RBD vs FTA vs FMEA

FMEAasks “what can fail, and what happens if it does?” — bottom-up. FTAasks “how can this specific bad outcome happen?” — top-down. RBDasks “given how everything is connected, how available is the whole system?” — success-oriented. They are complementary, not competing.

Failure Modes and Effects Analysisworks upward from the parts. You list each component’s failure modes, trace the effect of each on the system, and rank them — by criticality (FMECA) or a risk priority number. It is the inductive, what-could-go-wrong inventory, excellent early in design for surfacing weaknesses and feeding data to the other methods. It does not, by itself, compute a system availability number. (Reference: IEC 60812.)

Fault Tree Analysis starts from a single undesired top event — a hazard or a major failure — and decomposes it through Boolean AND/OR gates into the combinations of basic causes that produce it. It is deductive, good for analysing one specific outcome in depth, finding minimal cut sets, and is widely used in safety work. (Reference: IEC 61025.)

A Reliability Block Diagram represents the system as success paths: if a path of working blocks connects start to end, the system is up. It is the natural fit for availability and production questions — redundancy, k-of-n, standby, and (with simulation) the time-dependent behaviour of maintenance, spares and crews. (Reference: IEC 61078.)

A typical programme uses all three: FMEA to enumerate failure modes and their data, FTA to drill into critical events, and an RBD to roll the whole system up into an availability or production figure.

Ramly is a reliability block diagramtool — the success-path, system-availability lane — evaluated either by Monte Carlo simulation or an exact analytical (Markov) solver. That is deliberately one modelling method, done well: redundancy and standby, maintenance and spares, throughput and loss contributors, with time-dependent behaviour that static methods can’t capture.

Ramly does not build fault trees or FMEA worksheets. Those are different methods for different questions; the output of an FMEA (the failure modes and their rates) is exactly the kind of data you then bring into an RBD. If your question is system availability and where the downtime comes from, the RBD is the right tool — and that is what Ramly is for.