Due to the internal complexity, the human intent and the large physical distribution of a facility, a classification of its vulnerability to intentional EMI founded on shielding effectiveness is deemed inadequate. In the power network of a facility, low-level differential mode ultra wideband transients were injected and the received voltages at different ports (power sockets) were measured. The results were processed using statistical methods and were used to form an alternative method for classifying facilities or large distributed systems.