Ques.41. Copper as conductor for cables is used as
Hard drawn
Annealed
Hardened and tempered
Alloy with chromium
Answer.2. Annealed
The conductor is defined as materials that allow electricity. Conductors are usually applied to materials, generally, metals, used to conduct electric current, though heat conductors and sound conductors have important uses. Good conductors of electricity tend to be good conductors of heat, too. Tough-pitch copper is the standard conductivity metal, and it is designated as the International Annealed Copper Standard (IACS).
Cold-drawn copper is used for conductors in overhead transmission lines, bus bars, etc. where high mechanical strength is needed. Annealed copper due to its flexibility is used in cables.
Ques.42. Cables, generally used beyond 66 kV are
Oil filled
Belted
S.L type
None of the above
Answer.1. Oil Filled
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.
Ques.43. The thickness of metallic shielding on cables is usually
0.04 mm
0.2 to 0.4 mm
40 to 60 mm
3 to 5 mm
Answer.4. 3 to 5mm
Metallic Sheath:- For protecting the insulation material from moisture, gases and any other harmful liquids in the soil, a metallic sheath is provided over the insulation. It also protects the insulation from mechanical damage. The metallic sheath is usually of lead or lead alloy. Recently, aluminum is also being used as a metallic sheath because it has greater mechanical strength, low weight and cost when compared to lead sheath. The thickness of the metallic shield is between 3 to 5 mm.
Ques.44. The surge resistance of the cable is
20 Ohms
50 Ohms
40 Ohms
1 Ohms
Answer.2. 50 ohms
The characteristic impedance of a lossless line is a real quantity. It has the characteristics of resistance and the dimensions of an ohm. Therefore, it is also called characteristic resistance or surge resistance. The surge resistance is denoted by R and is the ratio of the square root of Inductance of line to the capacitance of the line is R = √L/C
The surge resistance for cables is between 40-60Ω.
The surge impedance of the overhead transmission line is 400-600 Ω.
Ques.45. PVC stands for
Polyvinyl chloride
Post varnish conductor
Pre varnish conductor
Power voltage conductor
Answer.1. Polyvinyl chloride
Polyvinyl chloride (PVC) is the most widely used nonmetallic jacketing material in the wire and cable industry.
Ques.46. The advantage of underground cables over overhead transmission lines is
Easy maintenance
Low cost
Can be used in congested areas
Can be used in high voltage circuits
Answer.3. Can be used in congested areas
Advantages of underground cables
The underground cable is not affected by lightning, thunderstorms and other severe weather conditions. So, there are no interruptions in service to consumers.
They can be used in congested areas where the overhead transmission line is difficult.
Accidents caused by the breaking of the conductors will be reduced.
The maintenance cost of underground system is very low because of less chance of fault occurrence.
Because of less spacing between the conductors in an underground system, inductance is very low as compared to overhead lines; therefore, the voltage drop is less.
In an underground system, surge effect is smoothened down as the sheath absorbs surge energy.
In an underground system, there is no interference to communication lines.
The underground system of distribution or transmission is more aesthetic because no wiring is visible
Ques.47. Elastomer cables are popularly known as _____
PVC cable
Rubber cables
Internal cables
Power Cable
Answer.2. Rubber cables
Elastomer cables are popularly known as Rubber cables and probably the first type of insulation in the world. Originally it was with natural rubber for both insulation & Sheath, however, in the late 70s, it has been replaced by synthetic rubber for insulation & sheath. Elastomer cables are preferred for flexible application and in congested locations where the bending radii are very small. Elastomer cables are available from low voltage up to 33 kV (extra high Tension) grade.
Ques.48. The relative permittivity of rubber is
Between 2 and 3
Between 5 and 6
Between 8 and 10
Between 12 and 14
Answer.1. Between 2 and 3
Rubber is the most commonly used insulating material. Natural rubber is produced from the latex of the rubber tree. Synthetic rubber is produced from alcohol or oil products. Its relative permittivity is between 2 and 3 and dielectric strength is between 30 and 35 kV/mm. It absorbs moisture slightly and the maximum safe temperature as low as around 38°C.
Ques.49. Which of the following protects a cable against mechanical injury?
Bedding
Sheath
Armouring
None of the above
Answer.3. Armouring
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).
Ques.50. The disadvantage with paper as an insulating material is
It is hygroscopic
It has high capacitance
It is an organic material
None of the above
Answer.1. It is hygroscopic
Paper insulation: Dry paper is an excellent insulator but loses its insulating properties if it becomes wet. Dry paper is hygroscopic absorbing moisture from the air. So, paper insulation must be sealed to avoid any contact with air. Because of this, paper insulated cables are sheathed with impervious materials (for example lead). Paper insulated lead-covered (PILC) cables are used for carrying very large power. The performance of the paper insulation is improved by impregnating it with mineral oil or a non-draining compound.
