Sensor nets are being widely proposed as a solution technology in a wide number of applications, e.g. health care. As part of this work some key challenges for the safety and sensor net communities are established in part by developing parts of a safety case for a fire detection system in a skyscraper. We then demonstrate how some of these issues can be resolved by modifying earlier work on Run Time Assurance of applications to satisfy some key safety and dependability requirements in the context of a sensor net used as part of a fire fighting system.