This paper summarises the experience gained during the development of the personal protection systems of the LHC and more particularly the feedback of the use of the IEC 61508 functional safety standards. The paper drafts also the guidelines for the development of the future functional safety systems at CERN. After an introduction on the legal aspects and responsibility of the various stakeholders implied in the development of a safety system, the paper describes the functional safety life cycle applied and experience gained in each stage of the process; covering topics such as : the preliminary risk analysis, the definition of the safety functions, the probabilistic analysis of the architecture implementing the safety function, the verification and validation process, the maintenance strategy, and the validation of the system by the external safety Authority. The perspective of the applicability of the new nuclear branch safety standard IEC 61513 is also introduced.