D. All of the above
Answer: D. All of the above
Explanation:
Insulating material for a cable
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
- 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.
- Nonhygroscopic, that is, 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.
- Noninflammable.
- Low cost so as to make the underground system a viable proposition.
- Unaffected by acids and alkalies to avoid any chemical action.
3. In capacitance grading ______ dielectric is used.
A. Homogeneous
B. Porous
C. Hygroscopic
D. Composite
Answer:D. Composite
Explanation:
- The process of achieving uniformity in the dielectric stress by using layers of different dielectrics 1s known as capacitance grading.
- In capacitance grading, the homogeneous dielectric is replaced by a composite dielectric. The composite dielectric consists of various layers of different dielectrics in such a manner that the relative permittivity of any layer is inversely proportional to its distance from the center. Under such conditions, the value of potential gradient at any point in the dielectric is constant and is independent of its distance from the center.
- In other words, the dielectric stress in the cable is the same everywhere and the grading is an ideal one. However, ideal grading requires the use of an infinite number of the dielectric which is an impossible task. In practice, two or three dielectrics are used in the decreasing order of permittivity. The dielectric of the highest permittivity being used near the core.
4. Underground cables are laid at sufficient depth
A. To minimize the effect of shocks and vibrations due to gassing vehicles, etc.
B. To avoid being unearthed easily due to removal of soil
C. To minimize temperature stresses
D. For all of the above reasons
Answer: A. To minimize the effect of shocks and vibrations due to gassing vehicles, etc.
Explanation:
- In urban areas, underground cables form the bulk of connections for the electricity distribution system. 240 volts applies generally for single domestic supplies, and 440 volts for three-phase supplies, largely for industrial uses.
- The depths at which cables are laid generally increase with the voltage with the aim being to provide a sufficient depth of cover over the cables so as to minimize potential contact by third parties, vehicles which can lead to serious hazards such as electric shock. At higher voltages cables are more likely to be installed in ducts and in all cases would be surrounded by sand to avoid the impact of bricks and stones damaging the cable when the trench is backfilled.
5. The material for armoring on cable is usually
A. Steel tape
B. Galvanized steel wire
C. Any of the above
D. None of the above
Answer:C. Any of the above
Explanation:
Armouring and covering:
- Armouring is provided over the bedding to protect the cable from mechanical injury while laying and handling it. It consists of one or two layers of galvanized steel wires or two layers of steel tape. The steel tapes are coated with preservative compounds and are helically wound on the cable insulation. Single-core cables in AC systems are not provided with armouring because eddy currents induced in the steel armour producing high loss. In these cables, plastic wrap is used for mechanical protection. Power cables are usually protected by bituminous compound wrapping (covering).
6. The cable used for high voltage application is
A. Vulcanized Indian Rubber (VIR) cables
B. Elastomer Insulated cable
C. Polythene Insulated cable
D. Gas-filled cable
Answer: D. Gas-filled cable
Gas pressure cables:
- Paper impregnated with petroleum jelly is normally used as the dielectric in gas-filled cables. The space between layers of paper insulation is filled with dry nitrogen gas with a pressure of 1400 kN/m2. This pressure is maintained constantly by pumps and by the lead sheath inside the cable. These cables are usually single-core cables and a small clearance is left to allow the gas to flow axially. In the case of three-core cables, clearance is not necessary because the filler spaces and strands allow the gas to flow.
High-pressure gas-filled cables:
- For this type of cable, the space for the gas is provided in the dielectric itself, which is filled up by inert gases like nitrogen at a pressure of about 6 atmospheres for extra high-tension voltage cables and 12 atmospheres for super-voltage cables. This facilitates the axial flow of gas, which also passes along the un-impregnated strand. This clearance is not essential in the case of multi-core cables because the filler spaces and strands provide a low resistance path for the flow of gas.
The advantages of gas-filled cables are:
(a) No external accessories are required.
(b) The cable can be used for the vertical run without any fear of leakage with suitable designs.
(c) With an increase in pressure, there is an improvement in the power factor of the cable dielectric.
(d) The extra super-voltage power cables are used for operating voltage beyond 132 kV.
7. In the cables, sheaths are used to
A. Prevent the moisture from entering the cable
B. Provide enough strength
C. Provide proper insulation
D. None of the above
Answer: A. Prevent the moisture from entering the cable
Explanation:
Metallic sheath:
A metallic sheath is provided over the insulation to protect the insulation material from moisture, gases, and any other harmful liquids in the soil. It also protects the insulation from mechanical damage. The metallic sheath is usually made of lead or lead alloy. Nowadays, aluminum is also being used as a metallic sheath because of its greater mechanical strength, low weight, and cost as compared to lead sheath.
8. In a cable immediately above metallic sheath, ________ is provided.
A. Earthing connection
B. Bedding
C. Armouring
D. None of the above
Answer: B. Bedding
Explanation:
A layer of bedding is provided over the metallic sheath to protect it from corrosion and mechanical injuries due to armoring. Generally, bedding consists of paper tape which is compounded with fibrous materials such as jute or hessian tape.
9. Electric strength in a cable is more on
A. Layer on the conductor
B. Outside of the conductor
C. The outer side of the armoring
D. None of these
Answer: A. Layer on the conductor
Explanation:
- The electric strength of insulation material is the ability to withstand an applied voltage without a breakdown. The voltage between the conductors at which the surrounding air is ionized is called the breakdown voltage or disruptive voltage. Break down voltage depends on atmospheric conditions and also surface conditions of the conductor.
or
- Electric strength is the maximum voltage that an insulating material can take, after which it loses its insulating properties. The higher the electric strength, the more useful is the material as an insulator. Hence the cable must be designed so that the electric field strength, or maximum electric stress, at the surface of the conductor, does not exceed that required to break down the insulation.
- When the potential is applied to the conductors, a potential gradient is developed in the air. This potential gradient is maximum around the surface of the conductors. Due to the potential gradient, the free electrons will start moving with a certain velocity which depends on the field strength. The greater is the applied voltage, the higher is the potential gradient and the velocity acquired by the free electrons.
- If the voltage is higher than the electric strength, an electric breakdown occurs. No matter which mechanism initiates the breakdown, the result is a sudden discharge of the voltage through the insulation. In cable insulation, such an event inevitably leads to a complete failure and an outage of the power line. The cable insulation must be designed such that it can withstand all expected voltages during the lifetime of the cable.
- The electric strength is a material property given in kV/mm. There are no fixed values for solid insulation materials, even if cables with electric strength values can be found in many textbooks and journal articles. The electric strength of a given material is depending on a number of parameters under which the electric strength has been measured. Sometimes, the concept of the “intrinsic dielectric strength” of a material is used.
Important Points:
- When we want a low electric field strength at the conductor surface, we have to choose for thick conductors. By increasing the diameter of the conductor, the distance between the equipotential lines increases, and the electric field strength at the surface of the conductor, decreases. However, thick conductors are heavy and, besides the fact that more material is needed to manufacture them, they require more rigid tower structures which makes this solution an expensive one. A much better solution is to divide the conductor into bundles. Instead of one conductor per phase, multiple conductors are used per phase.
10. Pressure cables are generally used beyond
A. 11 kV
B. 33 kV
C. 66 kV
D. 132 kV
Answer: C. 66 kV
Explanation:
- For voltages beyond 66 kV, solid-type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. In such cables, voids are eliminated by increasing the pressure of the 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.
11. The thickness of the layer of insulation on the conductor, in cables, depends upon
A. Reactive power
B. Voltage
C. Current carrying capacity
D. Power factor
Answer.2. Voltage
Explanation:
The thickness of the layer of insulation on the conductor in cables depends upon voltage. Insulation covers the base conductor. The material used in insulation have greater dielectric strength and absorbs all the voltage gradient just within a few mm or cm distance. As voltage increases the thickness of insulation increases.
12. Solid type cables are considered unreliable beyond 66 kV because
A. There is a danger of breakdown of insulation due to the presence of voids
B. Of corona loss between conductor and sheath material
C. Skin effect dominates on the conductor
D. Insulation may melt due to higher temperature
Answer: A. There is a danger of breakdown of insulation due to the presence of voids
Explanation:
The voltage limit for solid type cables is 66 kV due to the following reasons:
- As a solid cable carries the load, its conductor temperature increases and the cable compound (i.e., insulating compound over paper) expands. This action stretches the lead sheath which may be damaged.
- When the load on the cable decreases, the conductor cools and a partial vacuum is formed within the cable sheath. If the pinholes are present in the lead sheath, moist air may be drawn into the cable. The moisture reduces the dielectric strength of insulation and may eventually cause the breakdown of the cable.
- In practice, voids are always present in the insulation of a cable. Modern techniques of manufacturing have resulted in void free cables. However, under operating conditions, the voids are formed as a result of the differential expansion and contraction of the sheath and impregnated compound. The breakdown strength of voids is considerably less than that of the insulation. If the void is small enough, the electrostatic stress across it may cause its breakdown. The voids nearest to the conductor are the first to break down, the chemical and thermal effects of ionization causing permanent damage to the paper insulation.
13. The insulation resistance of the cable decreases with
A. The increase in length of the insulation
B. The decrease in the length of the insulation
C. The decrease in the temperature
D. Any of the above
Answer:A. The increase in length of the insulation
Explanation:
- Insulation resistance is the resistance offered by the cable insulation to the leakage current flowing from the conductor to the sheath.
- The insulation resistance of a cable is inversely proportional to the length of the cable.
- As the length increases, the leakage current flowing through the insulation from the conductor to the earthed sheath increases, and hence, the effective insulation resistance decreases.
14. If a power cable and a communication cable are to run parallel the minimum distance between the two, to avoid interference, should be
A. 2 cm
B. 50 cm
C. 400 cm
D. 10 cm
Answer: B. 50 cm
Explanation:
- It is possible for 50 Hz currents to be induced in the metallic conductors of communications cables where they run parallel to high-voltage power lines carrying low-frequency (50 Hz) currents.
- The voltage induced in the telephone line from the power cables happens in a manner similar to the operation of a step-down transformer.
- A higher potential on the primary circuit results in a higher potential on the secondary circuit. Hence, low-voltage ac power lines (less than 1000 volts) pose a threat for direct contact, but less of a threat for dangerous voltage being induced on the telephone line.
- It is important to maintain spatial separation between conductive telecommunications cables and HV power cables to ensure that the possibility of induced voltage on the communications cables does not exceed 430 V ac.
- The minimum distance between power and communications lines for parallel pole routes exceeding 330 kV is 50 m.
15. The inter sheaths in the cables are used to
A. Minimize the stress
B. Provide proper stress distribution
C. Avoid the requirement of good insulation
D. None of the above
Answer: B. Provide proper stress distribution
Explanation:
Intersheath grading:-
The process of achieving uniform distribution in dielectric stress by providing a metallic inner sheath between successive layers of the same dielectric materials and maintaining the appropriate potential level at the inner sheath.
16. Empire tape is
A. Impregnated paper
B. Vulcanized rubber
C. Varnished cambric
D. None of the above
Answer: C. Varnished cambric
Explanation:
Varnished cambric (or empire tape):-
- This is a cotton cloth impregnated and heated with varnish. The cambric is lapped onto the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent.
- Its dielectric strength is about 4 kV/mm.
- Its permittivity is 2.5 to 3.8.
- Such cables do not require sealing.
17. Conduit pipes are normally used to protect ________ cables.
A. PVC sheathed cables
B. Armoured
C. Unsheathed cables
D. All of the above
Answer: C. Unsheathed cables
Explanation:
Conduit wiring system
- In conduit wiring, cables and Unsheathed cables wires are carried through plastic giving good protection from mechanical injury or fire risks. This system is the best and most desirable system of wiring for workshops and public buildings.
- It gives a very good appearance when concealed. In this wiring, the pipes are cut with the hacksaw and are threaded tee junction box, etc., and are then fixed on the walls on wooden gutties or plugs with saddles. Then wires are drawn with the help of fish wire (steel wire).
- Nowadays P.V.C. conduit pipes are also available which do not require threading. Jointing is done with a special mode solution. These are flexible and can be bent easily. For concealed wiring, the pipes are directly buried in the wall and roofs, and then wires are drawn through them.
Advantages of Conduit Wiring Systems
- It is the safest wiring system (Concealed conduit wring)
- Appearance is very beautiful (in case of concealed conduit wiring)
- No risk of mechanical wear & tear and fire in case of metallic pipes.
- Customization can be easily done according to the future needs.
- Repairing and maintenance are easy.
- There is no risk of damage the cable’s insulation.
- it is safe from corrosion (in case of PVC conduit) and risk of fire.
- It can be used even in humidity, chemical effect and smoky areas.
- No risk of electric shock (In case of proper earthing and grounding of metallic pipes).
- It is reliable and popular wiring system.
- sustainable and long-lasting wiring system.
- Disadvantages of Conduit Wiring Systems
- It is expensive wiring system (Due to PVC and Metallic pipes, Additional earthing for metallic pipes Tee(s) and elbows etc.
- Very hard to find the defects in the wiring.
- installation is not easy and simple.
- Risk of Electric shock (In case of metallic pipes without proper earthing system)
- Very complicated to manage additional connection in the future.
18. _______ cables are used for 132 kV lines.
A. High tension
B. Super tension
C. Extra high tension
D. Extra super voltage
Answer: D. Extra super voltage
Explanation:
Depending on the operating voltage, the cables are classified as follows:
- L.T. (Low tension) cable – up to 1 kV
- H.T (High tension) cable- up to 11kV
- S.T. (Super tension)cable- from 22 kV to 33 kV
- E.H.T (Extra high tension) cable- from 33 kV to 66 kV
- Extra super voltage cable- beyond 132 kV
19. High tension cables are generally used up to
A. 132 kV
B. 66 kV
C. 33 kV
D. 11 kV
Answer:D. 11 kV
Explanation:
Depending on the operating voltage, the cables are classified as follows:
- L.T. (Low tension) cable – up to 1 kV
- H.T (High tension) cable- up to 11kV
- S.T. (Super tension)cable- from 22 kV to 33 kV
- E.H.T (Extra high tension) cable- from 33 kV to 66 kV
- Extra super voltage cable- beyond 132 kV
20. A cable carrying alternating current has
A. Hysteresis, leakage, and copper losses only
B. Hysteresis losses only
C. Hysteresis and leakage losses only
D. Hysteresis, leakage, copper and friction losses
Answer: C. Hysteresis and leakage losses only
Explanation:
The losses occurring in a cable dielectric include leakage loss and hysteresis loss.
Leakage Losses:-
- Since insulating materials are not truly perfect insulators there is always some current flow across the material.
- However, the conductivity is so low that this current can be neglected.
- The current flow through insulation is called leakage current.
- It occurs in both AC and DC voltages
Hysteresis Loss:-
- The additional dielectric loss under AC is caused by the hysteresis involved in the process of dielectric polarization.
- The increase in the power loss due to dielectric hysteresis is much greater than leakage loss. The dielectric hysteresis loss cannot be measured separately.
- The total dielectric loss, consisting of dielectric hysteresis loss and the power loss due to leakage current flowing through the insulation resistance, can be measured by means of the Schering bridge.