Underground cables

1. Underground cables
Prepared by Souvik Dutta

2. construction
Underground cables have
several advantages over
over headlines;
• they have smaller
voltage drops
• less chances of
developing faults
• low maintenance cost.

3. Type‘s of Underground cable‘s
1. Low tension(L.T.) cables – upto 1000v
2. High tension(H.T.) cables –upto 11000v
3. Super tension(S.T) cables – from 22kv to 33 kv
4. Extra high tension(E.HT.) cables – from 33kv to 66kv
5. Extra super voltage cables- beyond 132kv

4. Construction of single core
shows the constructional details of a single-core
low tension cable. The cable has ordinary
construction because
the stresses developed in the cable fo
r low voltages (upto
6600 V) are generally small.
It consists of one circular core of
tinned stranded copper (or aluminium)

5. Belted Cables
• The cores in the belted underground cables are
not circular and are insulated by impregnated
paper .
• The cores are generally stranded and may be of
non-circular shape to make better use of available
space.
• The gaps between the conductors and the paper
insulation are filled with fibrous material such as
the jute.
• Disadvantages: Under high voltages the air may be
ionized and cause deterioration and breakdown of
insulation. For this reason, the belted cables are
only suitable for voltages up to 11KVa and not
higher.

6. H-type cables
Advantages of H-type cables
• Metallic screens improve the heat
dissipation of the cable
• No formation of air pockets and
voids in the dielectric, hence a high
breakdown strength and less
dielectric losses
Disadvantages H-type cables : the
cables are only suitable for low and
medium voltages of up to 33KV, but
can reach 66KVA at times.

7. S.L Type screened cable
Advantages of S.L type Screened cables:
• The use of separate sheaths reduces
chances of core-to-core breakdown
• Easy to bend the cable
Disadvantages of S.L type Screened
cables
• Thinner lead sheaths are used hence
need for greater care in
manufacturing and handling
• Only suitable for low and medium
voltages of up to 33KV