Recent developments facilitated the integration of industrial robots into hybrid work cells in order to share a common workspace with the human worker. Besides, the worldwide demographic change leads to an increasing need for hybrid assembly solutions that represent a promising solution for encountering the challenges of the future and the rising demand for flexible and reactive production systems. Nevertheless, the physical human safety within human-robot co-operation represents the key requirement. This paper introduces a model-based safety-system for co-operative robots using an onlinemodel of the robot work cell. Based on a test bench, generic functional specifications and important safety aspects are derived, highlighted and discussed, namely data processing, hardware defects and test structures are covered.