Which of the following methods is not permitted for the protection of equipment in electrical vaults?

The method that is not permitted for the protection of equipment in electrical vaults is the use of stud walls covered with wallboard with a 3-hour rating. This is due to specific regulations that aim to ensure the highest level of fire safety and structural integrity in environments where sensitive electrical equipment is stored.

In electrical vaults, it is crucial to provide robust construction that can withstand potential fire hazards and protect equipment from external damage. While metal enclosures, concrete block walls, and fire-rated doors contribute to creating a secure and fire-resilient environment, stud walls, even if covered with fire-rated wallboard, may not provide adequate protection compared to the more robust materials specified in code requirements.

The use of permanent wall systems, such as concrete block walls and metal enclosures, is preferred in these scenarios because they offer superior durability and resistance to both physical impact and fire, ensuring that electrical equipment remains safe and functional during emergencies. Thus, the preference for more substantial wall constructions over stud walls highlights the necessity for stringent safety measures in electrical installations.