Final answer:
Sifted sand is used when burying PVC/SWA cables in trenches to protect the cable from physical damage, absorb shocks, and provide a stable thermal environment.
Step-by-step explanation:
When a PVC/SWA (Polyvinyl Chloride/Steel Wire Armoured) cable is buried in a trench, it is laid upon and then surrounded by sifted sand to protect the cable from damage. This includes protection from sharp stones, rocks, and other objects that could potentially pierce the cable's outer sheath. The sifted sand acts as a cushioning and shock-absorbing layer. It also ensures stable and consistent bedding for the cable, maintaining a uniform thermal environment to help with heat dissipation. The sand surrounding the cable should be free of large stones and impurities to ensure this protective layer is effective.