What types of conduits can be used for underground installations?

For underground installations, specific types of conduits are required to ensure safety and durability. Rigid non-metallic conduits, like PVC (polyvinyl chloride), are commonly used because they are resistant to moisture and corrosive soil conditions. These materials also provide good mechanical protection for the cables inside.

Rated metal conduits, such as EMT (electrical metallic tubing) or steel conduit, are also suitable for underground use when they are properly rated for the conditions, as they offer robust protection against physical damage and environmental hazards.

On the other hand, flexible conduits are generally not recommended for permanent underground installations due to their susceptibility to damage and lack of structural strength compared to rigid conduits. Additionally, while plastic conduits are part of the correct answer since they include rigid non-metallic options, the inclusion of rated metal conduits is crucial for ensuring that the installation can withstand various environmental factors, which is why the specific combination of rigid non-metallic and rated metal conduits is the most appropriate choice for underground applications.