The use of shield topology Co partition large systems into tractable pieces is discussed. The interactions of externally generated transients with large systems are discussed from the aspect of the numbers of operations that must be considered. The segments of interaction associated with multiple shield imperfections, multiple levels of shields, system states, and abnormal modes lead to enormous numbers of analytical operations that must be performed to evaluate a family of responses. Practical reduction in this number, by invoking the principle of effectively impervious shields, is described.