Abstract: The present invention shows an apparatus and method for the determination of clearances for safety of personnel and safe storage of critical assets within a structure. The method comprises the steps of choosing appropriate test locations on top of the structure, using a test apparatus including a signal generator, a receiver and a computer, injecting low-level test currents at each test locations, measuring the resulting electromagnetic fields; calculating the transfer functions for the structure, and determining the internal energy levels that lightning strikes would cause inside the structure in order to determine a safe stand-off distance for critical assets or personnel within the structure.