Historical
NFPA 326-1999
NFPA 326: Standard for the Safeguarding of Tanks and Containers for Entry, Cleaning, or Repair, 1999 Edition
Applies to the entry of underground storage tanks, operating at nominal atmospheric pressure, that have contained flammable or combustible liquids, and that might contain flammable or combustible vapors or residues.
National Fire Protection Association [nfpa]