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Class 12 Power Chapter 2 Maintenance of the Cable System
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Maintenance of the Cable System
Chapter: 2
| SESSION 1: LAYING OF UNDERGROUND CABLE |
| Exercise |
A. Fill in the blanks with Suitable answers.
1. ____________ is used for branching of a service cable from a main cable.
Ans: T-Joints.
2. Three faults that cables are tested for are _________, discontinuity and earth fault.
Ans: Short circuiting.
3. __________. are overhead power lines using several insulated phase conductors, contrasting with the traditional practice of using un-insulated conductors separated by air gaps.
Ans: Aerial Bundled Cables (ABC).
B. Choose the correct answer from the following options.
1. Mid – span joining should be avoided in the case of HT lines.
(a) True.
(b) False.
Ans: (a) True.
2. Which of these are disadvantages of AB Cables (you can choose more than 1):
(a) Additional cost.
(b) Electricity theft is easy from these cables.
(c) Increased weight.
(d) Cluttered appearance.
Ans: (a) Additional cost.
(c) Increased weight.
(d) Cluttered appearance.
4. Laying the cables is important for power distribution. So, no clearances are required.
(a) True.
(b) False.
Ans: (b) False.
C. Match the elements of Group A with Group B.
| Group A | Group B |
| 1. A cable test | (a) Yarlay Loop Test |
| 2. This test is used to locate the fault of the long length of Cables. | (b) AB cables |
| 3. Short spans, more poles Needed | (c) Murray Loop Test |
Ans:
| Group A | Group B |
| 1. A cable test | (c) Murray Loop Test |
| 2. This test is used to locate the fault of the long length of Cables. | (a) Yarlay Loop Test |
| 3. Short spans, more poles Needed | (b) AB cables |
D. Answer the following:
(a) Classify various types of cables.
Ans: Types of cables are classified in different ways such as based on their use, voltage and type of insulation used for their construction.
(i) PVC (Poly Vinyl Carbide Cable).
(ii) PILCA (Paper Insulated Lead Covered Armoured Cable).
(iii) XLPE (Cross Linked Poly Ethylene Cable).
(b) List tools and equipment used in cable laying activities.
Ans: 11 kV cable laying from cables drums using erection stools & pulling pulleys.
Other tools for cable laying are: Cable pulling winch, Cable guiding device and Cable pulling grip etc.
Additional
A. Short Answer Questions:
1. What are the conductors used in underground cables?
Ans: Conductors used in underground cables are aluminium and copper.
2. Name the three types of cables classified according to insulation used.
Ans: (i) PVC (Poly Vinyl Carbide Cable).
(ii) PILCA (Paper Insulated Lead Covered Armoured Cable).
(iii) XLPE (Cross Linked Poly Ethylene Cable).
3. Question: What are the three major components of power cables?
Ans: Cables consist of three major components: conductors, insulation, and protective jacket.
4. Question: What are the three types of cable joints?
Ans: Cable joints are of three types: Straight through Joint, T-Joint and Terminal Joint.
5. Name any two tests used for testing underground cables.
Ans: (A) Murray Loop Test (B) Yarlay Loop Test.
B. Long Answer Questions:
1. Question: Explain the disadvantages of PILCA cables.
Ans: (a) Heavy due to lead sheathing.
(b) Less flexibility.
(c) Poor resistance to vibration.
(d) Difficulty in sealing and jointing.
2. Describe the advantages and disadvantages of the underground cable system.
Ans: Advantages:
(i) Selection of route length is easy and simple.
(ii) Safety factor is very high.
(iii) Maintenance cost is almost negligible.
(iv) The system has good power factor.
(v) The system can be laid across zigzag and sharp bend routes with care.
Disadvantages:
(i) In case of any fault, the time taken for repairs will be too much.
(ii) Very costly in comparison to the O.H. system because of its initial cost and laying methods.
(iii) Additional load cannot be taken up by the existing cables hence necessitates the laying of new cables in case of load expansion.
3. Question: Explain the constructional parts of a High-Tension (PILCA) Cable.
Ans: (a) Conductor – Copper or Aluminium, stranded, sector shaped.
(b) Insulation over core – Impregnated Paper insulation wrapped over each conductor.
(c) Fillers – placed in between the cores, made of Jute.
(d) Insulation over all cores – Belted paper insulation wrapped over all the cores.
(e) Lead sheath – To protect the paper insulation from foreign elements and mechanical shock.
(f) Bedding – Protects the lead sheath against corrosion. Consists of bitumen compound and impregnated cotton tapes.
(g) Armour – Provides mechanical strength to the cable, made of steel tape or round galvanised wire.
(h) Serving – protects the armour from corrosion, made of Jute yarns coated with bitumen compound.
4. Question: Explain the Straight-Through Joint and its features.
Ans: Straight Through Joints form an integral part of today’s power cable networks. These joints offer reliability and flexibility to meet the demands of cable network operators. Straight through Joint provides: Quick cable preparation, High electrical insulation, No moisture ingress, Good mechanical strength, Compact dimensions, Suitable for all conductor, shape and material. Straight through Joints are made by metal joining processes such as welding and soldering.
5. Question: Explain the Murray Loop Test used for testing underground cables.
Ans: The Murray Loop Test is based upon the principle of Wheat-stone bridge, and bridge megger is used for this purpose. Use: The Murray Loop Test is used for the location of faults on lines of low resistance such as power cables and telephone cables. Operation: The value of the resistance box (R.B) and resistance dial of the instrument is changed till the galvanometer shows null point i.e. zero defection. With this method we will be able to locate the distance of faulty section of the cable (‘X’ as shown in the figure below). Connections: The conductors are shorted at its far end by thick wire. Precautions: 1. For carrying out Murray Loop Test the only requirement is that all the core conductors should be of same cross-section. 2. If the sound and faulty conductors are of different sizes, it should be taken into account. In high voltage cables, a high voltage D.C. is applied to the bridge network.
| SESSION 2: LAYING OF AB CABLES |
CHECK YOUR KNOWLEDGE
A. Tick the correct answer:
1. Aerial Bundled Cables are the same as bundle conductors.
(a) False.
(b) True.
Ans: (a) False.
2. Cable joints should be near the support.
(a) False.
(b) True.
Ans: (b) True.
3. Messenger wire is not used as earth wire.
(a) False.
(b) True.
Ans: (a) False.
4. Write short note on the following :
(a) Jointing of ABC cable.
Ans: Mid-span jointing is allowed in LT ABC systems but recommended to be at the supports. Mid-span jointing is not recommended for HT lines. If unavoidable, tapping at support points through suitable clamp connectors may be allowed. Conducting screen continuity must be maintained, and 3-phase screens may be earthed through surge arrestors.
(b) Types of AB cables.
Ans: (i) LT Lines:
Three Phase Four Wire: 3×150mm² + 1×95mm² (neutral) + messenger wire of 25mm².
Three Phase Five Wire (for street light): 3×150mm² + 1×95mm² + messenger wire 25mm² + 1×16/25mm² (street light).
Other type: 3×95mm² + 1×50mm² + messenger wire 25mm².
(ii) HT Lines (11kV):
3×150mm² + messenger wire 25mm²
3×95mm² + messenger wire 25mm²
B. Explain the following:
(a) Explain the importance of various cable joints.
Ans: Cable joints are essential to connect or extend cables safely and maintain electrical continuity. Proper jointing at support points ensures minimal stress, avoids mid-span failures, and maintains insulation and conductor integrity in both LT and HT lines.
(b) List the advantages of aerial bundled cables.
Ans: Advantages of Aerial Bundled Cables:
(i) Relative immunity to short circuits.
(ii) Safe near trees—no sparking if touched.
(iii) Simpler installation (no insulators/crossbars).
(iv) Less cluttered appearance.
(v) Requires narrower right-of-way.
(vi) Fewer splices at junction poles.
(vii) Increased safety against neutral break.
(viii) Electricity theft is harder and easily detectable.
| SESSION 3: CONDUCTOR JOINTING |
| Exercises |
A. Fill in the blanks with Suitable answers.
1. ____________ is used for branching of a service cable from a main cable.
Ans: T-Joints.
2. Three faults that cables are tested for are _________, discontinuity and earth fault.
Ans: Short circuiting.
3. __________. are overhead power lines using several insulated phase conductors, contrasting with the traditional practice of using un-insulated conductors separated by air gaps.
Ans: Aerial Bundled Cables (ABC).
B. Choose the correct answer from the following options.
1. Aerial Bundled Cables are the same as bundle conductors (True/False)
Ans: False.
2. Mid – span joining should be avoided in the case of HT lines. (True/False)
Ans: True.
3. Laying the cables is important for power distribution. So, no clearances are required. (True/False)
Ans: False.
C. Which of these are disadvantages of AB Cables (you can choose more than 1):
(a) Additional cost.
(b) Electricity theft is easy from these cables.
(c) Increased weight.
(d) Cluttered appearance.
Ans: (a) Additional cost.
(c) Increased weight.
(d) Cluttered appearance.
D. Match the elements of Group A with Group B.
| Group A | Group B |
| 1. A cable test | (a) Yarlay Loop Test |
| 2. This test is used to locate the fault of the long length of Cables | (b) AB cables |
| 3. Short spans, more poles Needed | (c) Murray Loop Test |
Ans:
| Group A | Group B |
| 1. A cable test | (a) Yarlay Loop Test |
| 2. This test is used to locate the fault of the long length of Cables | (c) Murray Loop Test |
| 3. Short spans, more poles Needed | (b) AB cables |
E. Answer the following:
(a) Explain the different types of LT cable jointing.
Ans: Different types of conductor jointing are as follows:
(i) Britannia Joint: Used only for solid conductors (not stranded).
Conductors are cleaned, bent, and bound by copper wire for a strong electrical connection.
(ii) Telephone (Western Union) Joint: Used for solid conductors of size 8 No. or higher.
Ends bent and twisted together after cleaning 200–250 mm of conductor.
(iii) Married Joint: Made between copper conductors having G.I. central strand.
Strands are unwound, cleaned, woven together, and soldered. Used for small span length.
(iv) ‘T’ Joint: Used for stranded conductors in jumpers or tapping at substations.
Strands separated and twisted over the horizontal conductor.
(v) Sleeve Joint: Used for any type of Aluminium conductor.
Cleaned with caustic soda, greased, and twisted with Al sleeves by wrench.
(vi) Compression Joint: Used for large-size conductors (>0.06 cm²).
Uses steel and aluminium sleeves, compressed with a compression machine for strong and continuous connection.
(b) List steps used in preparing the cable.
Ans: (i) Clean the conductor with wire brush or sandpaper to remove rust.
(ii) Wash with soft soda water or caustic soda solution (for Aluminium).
(iii) Dry the conductor properly.
(iv) Apply graphite grease over the conductor (for sleeve joints).
(v) Measure and mark sleeve lengths accurately before compression.
(vi) Ensure all joints are tight and compressed properly using a wrench or compression machine.
(vii) Close holes in sleeve by rivets and hammer for complete sealing.
| SESSION 4: CABLE JOINTING |
| Exercise |
1. Write a short note on:
(a) LT cables Joints.
Ans: LT (Low Tension) cable joints are used to connect or extend cables in electrical installations. These joints can be straight or branch type depending on the need. They ensure proper electrical connection, mechanical strength, and insulation between cables. The methods used for LT cable jointing include Western Union splice joint, Fixture joint, Knotted tap joint, joints using wire nut and split bolt, and joints for armoured cables.
(b) Western union splice joint.
Ans: This is a straight joint used for small solid cables.
Steps: (i) Remove the insulation.
(ii) Bring the two conductors to a crossed position and then make a long bend or twist in each wire.
(iii) Wrap the end of one of the wires around the straight portion of the other wire, and then do the same for the other wire (4–5 times).
(iv) Press ends of the wires down close to the straight portions to prevent the ends from piercing through the insulation tape.
(v) Insulate the joint using tape.
(c) Knotted tap joint.
Ans: The knotted tap joint is used for branch joints to connect a branch wire to a continuous wire.
Steps: (i) Remove about 1 inch of insulation from the main wire and about 3 inches from the branch wire.
(ii) Place the branch wire behind the main wire so that three-fourths of its bare wire extends above the main wire.
(iii) Bring the branch wire over the main wire, around itself, and finally over the main wire so that it forms a knot.
(iv) Wrap the wire around the main conductor in short, tight turns and trim its end.
2. Mark True or false:
1. Western Union joint are used for all conductor (True/False).
Ans: False.
2. Crimping is necessary for jointing the cable (True/False).
Ans: True.
3. Answer the following:
(a) List steps used in preparing cable.
Ans: (i) Remove the over sheath and the wire armor.
(ii) Separate the wire armor and bend the wires away from the cable, place the support ring under the armor at each side of the joint.
(iii) Cut back the cable insulation.
(iv) Remove the insulation from each of the conductors.
(b) Explain different types of LT cable jointing.
Ans: The different types of LT cable jointing are:
(i) Western Union Splice Joint – a straight joint for small solid cables.
(ii) Fixture Joint – a branch joint connecting a small-diameter wire to a large conductor (used in lighting fixtures).
(iii) Knotted Tap Joint – a branch joint connecting a branch wire to a continuous wire.
(iv) Wire Nut Joint – a plastic-insulated connector used instead of a rattail joint.
(v) Split Bolt Connector Joint – used for large conductors and can join three ends or a branch wire to a continuous conductor.
(vi) Joints for Armoured Cables – made using crimp connectors, mesh tape, constant force springs, and PVC tape for mechanical and electrical strength.
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