Which type of underground structures must be installed to ensure accessibility without excavation?

The correct choice is handhole enclosures, which are specifically designed to provide accessibility to underground electrical systems without the need for excavation. These enclosures allow for maintenance and troubleshooting directly within the underground electrical infrastructure, enabling utility workers to access cables and connections safely and efficiently.

Handhole enclosures typically feature removable covers that allow for easy entry. This is particularly advantageous when there is a need for frequent access or when repairs are required, as it minimizes the disruption and time involved in excavating to reach underground components.

Other types of structures, while useful, do not provide the same level of accessibility without excavation. For instance, conduit enclosures are primarily used to protect cables and conductors running underground but do not offer direct access points like handhole enclosures do. Transformer vaults are designed to house transformers and are more focused on protection and efficiency, rather than accessibility. Cable trays serve a different function, primarily for supporting and organizing wire and cable runs but do not provide access to components buried underground.