The object of the article is the estimation of Common Cause Failures (CCF) in digital systems, e.g. protection system of nuclear plants. The system under study is composed of four divisions, 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 the CCF impact on the system dependability. Assumptions related to hardware reliability and system logic, maintenance and repairs are taken into account in the model that is thus dynamic. The simulation, based on a CPN model and the assumptions of the Atwood model, permits to compare estimators of CCF parameters. An example of comparison is presented in this article, based on the Impact Vectors approach. Finally, some conclusions are presented.