Ans : The cable buried in ground for transmission and distribution of electrical power is known as underground cable. These are used in large cities and other densely populated areas where overhead construction in such area is practically impossible.

Ans : The advantages of the underground cable are as follows :-

1. Being exposed to atmosphere cables are free from the interruptions of supply caused by lightning or thunderstorm or birds or other severe weather conditions.
2. It reduces accident due to breaking of conductors.
3. Maintenance cost is low.
4. It does not spoil the beauty of the cities.
5. These are very much useful in densely populated areas.

Ans : When a there phase, four core cable is used as a distributor it is known as L.T. mains and when a single phase two core cable is used as a distributor it is known as L.T. submains.

Ans : (a) For low & medium voltage lines P.V.C cables are used. (b) For high voltage lines XLPS and PILC cables are used. (c) For extra high voltage lines XLPE, oil filled, gas filled and gas pressure or compression cables are used.

Ans :

1. Low tension cables – (1) 250-440 volts (2) 650-1100 volts.
2. High tension cables – (1) upto 3.3 kV. (2) up to 6.6 kV (3) up to 11 kV.
3. Super tension cables – (1)66 kV. (2) 132 kV.
4. Extra super voltage power cables – above 132 kV.

Ans : P.V.C. cable – Poly Vinyl Chloride insulated cable.

P.I.L.C. cable – Paper Insulated Covered cable.

P.I.L.C.S.W.A. cable – Paper Insulated Lead Covered Steel Wire Armoured cable.

P.I.L.C.D.S.T.A. cable – Paper Insulated Lead Covered Double Steel Tape Armoured cable.

XLPE cable – Cross Linked Polyethylene Insulated cable.

M.I.N.D. cable – Mass Impregnated Non Draining cable.

Ans : The essential of an underground cable are the conductor, the insulation, the screening, the sheathing, the bedding, the protection and the serving.

Ans : Stranded conductors are used to increase the surface area of the conductor and also to make the cable flexible and more workable.

Ans : The interstices between the cores and overall diameter are reduced.

Ans : The normal cross section is the area of the cross section of one conductor in a plane perpendicular to its length multiplied by the number of conductors.
The actual cross section is the area of the oblique cross section of a conductor produced by cutting the stranded conductor by a plane perpendicular to the core of the cable multiplied by the number of conductors, which is also greater than the nominal cross section.
The equivalent cross section is the cross section of a solid conductor of the same length as the cable and having the same resistance at the same temperature, which is less than the nominal cross section because of the incase in resistance due to spiraling.

Ans : P.V.C. Impregnated paper, Vulcanized rubber, Varnished cambric are used in L.T. cables while Impregnated paper, Cambric, XLPE are used in H.T. cables.

Ans : Belt paper is an oil impregnated paper, the principal insulating part of the cable, which is essentially composed of cellulosed fibres felted to from a mechanically strong sheet.

Ans : To protect the insulation against ingress of moisture.

Ans : Lead, Lead alloy and Aluminium are generally used for metallic sheath.

Ans : Bedding means a layer of compounded jute or hessian applied over the lead sheath.

Ans : To protect the metallic sheath against the corrosion and from mechanical injury from the armouring.

Ans : Armouring of a cable means the provision of layers of galvanized or ungalvanised steel wires or steel wires or steel tapes over the bedding.

Ans : Armouring is provided to protect the cable against mechanical damages caused by direct blows, abrasion, pink-axeing, vibration and longitudinal stresses or any accidental digging. If there is no chance of mechanical damages whatsoever the nuarmoured cables can be used.

Ans : The different types of armouring are as follows :
(a) Galvanised or ungalvanised steel tape, (b) Galvanised round steel wire, (c) Galvanised flat steel strip and (d) Hare drawn aluminium wire armour.

Ans : (a)Double steel tape armour –
Merits : It is gives a good protection against impact, mechanical damage and any penetration of sharp bodies.
Demerits : It is inflexible. It has grater loss rather than wire armour.
(b) Single wire armour –
Merits : It is higher longitudinal strength, more flexibility and greater electrical conductivity with minimum loss rather than steel tape armour. It can be easily handled with short bend.
Demerits : Mechanical protection is poor rather than steel tape armour.

Ans : Hard drawn aluminium wire armour is found.

Ans : Wire armoured cables can take bends of shorter radius due to higher longitudinal strength of wire armouring than that of tape armouring.

Ans : Single steel tape armouring:Single steel tape armouring is used principally for parkway cables, cables suspended on a messenger or clamped at intervals in shaft and tunnel installation or cables buried directly underground where a mechanical outer protection is required but longitudinal are not involved.

Double steel tape armouring: Double steel tape armouring is used particularly for low tension cables installed indoors or aerially outdoors or directly buried underground where a good protection against crushing or penetration by sharp bodies is required and not likely to be subjected to tensile stresses.

Single wire armouring: Single wire armouring is used in cables where the armouring is subjected to have longitudinal strength such as for cables pulled into duct, used for river crossing, laid in marshy ground or in colliery districts where soil subsidence may occur, vertical cable runs and for cables hung on brackets along side railways or tunnels etc.

Double wire armouring: Double wire armouring is used chiefty for vertical run of large size cables as in a mine shaft, submarine or dredger where easy handling and greater flexibility for the same conductivity with high longitudinal strength is required.

1. What is called “Barrier joint” of cable ?
2. Interview Question of Underground Cable
3. What are varnished cambric insulated cables ?
4. What do you mean by XLPE cable and what are its advantages over conventional cables ?
5. What do you mean by MIND cable and where is it used ?
6. What do you mean by the term “Sheath losses” ?
7. What is the procedure of reducing the sheath losses in sheaths of the three single core cables in a 3-phase system ?
8. What do you mean by the term “Dielectric losses” ?
9. What do you mean by thermal resistance of a single core cable ?
10. What are the load conditions of an underground cable ?
11. what special cares are to be taken for storage of cable drum ?
12. What are the methods of cable laying ?
13. What is soil thermal resistivity ?
14. What is proximity effect ?
15. What are the procedures to be adopted for determining the types of cable faults ?
16. What precaution should be taken while taken a cable through trenches in the following locations (a) Road crossing, (b) Railway crossing & (c) Crossing of water mains ?
17. What is called “Heat shrinkable joint” of cable?
18. What can be the causes of damage to PVC cables and paper insulated cables?