Electrical Cables
Underground cables are laid at sufficient depth

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

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

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

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