Screened cable
The screened cables are used for voltage up to 33 kV,
but it may be applicable for 66 kV also. There are two types of screened cable
: H type cable and SL type cable.
H type cable
- This type of cable is first designed by H. Hochstadter therefore it is called as H type cable.
- The construction of H type cable is shown in the figure.
- Each core of cable is insulated by impregnated paper.
- The insulation of each core is surrounded by metallic screen which is usually made of aluminum foil.
- The cores of cable are placed such that the entire three metallic screens touch together as shown in the figure.
- These three metallic screens are wrapped by copper woven fabric tape. The entire core metallic screen is in contact with conducting belt and lead sheath.
- All three core, metallic screen and lead sheath are at earth potential therefore the electric stresses are radial resulting dielectric losses are reduced.
Advantages
- Air pockets or voids in the dielectric are eliminated due to metallic screen.
- If voids may present, it reduce the breakdown strength of cable and may damage paper insulation.
- The metallic screen increases the heat dissipating area of cable.
SL type cable
- Each core of the three core cable is separately covered with paper insulation and all cores are separated wrapped with lead sheath.
- Therefore each core works as separate cable in the three core cable.
- There is no lead sheath is used surround all three cores in addition to their individual lead sheath.
Advantages of SL type cable over H type cable
- Bending of cable is possible because no overall lead sheath is used in the cable
- Minimum chances of core to core breakdown due to presence of separate lead surround individual core
Disadvantages of SL cable over H type
cable
- Manufacturing of SL cable is difficult due to thinner lead sheath of individual core of cable
Write the function of metallic screen
in the H type cable.
- As the metallic sheath, three cores and lead sheath are at earth potential, the electrical stresses are radial therefore the dielectric losses are reduced.
What is reason behind the name –
Screen cable?
- As there is metallic screen is used around all the three cores, it is called as screen cable.
Whether insulating belt is used in
the H type cable?
- No
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