CABLE LAYING 101: A COMPREHENSIVE GUIDE

MINIMUM PERMISSIBLE BENDING RADII OF CABLE LAYING.

The electrical power cables should not be bent in a very sharp radius in the process of cable laying. The minimum bending radius for cable laying as per IS-1255 is listed below. It is to be noted that D is the outer diameter of the cable.

Power Cables Up to 1.1KV

• The minimum bending radius for PILC cable laying with a single core is 20D and the multi-core is 15D.
• The minimum bending radius for PVC and XLPE cable laying with a single core is 15D and for multicore cable it is 12D.

Power Cables above 1.1 KV to 11KV

• The minimum bending radius for PILC cable laying with a single core is 20D and multi core is 15D.
• The minimum bending radius for PVC and XLPE cable laying with the single core is 15D and for multi-core cable, it is 15D.

Power cables above 11KV

• The minimum bending radius for PILC cable laying with a single core is 25D and multi core is 20D.
• The minimum bending radius for PVC and XLPE cable laying with the single core is 20D and for multi-core cable it is 15D.
• At joints and termination of cables, in cable laying, the bending radius should be above 12 times the diameter of the insulation.

MAXIMUM PERMISSIBLE TENSILE STRENGTH (PULLING) OF POWER CABLES

Considering, P= Pulling force in Newtons and D= The diameter of the cable in millimeters, and the maximum permissible tensile strength of cables are:

• For PVC and XLPE insulated armoured cable: P = 9D²
• For PVC and XLPE insulated un-armoured cable: P = 5D²
• Paper insulated armoured power cable belted and H type: P = 3D²
• HSL cables: P = D²

For cables pulled by gripping the conductors directly by pulling eyes, the maximum tensile strength for the cross-section of,

• Aluminum conductors: 30 N/mm²
• Copper conductors: 50 N/mm²

Expected percentage pulling force when cables are pulled by a winch

• In trenches with large bends: 15-20%
• In trenches with 1-2 bends with 90 degrees each: 20-40%
• In trenches with 3 bends of 90 degrees each assuming supports at corners: 50-60%
• In trenches for bends totalling 360 degrees: 100%

Belt type power cables:

In these types of cables, the conductors are bunched together, impregnated paper insulated, and bonded by a paper belt. Fibrous dielectric materials like jute are used to fill the gap between the insulation and the belt and then armoured with a metallic screen.

H-type power cable:

These have three paper-insulated cores covered by perforated metallic screens which are grouped together by a metallic tape surrounded by a lead sheath. Both the sheath and metallic screen are grounded.

S-L type power cable:

These are similar to H-type cables with lead sheaths for individual cores. The advantage is that core-to-core breakdown is minimized in this type of construction. However if subjected to moisture, the dielectric strength may suffer badly as the individual seaths are leanner.

DEPTH OF POWER CABLES LAYING:

The desired minimum depth of laying cable from the ground surface to the top of the cable should be as below:

• For cables, 3.3 KV to 11 kV Voltage rating: 0.9m
• Cables, 22 kV, 33 kV Voltage rating: 1.05 m
• Low voltage and control cables = 0.75 m
• Cables at road crossings: 1.00 m

(Measured from the bottom of sleepers to the top of the pipe)

CLEARANCES IN CABLE LAYING:

The desired minimum clearances are as follows.

Power cable to power cable – Clearance is not necessary. However, the larger the clearance, the better would be current carrying capacity. This is because heat can be transferred by conduction, convection, and radiation. And heating of cables lowers its current carrying capabilities.

• Power cable to control cables: 0.2m
• Power cable to communication cable: 0.3m
• Power cable to gas/ water main: 0.3m.

CABLES LAYING ACROSS SERVICE ROADS:

• Hume pipe/ ‘B’ grade GI pipe of suitable size shall be used where the cable crosses roads. Spare ducts for future extensions shall also be provided.
• The duct/ pipe joints shall be covered by collars to prevent settlement in between pipes.
• The diameter of the cable conduit or pipe/duct shall be at least 1.5 times the outer diameter of the cable. The ducts/pipes shall be mechanically strong to withstand forces due to heavy traffic when they are laid across the road/ railway tracks.
• The cable entry and exit shall be through bell mouth or padding.
• The bending radii of steel or plastic ducts shall not be less than 1.5m.
• Single core cables shall not be laid individually in steel ducts but instead, all three cables of the same system shall be laid in one duct.

CABLE LAYING ON RACKS

• The space for cable racks has to be sufficient as they are fixed to the walls with the help of a support structure all fabricated for easy installation and easy replacement.
• The vertical clearance between two racks must be 0.3 meters and the distance between the cable and the wall must be at least 25mm. The width of the rack or tray shouldn’t exceed 0.75 meters.
• Each tray should contain one layer of cable with spacers or without. Stacking cables one above other in 2-3 layers on a single tray or rack can potentially decrease the current carrying capacity of the conductors greatly.
• Ungalvanized steel used in the fabrication of the racks or trays should be painted with primer and then anti-corrosive paint should be layered for proper finishing.
• Only single-core cables can be clamped at suitable intervals on horizontal racks. Multicore cables need no clamping. The distance between vertical clamps must be less than 2 meters.

Cables can also be laid with the help of J-hooks mounted on the walls or held by catenary wires.

When cables are laid in unventilated tunnels, the heat developed cannot escape the tunnel, which reduces the current carrying capacity of the cable. The heating of the air depends on the power loss from the cable and can be calculated by:

T=W/3P, where T is the rise in temperature above the ambient in degrees Celsius, W is the power lost per meter length of the cable and P is the perimeter of the tunnel.

It must be noted that only the surfaces that dispose of the heat should be taken in the perimeter calculation.