The design of explosion isolation barriers is an important part of ascribing overall plant explosion protection. From a detailed understanding of the course of flame propagation into a pipeline or duct, algorithms are derived that establish the basis for barrier design. It is shown that barrier efficacy is critically dependent on both the selected hardware and the assumptions regarding explosion intensity and ignition location. It is shown that the selection of both flame and pressure detection in OR logic results in an ability to ascribe shorter barrier distances than is the case with a single detection method. The implicit residual risk (limitation of efficacy) of explosion isolation barrier designs can thus be assessed.