Reliability Models used to estimate the PFH

Last edit: 26/02/2024

In high demand mode, the two standards, ISO and IEC, use different models to come to the estimation of the unreliability function.

IEC 62061 uses the Reliability Block Diagram method and it assumes the systems (Architectures) as non-repairable. ISO 13849-1 uses Markov Chains and it assumes the systems (Categories) as repairable.

That seems a major difference that makes the two approaches irreconcilable. In reality that is not the case and the reason is that in high demand, normally, the safety control system is the ultimate safety layer: that is the assumption in both ISO 13849-1 and IEC 62061. Where a safety-related control system is working in high demand or in continuous mode and it is the ultimate safety layer then, the overall safety-related control system failure will lead directly to a potentially hazardous situation, regardless if it is considered repairable or non-repairable.

In case the safety-related control system is the ultimate safety layer w(t) = f(t); that means:


Therefore, PFH(T) is the average unavailability of F(t).

Supposing a constant failure rate λ and λ·t<<1 :