Recent questions tagged cables

Description : What is the type cable used in radiation areas?

Last Answer : Mineral insulated (MI) cables. 

Description : Why bimetallic washers provided in aluminium copper transition joints?

Last Answer : To avoid galvanic corrosion failure.  

Description : Why 1.1 kV grade cables used for 415V?

Last Answer : To take care of the both earthed / unearthed systems.  

Description : What are the types of cables used in 6.6 kV and 415 V system voltages? 

Last Answer : In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors cables are used. Their insulation's are as follows. 1. XLPE - Crossed linked polyethylene insulation. 2. FRLS PVC - Fire ... V grade copper or aluminium stranded cables are used. Their insulations are HR PVC and FS type. 

Description : How the required conductor size can be reduced by use of HRC fuses?

Last Answer : HRC fuse limits the peak amplitude of fault current. HRC fuse melts at prospective current but not allows circuit to pass their high rupturing capacity that is it’s kA. 

Description : Why armouring done?

Last Answer : For mechanical strength, protect against damage by impact of an object.  

Description : How armours /sheaths grounded? 

Last Answer : 1 core cables -- sheaths/shields/armours grounded at one end only, other end insulated to prevent the circulating current through sheaths. 3 core cables -- grounded at both ends but not including core balance CT’s, since even small induced current causes 50N operation. 

Description : What is the specified cable life? 

Last Answer : 50 years.  

Description : What are the advantages of XLPE? 

Last Answer : a. Easy routing at heights b. Easy maintenance c. Large current (90 ºC) d. No sheath (no fatigue) e. No paper tape wrapping technique f. High dielectric strength g. Very little deformation ... i. Low tan delta and hence suits long routes j. Very light k. Good mechanical properties. 

Description : What are the disadvantages of paper insulation? 

Last Answer : a. Absorbs moisture. b. Cable termination/sealing problems. 

Description : What are the purposes of cable trays? 

Last Answer : a. Avoid sagging of cables b. Give mechanical support 

Description : The metallic sheath may be made of lead or lead alloy or of aluminium. (a) Yes (b) No

Last Answer : (b) No

Description : Polyethylene has very poor dielectric and ageing properties. (a) Yes (b) No

Last Answer : (b) No

Description : Rubber is most commonly used insulation in cables. (a) Yes (b) No

Last Answer : (a) Yes

Description : Natural rubber is obtained from milky sap of tropical trees. (a) Yes (b) No

Last Answer : (a) Yes

Description : In congested areas where excavation is expensive and inconvenient 'draw in system' of laying of underground cables is often adopted. (a) Yes (b) No

Last Answer : (a) Yes

Description : In capacitance grading a homogeneous dielectric is used. (a) Yes (b) No

Last Answer : (b) No

Description : It is difficult to maintain oil filled cables. (a) Yes (b) No

Last Answer : (a) Yes

Description : The breakdown voltage of a cable depends on (a) presence of moisture (b) working temperature (c) time of application of the voltage (d) all of the above

Last Answer : (d) all of the above

Description : The disadvantage with paper as insulating material is (a) it is hygroscopic (6) it has high capacitance (c) it is an organic material (d) none of the above

Last Answer : (a) it is hygroscopic

Description : The advantage of oil filled cables is (a) more perfect impregnation (b) smaller overall size (c) no ionisation, oxidation and formation of voids (d) all of the above

Last Answer : (d) all of the above

Description : The insulating material should have (a) low permittivity (b) high resistivity (c) high dielectric strength (d) all of the above

Last Answer : (d) all of the above

Description : Copper as conductor for cables is used as (a) annealed (b) hardened and tempered (c) hard drawn (d) alloy with chromium

Last Answer : (a) annealed

Description : If a power cable and a communication cable are to run parallel the minimum distance between the two, to avoid interference, should be (a) 2 cm (b) 10 cm (c) 50 cm (d) 400 cm

Last Answer : (c) 50 cm

Description : Is a cable is to be designed for use on 1000 kV, which insulation would you prefer ? (a) Polyvinyle chloride (b) Vulcanised rubber (c) Impregnated paper (d) Compressed SFe gas (e) none of the above

Last Answer : (d) Compressed SFe gas

Description : Cables for 220 kV lines are invariably (a) mica insulated (b) paper insulated (c) compressed oil or compressed gas insulated (d) rubber insulated (e) none of the above

Last Answer : (c) compressed oil or compressed gas insulated

Description : The thickness of metallic shielding on cables is usually (a) 0.04 mm (b) 0.2 to 0.4 mm (e) 3 to 5 mm (d) 40 to 60 mm

Last Answer : (a) 0.04 mm

Description : The advantage of cables over overhead transmission lines is (a) easy maintenance (b) low cost (c) can be used in congested areas (d) can be used in high voltage circuits

Last Answer : (c) can be used in congested areas

Description : Underground cables are laid at sufficient depth (a) to minimise temperature stresses (b) to avoid being unearthed easily due to removal of soil (c) to minimise the effect of shocks and vibrations due to gassing vehicles, etc. (d) for all of the above reasons

Last Answer : (c) to minimise the effect of shocks and vibrations due to gassing vehicles, etc.

Description : Capacitance grading of cable implies (a) use of dielectrics of different permeabilities (b) grading according to capacitance of cables per km length (c) cables using single dielectric in different ... introduced at different lengths to counter the effect of inductance (e) none of the above

Last Answer : (a) use of dielectrics of different permeabilities

Description : In a cable the voltage stress is maximum at (a) sheath (6) insulator (e) surface of the conductor (d) core of the conductor

Last Answer : (d) core of the conductor

Description : A cable carrying alternating current has (a) hysteresis losses only (b) hysteresis and leakage losses only (c) hysteresis, leakage and copper losses only (d) hysteresis, leakage, copper and friction losses

Last Answer : (b) hysteresis and leakage losses only

Description : The insulation of the cable decreases with (a) the increase in length of the insulation (b) the decrease in the length of the insulation (c) either (a) or (b) (d) none of the above

Last Answer : (a) the increase in length of the insulation

Description : The breakdown of insulation of the cable can be avoided economically by the use of (a) inter-sheaths (b) insulating materials with different dielectric constants (c) both (a) and (b) (d) none of the above

Last Answer : (c) both (a) and (b)

Description : The electrostatic stress in underground cables is (a) same at the conductor and the sheath (b) minimum at the conductor and maximum at the sheath (c) maximum at the conductor and minimum at the sheath (d) zero at the conductor as well as on the sheath (e) none of the above

Last Answer : (c) maximum at the conductor and minimum at the sheath

Description : The intersheaths in the cables are used to (a) minimize the stress (b) avoid the requirement of good insulation (c) provide proper stress distribution (d) none of the above

Last Answer : (c) provide proper stress distribution

Description : In the cables, sheaths are used to (a) prevent the moisture from entering the cable (b) provide enough strength (e) provide proper insulation (d) none of the above

Last Answer : (a) prevent the moisture from entering the cable

Description : If a cable of homogeneous insulation has a maximum stress of 10 kV/mm, then the dielectric strength of insulation should be (a) 5 kV/mm (b) 10 kV/mm (a) 15 kV/mm (d) 30 kV/mm

Last Answer : (b) 10 kV/mm

Description : A certain cable has an insulation of relative permittivity 4. If the insulation is replaced by one of relative permittivity 2, the capacitance of the cable will become (a) one half (6) double (c) four times (d) none of the above

Last Answer : (a) one half

Description : In cables the charging current (a) lags the voltage by 90° (b) leads the voltage by 90° (c) lags the voltage by 180° (d) leads the voltage by 180°

Last Answer : (b) leads the voltage by 90°

Description : If the length of a cable is doubled, its capacitance (a) becomes one-fourth (b) becomes one-half (c) becomes double (d) remains unchanged

Last Answer : (c) becomes double

Description : Solid type cables are considered unreliable beyond 66 kV because (a) insulation may melt due to higher temperature (b) skin effect dominates on the conductor (c) of corona loss between conductor and sheath material (d) there is a danger of breakdown of insulation due to the presence of voids

Last Answer : (d) there is a danger of breakdown of insulation due to the presence of voids

Description : The relative permittivity of rubber is (a) between 2 and 3 (b) between 5 and 6 (c) between 8 and 10 (d) between 12 and 14

Last Answer : (a) between 2 and 3

Description : Cables, generally used beyond 66 kV are (a) oil filled (b) S.L. type (c) belted (d) armoured

Last Answer : (a) oil filled

Description : The material for armouring on cable is usually (a) steel tape (b) galvanised steel wire (c) any of the above (d) none of the above

Last Answer : (c) any of the above

Description : Pressure cables are generally not used beyond (a) 11 kV (b) 33 kV (c) 66 kV (d) 132 kV

Last Answer : (c) 66 kV

Description : In capacitance grading of cables we use a ______ dielectric. (a) composite (b) porous (c) homogeneous (d) hygroscopic

Last Answer : (a) composite

Description : In the cables, the location of fault is usually found out by comparing (a) the resistance of the conductor (b) the inductance of conductors (c) the capacitances of insulated conductors (d) all above parameters

Last Answer : (c) the capacitances of insulated conductors

Description : PVC stands for (a) polyvinyl chloride (b) post varnish conductor (c) pressed and varnished cloth (d) positive voltage conductor (e) none of the above

Last Answer : (a) polyvinyl chloride

Description : The surge resistance of cable is (a) 5 ohms (b) 20 ohms (c) 50 ohms (d) 100 ohms

Last Answer : (c) 50 ohms