These cables are used up to 33kv, but some particular cases it may be extended to operating voltages up to 66kv.
It is further classified into two, H type and SL type cables.
H-Type Cables:
It was first designed by M. Hochstadter.
The three cores are individually insulated with Impregnated paper and then covered by a metallic screen / cover. These metallic covers are perforated Aluminum foil. As a result, such construction allows the three metallic screens to touch each other.
These three metallic covers are then grouped together in a metallic tape (conducting belt) usually made of copper. A lead sheath surrounds this construction. The metallic covers and the sheath are grounded.
The cable has no insulating belt but lead sheath, bedding, armouring, and serving follow as usual.
The advantage is the electric stresses are radial, not tangential and hence of lesser magnitudes. Also, the metallic covers improve the heat dissipation.
The perforations in the metallic screens assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated.
S.L Type Cables:
It is similar to the H type cables, with the difference that each of the three cores has its own lead sheath (separate lead). With this provision, the need for the overall sheath used previously is eliminated.
The advantage of such a construction is that the chances of a core-to-core breakdown are greatly minimized. Also, the flexibility of the cable is improved.
The limitations are severe. Such construction is limited for voltages up to 66kV only.
Disadvantage of these cable is the individual sheaths are thinner, and if there are constructional defects, moisture may enter the cable and reduce its dielectric strength.
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