Under which circumstances is a safety harness required?

A safety harness is specifically required when working at heights greater than six feet above a lower level. This regulation is in place because falls from such heights can result in serious injuries or fatalities, making it critical to have appropriate fall protection measures in place. The use of a safety harness is an integral part of a fall protection program, as it helps to secure the worker and prevent falls when working on scaffolding, rooftops, or any elevated work surfaces.

The other scenarios presented do not universally necessitate a safety harness. Lifting heavy materials may require proper lifting techniques and equipment but does not inherently relate to height safety. Similarly, using power tools requires awareness of tool safety and proper usage protocols, but again, it does not tie directly to fall hazards. Working in confined spaces has its own set of safety regulations, primarily addressing atmospheric hazards and access but does not typically require a safety harness unless there is a risk of falling. Thus, the correct answer focuses on the essential nature of fall protection in elevated work environments.