Construction of H type cable and SL type cable

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

You may also like :

Why belted cable is not used above 22 kV?

How to reduce tempering in the Energy Meter?

Protection area covered by Lightning arrester

Voltage operated ELCB and current operated ELCB