86.Why cables are not used for long distance
are not used for long distance transmissions due to their large charging
87.Mention the 3 main parts of the cable?
88.What is the function of conductor?
provides the conducting path for the current.
89.What is the purpose of insulation in a cable?
insulation or dielectric withstands the service voltage and isolates the
conductor with other objects.
90.What is the function of sheath in a cables?
sheath does not allow the moisture to enter and protects the cable from all
external influences like chemical or electrochemical attack fire etc.
91.Mention the conductor materials in cables?
92.What is the purpose of stranding of conductors?
increases the resistance of the cable .It has flexibility.
93.Define the segmental conductors.
stranded wires which are compacted by the rollers to minimize the air spaces
between the individual wires are called segmented conductors .Here the
conductor size is reduced for a given conductance.
94.State the properties of insulating materials.
have high insulation resistance ,high dielectric strength ,good mechanical
properties ,non-hygroscopic, capable of being operated at high temperatures
,low thermal resistance and low power factor.
95.Mention the commonly used power cables.
paper, Polyvinyl chloride, polyethene
96.Mention the advantages of pvc over paper
cost and weight, Insulation is resistant to water, Simplified jointing,
Increased flexibility No plumbing required.
97. State the merits of paper insulated cables.
current carrying capacity ,long life and greater reliability
polythene cables are used?
being non-hygroscopic used in cables for
submarines and damp soil.
being lighter used as aerial cables for
advantages of polythene insulators.
non-hygroscopic, light in weight, low dielectric constant, low loss factorand
low thermal resistance.
100. By what materials cable sheaths are made?
sheaths and Aluminium sheaths.
101.In what way Al sheaths are superior to lead
sheaths are smaller in weight, high mechanical strength , greater conducyivity,
cheap,easy to manufacture and install, withstand the required gas pressure
102.Where CSA sheath is used in cables ?
seamless aluminium sheath is used in high voltage oil filled cables and
103.Why it is used?
used because it is very flexible and easily by repeated bending the sheath is
not distorted and it is not damaged. It has lesser weight and reduced
104.Why protective covering is done in cables?
protect the cables from mechanical damage , corrosion and electrolytic action
when laid direct in the ground the protective covering is made.
105.By what material protective covering is made?
&Bituminized materials, pvc and layers of fibrous materials.
106.What is meant by serving of a cable?
fibrous material permitted with waterproof compound applied to the exterior of
the cable is called serving of a cable.
107.Why armouring is done in the cables ?
protect the sheath from mechanical damage.
108.Why armouring is not done in single core
presence of magnetic material within the alternating magnetic field of a single
core cable produces excessive losses. Hence single core cables are left
unarmoured with non-magnetic materials like tin-bronze or silicon-bronze tapes
109.Why Al is used as an armour material?
It has non-magnetic
properties, high conductivity and mechanical strength.
110.What is meant by grading of cables?
method of equalizing the stress in the dielectric of the cable is called the
grading of cables.
111.Mention the methods of cables.
grading and inter sheath grading.
112.Why the capacitance of the cable is very high
than the capacitance of the overhead lines?
distance between the conductors are small. The distance between the cores and
the earthed sheath is also small. The permittivity of the cable insulation is 3
to 5 times greater than that of air insulation.
113.What is meant by charging current of a cable?
capacitance of a cable determines the charging current. The charging current
restricts the use of cables on EHV lines. The current carrying capacity of an
a.c cable is also reduced by the charging current.
power loss occurs in the dielectric of a cable?
due to conductivity of insulation
hysteresis or dielectric absorption.
the methods of laying the cables.
laying , draw in system and solid system
the advantages of direct laying of cables.
simple and cheaper method. It gives the best conditions for dissipating the
heat generated in the cables.
117.State any 2 disadvantages of direct laying
of fault is difficult . It cannot be used in congested areas where excavation
the disadvantages of pressure cables.
of the pressure tube is high.
the types of gas pressure cables.
and internal pressure cables.
the types of oil filled cables?
core conductor channel cables , Sheath channel cables and 3 core filler space
the types of pressure cables?
filled cables and gas pressure cables.
the operating range of pressure cables?
greater than 66 kv.
the advantages of SL cables over H- type cables?
possibility of core to core breakdown decreases to a large extent. Bending of
cables becomes easy owing to no overall lead sheath.
124. Mention the disadvantages of oil filled
, laying and maintenance of cables is quite complicated.
the types of screened cables?
and SL type cables.
working voltage level of belted cables is limited to 22 kv?
limited because beyond 22 kv tangential stresses acting along the layers of
paper insulation set up large current. This current causes local heating
resulting in the risk of breakdown insulation at any moment.
127. Up to what voltage range are belted cables
Up to 11
kv. In some extra ordinary cases they are used upto even 22 kv.
128. What are the different types of cables that
are generally used for 3 phase service?
cable , Screened cables and pressure cables.