An efficient, cost-saving system and method of designing, installing, testing, and monitoring a secondary containment system for new and existing Above Ground Storage Tanks (ASTs) that is comprised of a protective steel membrane for the floor of the tank, including the floor-to-wall juncture, and the lower portion of the tank shell such that the steel membrane creates one or more interstitial zones between the steel membrane and the tank shell. The steel membrane may be installed zone by zone such that the secondary containment system can encapsulate existing tank features by molding itself around the existing contours, such as repair plates, corners, sumps, etc. Furthermore, a unique testing method using a simple electronic device is provided for on-site testing of the integrity of the weld seams that may be efficiently used after each zone is installed without requiring that the entire tank be evacuated during the construction of other zones.