“Voltage & current are the parameters upon which selection of the cable depends !”
- The size of the conductor of a cable represents the cross sectional area of the conductor which is directly links to its current carrying capacity. (More the cross sectional area of the conductor; higher the current carrying capacity !)
- The insulation level of the cable is directly links with the voltage. (Higher voltage demands higher insulation!)
That’s the reason HT(high tension) cables contains high level of insulation in comparison with the low voltage cables.
In this post, we are going to discuss HT cable, HT cable configuration & its termination process.
HT Cables
Voltage ranges from 650volts to 33000volts is classified as high voltage in INDIA. To deliver the power at high voltage; HT cables are required. HT cables are available in single core & multicores. HT cables of same mm²(thickness) rating are thicker in size as compare to LT cable of same mm² rating. The reason of higher thickness is; layers of Safeties such as the layer of XLPE insulations, copper tape screen, inner sheath & armour makes the cable thicker.
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Where are HT Cables Used?
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Industrial plants
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Substations
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Commercial buildings
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Power distribution systems
HT cable configuration
Insulation materials are made up of two type of materials which are Thermoplastic & Thermosetting.
- Thermoplastic are those materials which can be heated, molded & shaped again & again by heating.
- On the other hand, Thermosetting are those materials which can be heated, molded & shaped only once, Its shape can not be altered like thermoplastic material.
High voltage needs high safety. The insulation around the conductor in HT cable is made up from thermosetting material which is crosslinked polyethylene(XLPE). The layer of XLPE insulation around the conductor is very thick But insulation thickness doesn’t guarantee the safety !
High Voltage stress in HT Cables
A potential always create an Electric field stress around the insulation & High potential means high Electrical field stress around the insulation .
High electrical stress around the insulation can damage the insulation . That’s why for the safety of cables, humans & equipment’s; Additional internal layer of earthing around the XLPE is provided to control the electrical field stress around each conductor.
XLPE Insulation of each core is coated with semi-conductive paint & Around that conductive paint; copper sheet is wrapped in helix shape. In cable termination process the copper sheet from all the cores & metallic armour is earthed from both the open ends of the cable.
” Earthing around each core diminish the Electric field stress & interaction in between the cores. “
Types of HT Cable
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Single core
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3 core
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Armoured / Unarmoured
Configuration of HT Cable
In 3 core HT cable,
Conductor is insulated from XLPE insulation. Insulation is further covered with semi conductive paint & copper wrap.
All the three cores are round that’s why Void get created in between the cores. To fill the void In between the cable cores; round PVC filler is provided to fill the void. This filler make the cable equally round from all the sides of cable. After that, inner sheath is wrapped around. Upon the inner sheath metallic armour is provided which provides mechanical strength to the cable. At last, outer sheath is provided.
8 Components of HT cable :
1. Conductor
2. XLPE Insulation
3. Semi- conductive paint
4. Copper sheet wrap
5. PVC filler
6. Inner sheath
7. Metallic armour
8. Outer sheath
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Termination of HT Cable
In HT cable whether it is single core or multi-core, A layer of earth very next to the XLPE insulation; is present. A little dust or moisture can damage the insulation & results in short circuit at anytime.
A protection layer must be there which protect the open ends of the cable because earthing is very next to the XLPE insulation; A little dust or moisture can damage the insulation that’s why a termination is required in HT cable. Termination act as a protective layer on both the open ends of the cable which protect it from moisture & dust. It act like a waterproof bandage on an open wound.
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Termination needs heat shrinkable termination kit which comes with package of 18 to 20 things. In this kit, earthing material, cleaning material & heat shrinkable tubes are presents.
EARTHING MATERIAL INCLUDES
- 3 Earth Braids
- Large Earth Braid with lug
- Binding wire
- Backup Ring
- Jubilee Ring
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TAPES & CLEANING MATERIALS INCLUDES
- Silicon grease
- Isopropyl alcohol
- Mopping cloth
- Emery paper
- Red mastic tape
- Black mastic tape
- Self bonding insulation tape
- PVC tape
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SHRINKABLE TUBES
- Breakout
- Anti-tracking tubes
- Lugs Sleeves
- Stress control tubes
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HT Cable Termination Process
- First step is; Cut the outer sheath of the cable as per the size of shrinkable tubes. After the outer sheath, metal armour stirps needed to be cut.
- Cut the armour strips 2inches lesser than outer sheath. Make sure all the metal strips are cut in equally sizes than fold extra 2inches metal strips down as shown in an image below.
- Clear the inner sheath & than cut the filler material which are being fitted in between the cores.
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- Copper foil is wrapped on all the three cores, Bind the small earth braid on copper foil on each core with the help of copper binding wire given in the kit. Binding of braids on all the cores should be tight & in such a position that it covers the copper wrap & bended armour completely.
- Place backup ring on bended armour strips & make sure all the three earth braids, One big size earth braid are tightened inside the back up ring with the help of jubliee clamp which is clearly explained in an image below:
HT Cable termination process
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- Behind the copper wrap, Black paint can be seen on all the three cores which is a semi-conductive paint. Use a piece of glass to clean the semi-conductive paint. Clean the paint gently & make sure that paint is cleaned homogenously without any deep cut.
- After cleaning the semi-conductive paint, clean the entire open end of the cable with alcohol chemical & mopping cloth. Then next, grease it completely.
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- Cover the metal clamp & jubliee ring with self bonding insulation tape & then with PVC tape so that no sharp edge get remains behind. A sharp edge can crack heat shrinkable tubes.
- Its time to shrink the shrinkable tubes but Before shrinking the tubes, make sure all the tubes are void or crack free. Put the base shrinkable breakout at the open end & use medium gentle flame to shrink the tube. Shrink the breakout tube from the middle from where all the cores are coming out. While shrinking, always go from one side to another side so that no air bubble remain inside the tube.
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- After shrinking the breakout, Shrink the black stress control tubes in the middle of all the three cores & place self bonding black tape in between the breakout & stress control tube.
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- Shrink the anti-tracking tubes on all the cores. Clean the XLPE insulation from the conductor as per size of the lugs. Crimp the lugs on the conductor with the help of crimping tool. Use red mastic tape on the lug crimped area so that sharp edges get eliminated. After then, shrink the lugs sleeve on the red mastic tape.
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NOTE-
- In the termination image–13 above, there is a gap where XLPE insulation is seen clearly between the anti-tracking tube & lugs sleeve which is prevented. We made this because we need long length cores for indoor purpose.
- We use this HT termination for indoor purposes that’s why rain sheds are not utilized.
Common Mistakes in HT Cable Termination
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Improper cleaning of semi-conductive layer
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Air bubbles in heat shrink
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Sharp edges left on armour
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Moisture entry
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Improper earthing
Safety Precautions for termination of HT cable
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Use proper PPE
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Work in dry condition
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Correct crimping tools
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Follow manufacturer kit dimensions
FAQ related to termination of HT cable
Q1. Why is HT cable termination required?
To protect cable ends from moisture, dust and electrical stress and to ensure proper insulation and earthing.
Q2. What is the voltage range of HT cable in India?
HT cables are used from 1kV to 33kV.
Q3. What material is used for HT cable insulation?
XLPE (Cross Linked Polyethylene).
Q4. What happens if HT cable termination is poor?
It may cause insulation failure, partial discharge, overheating and short circuit.
Q5. Which kit is used for HT cable termination?
Heat shrink or cold shrink termination kits.
CONCLUSION
- The size of the conductor of a cable represents the cross sectional area of the conductor which directly links to its current carrying capacity. (More the cross sectional area of the conductor; higher the current carrying capacity!)
- The insulation level of the cable is directly links with the voltage. (Higher voltage demands higher insulation!)
- HT cables contains high level of insulation.
- Insulation of HT cable is made form thermosetting material.
- Heavy insulation around the conductor do not guarantee the complete safety because High voltage create high electrical stress around the insulation Which can damage the insulation.
- To remove the electrical stress around the insulation, insulation of each core is layered with Earthing.
- For proper Earthing, insulation layer is painted with semi conductive paint & than a copper sheath is wrapped Which is then earthed from both the ends of the cable.
- Since HT cable is layered with semi conductive paint & copper sheath, Termination of both the ends of the cable should be done otherwise chances of short circuit will be there.
- Termination act as a protective layer on both the open ends of the cable which protect it from moisture & dust. It act like a waterproof bandage on an open wound.
Hope, you like the post. If you have any doubt, suggestion or query please do comments!