Underground Cables Interview Question - Day 1

[Q:-01] What do you mean by underground cable and where is it used ?

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.

[Q:-02] What are the advantages of using cables for transmission of electrical power as compared to overhead lines ?

Ans :

The advantages are as follows :

(i) Being not exposed to atmosphere cables are free from the interruptions of supply caused by lightning or thunderstorm or birds or other severe weather conditions.

(ii) It reduces accident due to breaking of conductors.

(iii) Maintenance cost is low.

(iv) It does not spoil the beauty of the cities.

(v) These are very much useful in densely populated areas.

[Q:-03] When is a cable termed as L.T. mains and L.T. sub-mains ?

Ans :

      When a three 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. sub-mains.

[Q:-04] What are the different types of cables used in transmission & distribution system ?

Ans :

(a) For low & medium voltage lines P.V.C cables are used.

(b) For high voltage lines XLPE and PILC cables are used.

(c) For extra high voltage lines XLPE, oil filled, gas filled and gas pressure or compression cables are used.

[Q:-05] What are the classifications of the underground cables according to the voltage range ?

Ans :

(a) Low tension cables—(i) 250-440 volts (ii) 650-1100 volts.

(b) High tension cables-(i) upto 3.3 kV. (ii) up to 6.6 kV. (iii) up to 11 kV.

(c) Super tension cables—(i) 22 kV. (ii) 33 kV.

(d) Extra high tension cables—(i) 66 kV. (ii) 132 kV.

(e) Extra super voltage power cables—above 132 kV.

[Q:-06] What are the full names of PVC, PILC, PILCSWA, PILCDSTA & S, XLPE and MIND cables ?

Ans :

P.V.C. cable—Poly Vinyl Chloride insulated cable.

P.I.L.C. cable—Paper Insulated Lead 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.LN.D. cable—Mass Impregnated Non Draining cable.

[Q:-07] What are the essential elements of an underground cable ?

Ans :

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

[Q:-08] Why are stranded conductors used in the cable of sizes above 16 sq mm. ?

Ans :

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

[Q:-09] What are the advantages of circular and sector shaped conductor in cable ?

Ans :

      The interstices between the cores and overall diameter are reduced.

[Q:-10] What do you mean by the terms 'the nominal cross section', 'the actual cross section' and 'the equivalent cross section' of the spiralled stranded conductors of a cable ?

Ans :

      The nominal 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 increase in resistance due to spiralling.

[Q:-11] What are the insulating materials used in underground cables ?

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.

[Q:-12] What are the essential properties of insulation a cable must have ?

Ans :

The insulating materials used in a cable must have the following properties :—

(1) High resistivity.

(2) High resistance against moisture, acids and alkalis.

(3) Non hygroscopic.

(4) Non inflammability.

(5) High flexibility.

(6) Capacity to withstand high rupturing voltages.

(7) Capacity to withstand high temperature without much deterioration.

[Q:-13] What do you mean by the belt paper insulation ?

Ans :

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

[Q:-14] What Is the purpose of metallic sheath in underground cables ?

Ans :

To protect the Insulation against ingress of moisture.

[Q:-15] What materials are generally used for metallic sheath ?

Ans :

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

[Q:-16] What do you mean by bedding of a cable ?

Ans :

      Bedding means a layer of compounded jute or hessain applied over the lead sheath.

[Q:-17] Why is a layer of bedding provided over metallic sheath In underground cable ?

Ans :

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

[Q:-18] What do you mean by the armouring of a cable ?

Ans :

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

[Q:-19] Why is armouring provided in cable ? Under what operating conditions unarmoured cables are used ?

Ans :

      Armouring is provided to protect the cable against mechanical damages caused by direct blows, abrasion, pick-axeing, vibration and longitudinal stresses or any accidental digging.

      If there is no chance of mechanical damages whatsoever the unarmoured cables can be used.

[Q:-20] What are the different types of armouring used in cables ?

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) Hard drawn aluminium wire armour.

[Q:-21] What are the merits and demerits of

(a) Double steel tape armour & (b) Single wire armour ?

Ans :

(a) Double steel tape armour—

Merits : It gives a good protection against impact, mechanical damage and any penetration of sharp bodies.

Demerits : It is inflexible. It has greater loss rather than wire armour.

(b) Single wire armour—

Merits : It has 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.

[Q:-22] What type of armour will you expect in a single core 11 kV cable ?

Ans :

Hard drawn aluminium wire armour is found. .

[Q:-23] Why can wire armoured cables take bends of shorter radius than tape armoured ones ?

Ans :

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

[Q:-24] What are the applications of single steel tape, double steel tape, single wire and double wire armouring ?

Ans :

      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 stresses are not involved.

      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 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 in tunnels etc.

      Double wire armouring is used chiefly 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.

[Q:-25] What do you mean by serving of cable ?

Ans :

      Serving of a cable means the provision of a layer of fibrous material like bedding over and above armouring in order to provide protection against corrosion. Normally impregnated hessian tape is used as serving. In chemical and fertilizer plants likely to be subjected to chemical agression the extruded P.V.C. jacket is used as serving.