The object of the article is the estimation of dependability and other probabilities of interest for digital systems, e.g. protection system of nuclear plants. The system under study is composed of four divisions, all with identical hardware. Colored Petri Nets are used because of their capability to model complex digital systems and assess their dependability. The Atwood model is also implemented into the CPN model. It represents Common Cause Failures that are contributing to the residual risk of unavailability. Assumptions related to hardware reliability and system logic, maintenance and repairs are taken into account in the model that is dynamic. We explain in this article how the Atwood model can be modified to "direct" non-lethal CCF on certain parts of the system and take into account the different possible origins of CCF. This extension permits to avoid the assumption of uniform distribution of non-lethal shock on the components and to represent some benefits of diversity and separation between divisions.