Electrical analogy and notions of closure path and minimum cross-section
Reliability block diagrams

The two-state variables mentioned above are reminiscent of the two states of an electrical switch, which can be closed or open. Hence the idea of representing blocks as switches.

This is illustrated in figure 5 , where a block in good working order is modeled by a closed switch and a faulty block by an open switch. The BDF representing a system thus becomes an electrical circuit. When this circuit is closed, it means that the system is working (s = 1) and when the circuit is open that it is faulty (s = 0). This is illustrated in figure 6 .