unless made in entirely stable rock, at what depth is a protective system required for a trench?

A protective system is required for a trench at a depth of 5 feet (1.5 meters) or greater, unless the excavation is made entirely in stable rock. For trenches less than 5 feet deep, a competent person may determine if a protective system is necessary based on site conditions. Additionally, if a trench reaches 20 feet deep or more, the protective system must be designed or approved by a registered professional engineer. This is according to OSHA regulations and standards aimed at preventing trench cave-ins and ensuring worker safety.