Cable laying abstract

Cable laying abstract

Cable abstract 

Abstract of Installation & Maintenance of Cable upto 33KV

1)    Route Indicator           

Power cable route indicators should be provided at an interval not exceeding 30 M and also at turning points of the power cable route wherever practicable.

2)    Electrolytic corrosion:

Where the possibility of electrolytic corrosion exists, for example, adjacent to dc traction system, the potential gradient along the pipe-line and the cable sheath should be specified.

3)    Neutral

The neutral point is earthed in such a manner that during a line-to-earth fault the highest rms voltage to earth of a sound phase(s) expressed as a percentage of the highest line-to-line voltage, does not exceed 80 percent, irrespective of the fault location,

4)    Earthing

The neutral point is not earthed but a device is installed which automatically and instantly cuts out any part of the system which becomes accidentally earthed. In case of ac systems only, the neutral point is earthed through an arc suppression coil with arrangement for isolation within one hour for the non-radial field cables and within 8 hours for radial field cables, of occurrence of the fault provided that the total of such periods in a year does not exceed 125 hours.

5)    Tensile Strength          

Maximum Permissible Tensile Strength for Cables: PVC and XLPE insulated armoured power cables P = 9 D2,

Where, P=Pulling Strength (N),

D=Outer Diameter of Cable (mm)

Maximum Permissible Tensile Strength for Cables: PVC and XLPE insulated unarmoured power cables P = 5 D2

Maximum Permissible Tensile Strength for Cables: Paper insulated armoured power cables P = 5 D3

6)    Cable Pulling  

For Cables Pulled by Pulling Eye:

Expected Pulling Force When Pulling Cables by Winch:

If the cables are pulled by gripping the conductor directly with pulling eye, the maximum permissible tensile stress depends on the material of the conductor and on their cross-section as given below:

§  Aluminium conductors 30 N/mm2

§  Copper conductors 50 N/mm2″

The following values of pulling force are expected = (approximately percentage of cable weight):

§  In trenches without large bends 15-20 percent.

§  In trenches with 1 or 2 bends of 90° each 20-40 percent.

§  In trenches with 3 bends of 90° each (assuming the use of easy-running support and corner rollers) 50-60 percent.

In ducts with bends totalling 360° Up to 100 percent”

7)    Laying Direct in Ground          

This method involves digging a trench in the ground and laying cable(s) on a bedding of minimum 75 mm riddled soil or sand at the bottom of the trench, and covering it with additional riddled soil or sand of minimum 75 mm and protecting it by means of tiles, bricks

Depth — the desired minimum depth of laying from ground surface to the top of cable is as follows:

a)      High voltage cables, 3.3 kV to 11 kV rating =0.9 m

b)      High voltage cables, 22 kV, 33 kV rating= 1.05 m

c)      Low voltage and control cables = 0.75 m

d)     Cables at road crossings = 1.00 m

8)    Cables at railway

Cables at railway level crossings (measured from bottom of sleepers to the top of pipe) =1.00m

9)    Cable Clearance          

Clearances — the desired minimum clearances are

Power cable to power cable = Clearance not necessary; however, larger the clearance, better would be current carrying capacity.

a)      Power cable to control cables = 0.2 m

b)      Power cable to communication cable = 0.3 m

c)      Power cable to gas/water main = 0.3m

 Inductive influence on sensitive control cable on account of nearby power cables should be checked. The power cable should not be laid above the telecommunication cable, to avoid danger to life of the person, digging to attend to the fault in the Telecommunication cable.

10) Crossing         

Cables laid across Roads, Railway Tracks and Water Pipe Lines:

Steel, cast iron, plastics, cement or earthenware ducts, or cable ducting blocks should be used-

a)      Where cables cross roads and railway tracks.

b)      Spare ducts for future extensions should be provided.

c)      Spare duct runs should be sealed off.

d)     Buried ducts or ducting blocks should project into footpath or up to the edge of road,

Where there is no footpath, to permit smooth entry of cable without undue bending

11) Diameter of Pipe         

The diameter of the cable conduit or pipe or duct should be at least 1.5 times the outer diameter of cable. The ducts/pipes should be mechanically strong to withstand forces due to heavy traffic when they are laid across road/railway tracks.

12) Bending Radius          

The bending radius of steel or plastics ducts should not be less than 1.5 m.

13) Over Bridge     

On bridges, the cables are generally supported on steel cable hooks or clamped on steel supports at regular intervals.

While designing a cable layout on a bridge; expansion of bridge due to changes in atmospheric temperature should be taken into account.

On most of the rail-cum-road bridges, the cables are subjected to vibrations.

For such conditions, round wire armoured and lead alloy ‘B’ sheathed cables are preferred.

Cables can be laid on bridges duly suspended from centenary wire at regular intervals

14) Railway Crossing        

When the cables are laid under railway tracks the cables should be laid in reinforced spun concrete or cast iron or steel pipes at such depths as may be specified by the railway authorities but not less than 1 m measured from the bottom of sleepers to the top of the pipe.

On long run ducts, it is desirable to apply lubrication to the lead or serving/outer sheath as it enters the duct.

Petroleum jelly or graphite powder or a combination of both is effective for this purpose and through lubrication will reduce the pulling tension by about 40 percent.

15) Laying on Racks in Air

The vertical distance between the two racks should be minimum 0.3 m and the clearance between the first cable and the wall (if racks are mounted on wall) should be 25 mm. The width of the rack should not exceed 0.75 m in order to facilitate installation of cables.

Ungalvanized steel work of cable racking/trays should be painted with a coat of primer and thereafter finished with suitable anti-corrosive paint.

Only single-core cables laid on horizontal racks need be clamped at suitable intervals. Multi-core cables need not be clamped. The distance between the vertical clamps should not be more than 2 m.

Laying Cables on Racks Inside a Tunnel: Horizontal distance between Two cable is min Diameter of Cable and vertical distance between two cable row is 30cm.In cable tunnel, the head room should not be less than 2 m and width sufficient to leave a free passage of at least 600 to 800 mm either from one side or in the middle. With temperatures below 3°C, the cables should be warmed before the laying out, since otherwise the bending would damage the insulation and protective coverings of cables. The cable laying must be carried out swiftly, so that the cable does not cool down too much.

 Identification strips/tags of metal or plastics should be attached to the cables, particularly if several are laid in parallel, 8 to 10 m apart. Identification tags should also be attached at every entry point into the buildings and at the cable end termination The spacing between three cables laid in one plane should be not less than the cable diameter.

When the cable run is several kilometres long, the cables should be transposed at one-third and at two-thirds of the total lengths.

16) Trefoil arrangement in ducts   

If several single-core cables are laid per phase, these should be arranged as follows to ensure balanced current distribution

In Horizontal direction: R-Y-B-Distance-B-Y-R, (Distance=2 X Diameter of Cable), vertical distance shall be 6 X Diameter of Cable

17) Insulation Colour          

For reduced neutral conductors, the insulation colour shall be black. For cables having more than 5 cores, the core identification may be done by numbers. In that case, the insulation of cores shall be of the same colour and numbered sequentially, starting with number 1 for the inner layer. The numbers shall be printed in Hindu-Arabic numerals on the outer surface of the cores. All The numbers shall be of the same colour which shall contrast with the colour of the insulation.

When the number is a single numeral, a dash shall be placed underneath it. If the number consists of two numerals, these shall be disposed one below the other and a dash placed below the lower numeral. The spacing between consecutive numbers shall not exceed 50 mm.

18) Type of Armouring:

Where the calculated diameter below armouring does not exceed 13 mm, the armour shall consist of galvanized round steel wires. Where the calculated Diameter below armouring is greater than 13 mm, the armour shall consist of either galvanized round steel wires or galvanized steel strips.

19) Type of Cable Legend:

Improved fire performance or Category C1  FR

Cables in constrained areas, does not propagate fire even when installed in groups in vertical ducts),

Improved fire performance for Category C2 FR—LSH (Cables in constrained areas with limited human activity and/or presence of sophisticated systems)

Aluminium conductor= A,

PVC insulation=Y, Steel round wire armour= W,

Steel strip armour= F,

Steel double round wire armour= WW,

Steel double strip armour =FF,

PVC outer sheath= Y

20) Cable Route Indicator (Up to 33KV)

Route indicators — Power cable route indicators should be provided at an interval not exceeding 200 M and also at turning points of the power cable route wherever practicable.

21) Cable Corrosion (Up to 33KV)

Electrolytic corrosion — where the possibility of electrolytic corrosion exists, for example, adjacent to dc traction system, the potential gradient along the pipe-line and the cable sheath should be specified.

22) Neutral (Up to 33KV)

The neutral point is earthed in such a manner that during a line-to-earth fault the highest rms voltage to earth of a sound phase(s) expressed as a percentage of the highest line-to-line voltage, does not exceed 80 percent, irrespective of the fault location,

23) Earthing   (Up to 33KV)

The neutral point is not earthed but a device is installed which automatically and instantly cuts out any part of the system which becomes accidentally earthed,

In case of ac systems only, the neutral point is earthed through an arc suppression coil with arrangement for isolation within one hour for the non-radial field cables and within 8 hours for radial field cables, of occurrence of the fault provided that the total of such periods in a year does not exceed 125 hours.

24) Cable Tensile Strength (Up to 33KV)

Maximum Permissible Tensile Strength for Cables: PVC and XLPE insulated armoured power cables

P = 9 D2,

Where P=Pulling Strength (N),

D=Outer dia of Cable (mm)

§  Maximum Permissible Tensile Strength for Cables: PVC and XLPE insulated unarmoured power cables P = 5 D2

§  Maximum Permissible Tensile Strength for Cables: Paper insulated armoured power cables P = 5 D3

25) Cable Pulling   (Up to 33KV)

For Cables Pulled by Pulling Eye — If the cables are pulled by gripping the conductor directly with pulling eye, the maximum permissible tensile stress depends on the material of the conductor and on their cross-section as given below: For aluminium conductors 30 N/mm2 and

 For copper conductors 50 N/mm2

Expected Pulling Force When Pulling Cables by Winch — the following values of pulling force are expected:

P = (approximately percentage of cable weight): In trenches without large bends 15-20 %

In trenches with 1 or 2 bends of 90° each 20-40 %

In trenches with 3 bends of 90° each (assuming the use of easy-running support and corner rollers) 50-60 %

In ducts with bends totalling 360° Up to 100 %

26) Cable Laying Direct in Ground (Up to 33KV)

This method involves digging a trench in the ground and laying cable(s) on a bedding of minimum 75 mm riddled soil or sand at the bottom of the trench, and covering it with additional riddled soil or sand of minimum 75 mm and protecting it by means of tiles, bricks

Depth — the desired minimum depth of laying from ground surface to the top of cable is as follows:

High voltage cables, 3.3 kV to 11 kV rating =0.9 m

High voltage cables, 22 kV, 33 kV rating= 1.05 m

Low voltage and control cables = 0.75 m

Cables at road crossings = 1.00 m

Cables at railway level crossings (measured from bottom of sleepers to the top of pipe) =1.00m

27) Cable Clearance (Up to 33KV)

Clearances: The desired minimum clearances are as follow:

Power cable to power cable = Clearance not necessary; however, larger the clearance, better would be current carrying capacity

Power cable to control cables = 0.2 m

Power cable to communication cable = 0.3 m

Power cable to gas/water main =0.3m

Inductive influence on sensitive control cable on account of nearby power cables should be checked

The power cable should not be laid above the telecommunication cable, to avoid danger to life of the person, digging to attend to the fault in the Telecommunication cable.

28) Crossing (Up to 33KV)

Cables Laid Across Roads, Railway Tracks and Water Pipe Lines: Steel, cast iron, plastics, cement or earthenware ducts, or cable ducting blocks should be used where cables cross roads and railway tracks. Spare ducts for future extensions should be provided. Spare duct runs should be sealed off. Buried ducts or ducting blocks should project into footpath or up to the edge of road, where there is no footpath, to permit smooth entry of cable without undue bending

29) Diameter of Pipe (Up to 33KV)

The diameter of the cable conduit or pipe or duct should be at least 1.5 times the outer diameter of cable. The ducts/pipes should be mechanically strong to withstand forces due to heavy traffic when they are laid across road/railway tracks.

30) Bending Radius (Up to 33KV)

The bending radius of steel or plastics ducts should not be less than 1.5 m.

31) Cable on Over Bridge (Up to 33KV)

Cable Over Bridges — on bridges, the cables are generally supported on steel cable hooks or clamped on steel supports at regular intervals. While designing a cable layout on a bridge; expansion of bridge due to changes in atmospheric temperature should be taken into account. On most of the rail-cum-road bridges, the cables are subjected to vibrations. For such conditions, round wire armoured and lead alloy ‘B’ sheathed cables are preferred. Cables can be laid on bridges duly suspended from centenary wire at regular intervals

32) Cable on Railway Crossing (Up to 33KV)

Cables Below Railway Crossing — When the cables are laid under railway tracks the cables should be laid in reinforced spun concrete or cast iron or steel pipes at such depths as may be specified by the railway authorities but not less than 1 m measured from the bottom of sleepers to the top of the pipe

33) Cable on Duct (Up to 33KV)

On long run ducts, it is desirable to apply lubrication to the lead or serving/outer sheath as it enters the duct. Petroleum jelly or graphite powder or a combination of both is effective for this purpose and through lubrication will reduce the pulling tension by about 40 percent.

34) Laying on Racks in Air (Up to 33KV)

Lying on Racks in Air-The vertical distance between the two racks should be minimum 0.3 m and the clearance between the first cable and the wall (if racks are mounted on wall) should be 25 mm. The width of the rack should not exceed 0.75 m in order to facilitate installation of cables.

Ungalvanized steel work of cable racking/trays should be painted with a coat of primer and thereafter finished with suitable anti-corrosive paint.

Only single-core cables laid on horizontal racks need be clamped at suitable intervals. Multi-core cables need not be clamped. The distance between the vertical clamps should not be more than 2 m

35) Laying Cables on Racks inside a Tunnel (Up to 33KV)

Laying Cables on Racks Inside a Tunnel: Horizontal distance between Two cable is min Diameter of Cable and vertical distance between two cable row is 30cm.In cable tunnel, the head room should not be less than 2 m and width sufficient to leave a free passage of at least 600 to 800 mm either from one side or in the middle.

With temperatures below 3°C, the cables should be warmed before the laying out, since otherwise the bending would damage the insulation and protective coverings of cables. The cable laying must be carried out swiftly, so that the cable does not cool down too much

Identification strips/tags of metal or plastics should be attached to the cables, particularly if several are laid in parallel, 8 to 10 m apart. Identification tags should also be attached at every entry point into the buildings and at the cable end termination.

The spacing between three cables laid in one plane should be not less than the cable diameter.

When the cable run is several kilometres long, the cables should be transposed at one-third and at two-thirds of the total lengths.

36) Trefoil arrangement in ducts (Up to 33KV)

If several single-core cables are laid per phase, these should be arranged as follows to ensure balanced current distribution in Horizontal direction: R-Y-B-Distance-B-Y-R, (Distance=2 X Diameter of Cable), vertical distance shall be 6 X Diameter of Cable.

 Note:

CCI has been manufacturing EHV cables from Mid 1980s and introduced 230 kV cables in India in the year 1993, presently upto 400 kV.

Book reference :
A practical guide to cable installation and tool box talk

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