Safety is increasingly important for software based, critical systems. Fault tree analysis (FTA) is a safety technique from engineering, developed for analyzing and assessing system safety by uncovering safety flaws and weaknesses of the system. The main drawback of this analysis technique is, that it is based on informal grounds, so safety flaws may be overlooked. This is an issue, where formal proofs can help. They are a safety techniques from software engineering, which are based on precise system descriptions and allow to prove consistency and other (safety) properties.