[1] Dougherty, Dorothy: “Permit-Required Confined Spaces,” Occupational Safety and Health Administration, Department of Labor, Dec. 12, 2011. https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9797&p_table=STANDARDS

[2] Swinderman, Marshall & Marti: Foundations for Conveyor Safety, Chapter 18,
pp. 249-251. https://hubs.ly/H0mLk2t0