Electrical Cables
Underground cables are laid at sufficient depth

To avoid being unearthed easily due to removal of soil
To minimize the effect of shocks and vibrations due to gassing vehicles, etc.
All of the listed here
To minimize temperature stresses

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Electrical Cables
Solid type cables are considered unreliable beyond 66 kV because

Of corona loss between conductor and sheath material
There is a danger of breakdown of insulation due to the presence of voids
Skin effect dominates on the conductor
Insulation may melt due to higher temperature

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