B.3:6.1 - Fully calculated case: two necessary independent conditions

Target claim: “The protection function succeeds on a demand.” Assurance use: a bounded reliability argument for a named design decision.

The domain model says that both independently tested conditions must succeed: sensor detection and actuator response. Each has estimated probability 0.9 under the same stated demand class and qualification window. Under the declared independence assumption, the joint probability is:

0.9 × 0.9 = 0.81

Using min(0.9, 0.9) = 0.9 would overstate this conjunction. If the conditions are dependent, even the product is not justified; the result must use the applicable conditional model or remain unresolved. The B.3 result therefore cites the two domain results, the series dependency structure, independence basis, product calculation, 0.81 conclusion, limitations, and the observation that reopens the independence assumption.

What changes in practice: the design decision is evaluated against 0.81, not a falsely “conservative” 0.9. No universal B.3 reliability formula is created.