L.T. (Low Tension) cables are widely used for low voltage (1.1kV). These cables must have proper protection from water, corrosion and mechanical strength must be high. These cables must sustain a longer life for that various protections are required. Let’s study what all things are required for the protection of cable.
In this type of cables: –
- The electrostatic stresses are less.
- The thermal conductivity is also very less.
The materials used for construction of L.T. cables are: –
- Paper and cotton tape
- Lead sheath
- Serving
- 1st lead sheath
- 2nd lead sheath
- Brass tape.
The conductors in the cable are surrounded by lead sheath with an insulating layer of paper insulation.
Compounded fibrous material is surrounded around the lead sheath. The fibrous compounded material & wire armored is covered around the cable.
Multi-core cables are of 2 types:
- Shaped conductors
- Round conductors
The copper space factor is good in case of shaped conductors & it is given by formula
In case of shaped conductors, the overall diameter is very less. The whole insulation is provided by impregnated paper only. A paper belt is provided around the conductors & the fillings between them are filled with fibrous insulating material. Again a lead sheath with fibrous material is provided with paper belt.
The standards applicable on these cables are
- IS 1554
- BS 6346
- IEC 60502
These cables are generally manufactured with copper conductor of size 1.5 sqmm, 2.5 sqmm, and 4 sqmm.
General Construction – LT XLPE / PVC Power Cables
Conductor
The conductor of power cables is made from EC grade aluminium or annealed electrolytic copper. All conductors conform to IS: 8130. The conductors are solid / stranded / compact circular / sector shaped.
Insulation
In PVC Power Cables the conductor is insulated with the poly vinyl chloride covering over conductor as insulation by extrusion process and In XLPE Power Cables cross linked polyethylene covering as insulation over conductor by extrusion process.
For identification of cores the following colour scheme is adopted.
| 1 Core | Red, Black, Yellow, Blue |
| 2 Cores | Red & Black |
| 3 Cores | Red, Yellow & Blue |
| 3.5 & 4 Cores | Red, Yellow, Blue & Black (Reduced Neutral Core in case of 3.5 Core |
General Construction
Laying Up
In multicore cables, cores are laid up together with a suitable lay, the outer most layer is laid up in right hand and successive layer with opposite lay. 
The interstices are filled with non-hygroscopic material wherever necessary to make the laid-up cores circular.
Inner Sheath
The laid up cores are provided with the PVC inner sheath by extrusion or by wrapping of PVC Tape. Single core cables do not have inner sheath.
Armouring
Armouring is applied over inner sheath in case the calculated diameter over inner sheath does not exceed 13 mm, the armour consists of galvanized round steel wires. Where the calculated diameter over the inner sheath is greater than 13 mm the armour generally consists of flat steel. If required, the cable can be manufactured with steel wire where calculated diameter of cable exceeds 13 mm. Single core cables are provided with Aluminium Wire/Strip armouring.
Outer Sheath
Outer Sheath is of PVC compound applied by extrusion over Insulation in case of unarmoured single core cables, over inner sheath in case of unarmoured multicore cables & over armouring in case of armoured cables.
| Constituents | Code |
| Aluminium conductor | A |
| XLPE insulation | 2X |
| PVC insulation | Y |
| Steel round wire armour | W |
| Steel strip armour | F |
| Steel double round wire armour | WW |
| Steel double strip armour | FF |
| Non-magnetic (A1) round wire armour | Wa |
| Non-magnetic (A1) strip armour | Fa |
| PVC outer sheath | Y |
Features
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LT-XLPE Cables have longer life as compared to conventional PVC Cables. |
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LT-XLPE Cables have a higher conductor temperature rating i.e. 90°C |
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LT-XLPE Cables have a higher emergency overload capacity 120°C. |
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Max. temperature limit under short circuit conditions for LT-XLPE Cables is 250°C |
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Hence XLPE Cables have higher short circuit rating. |
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Insulation resistance of LT-XLPE Cable is excellent & superior to Identical PVC Cables. |
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LT-XLPE Cables have high corrosion resistance in polluted atmosphere. |
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LT-XLPE Cables have better properties of resistance to chemical and corrosive gases. |
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LT-XLPE Cables have low installation cost because of light weight, dimensions and are far more flexible. |
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Jointing of LT-XLPE Cables is easier and quicker. |
General Construction – LT Copper Control Cables
VOLTAGE :
These cables can be used on AC Voltage up to & including 1100 V or DC up to & including 1500 V.
Size :
1.5 Sq. mm. & 2.5 Sq. mm. up to 61 Cores in Control Cable.
CONDUCTOR :
Annealed Bare Electrolytic Copper, Solid / Stranded conforming to IS:8130:1984.
INSULATION :
Conductor are insulated with PVC Compound as per IS:5831:1984. On demand insulation is provided with XLPE also.
COLOR OF CORES :
Cores are identified with a colour scheme as per IS:1554(Pt-I):1988 as under
1 Core – Red, Black, Yellow, Blue
2 Cores – Red & Black
3 Cores – Red, Yellow & Blue.
3 ½ & 4 Cores – Red, Yellow, Blue & Black. (Reduced Neutral Core in case of 3 ½ Core)
5 Cores – Red, Yellow, Blue, Black & Grey
In case of cable exceeding five cores, two adjacent (counting and direction cores ) in each layer shall be coloured Blue, Yellow and remaining cores grey / Cores can be identified by numbers by numerals on the outer surface of the core as per IS : 1554 : Part-1 : 1988 & IS : 7098 : Part-1 : 1988 or as per customer requirement.
LAYING Of CORES :
Cores are laid up with a suitable lay. The Final layer direction shall be kept right hand lay.
INNER SHEATH :
The Inner Sheath is applied over laid up of cores by extrusion.
ARMOURING :
Armouring is applied over inner sheath in case the calculated diameter over inner sheath does not exceed 13 mm, the armour consists of galvanized round steel wires. Where the calculated diameter over the inner sheath is greater than 13 mm the armour generally consists of flat steel. If required, the cable can be manufactured with steel wire where calculated diameter of cable exceeds 13 mm. Single core cables are provided with Aluminium Wire/Strip armouring.
OUTER SHEATH :
A final covering of PVC Compound, conforming to IS:5831:84, is applied over Armoring in case of Armored Cable or over Inner Sheath in case of Unarmoured Cable, called as “Outer Sheath”.
The Insulation, Inner Sheath & outer sheath can be HR PVC, FRLS PVC or FRHF Compound depending upon their application.
Applications of L.T. cables are: –
- Interconnection of process control in chemical plant, Petrochemical plant, Thermal plant etc.
- Communication & signaling
REFERENCES: –
- ELECTRICAL POWER SYSTEMS BY UPPAL , Sunil S RAO