The probabilistic safety analysis evaluates system reliability and failure probability by using statistics and probability theory. However, it cannot estimate the system safety uncertainties due to variabilities of probabilities in different system states. The article first summarizes how the information entropy expresses the probabilistic uncertainties due to nonuniform probability distributions of system states. Next, it argues that the conditional entropy of system operational and failure states appropriately describes both system redundancy and robustness. Finally, the article concludes that the probabilistic uncertainties of reliability, redundancy, and robustness jointly define the integral system safety. The concept of integral system safety allows for more precise definitions of efficient system functional properties, system configuration evaluation, optimal component selection, optimization, and decision making in engineering.