# An Accident Model with Considering Physical Processes for Indoor Environment Safety
## 1 Introduction
However, in a limited period, e.g., days, the outdoor climate can be considered with no big changes. Third, the role of people in the home environment. The characteristics of workers in workplaces and occupants in the home environment are different. In this paper, we extend the STAMP model with considering physical processes (hereafter denoted by STAMP-PP) to understand accident formation. Smart home systems interact with the home environment through its behaviors, e.g., warm up and cool down. Thus, the STAMP-PP connects the physical world through the behaviors of the systems. Under this consideration, accidents are the result of the violation of a set of constraints on the behaviors of the systems to cause abnormal changes in physical processes, and finally result in personal harm. The extended STAMP-PP model demonstrates accident formation with respect to system behaviors and physical processes. The system behaviors can be controlled either by the smart home system directly or by occupants indirectly. The information related to physical processes is important, and the abnormal behaviors of systems under specific operation scenarios must be known to select the appropriate reactions and precautionary measures. To this end, hazard analysis techniques [9; 11] that can assist in analyzing potential causes of accidents are required. We adopted a hazard analysis technique to identify causes of abnormal system behaviors under related operation scenarios, which can result in abnormal changes in physical processes. A new approach to hazard analysis, called System-Theoretic Process Analysis (STPA) [2; 9], is based on the STAMP model, and it is tailored and applied to the smart home system [12] for adjusting the indoor temperature, to demonstrate how to identify causes to abnormal system behaviors that result in physical process anomalies. The STPA can be used to identify unsafe control actions of a controller and are also the reasons why unsafe control actions can happen under specific scenarios. As abnormal behaviors of smart home systems that result in intermediate states of physical processes are also considered, the STPA is then tailored also for identifying causes to these abnormal behaviors. Landscape Genealogical Layout Documentation (LGLD) is proposed for documenting the analytical results, and the relations among the results are clearly and straightforwardly represented by comparing with conventional ways of documentation, i.e., tables and lists. We compared the results with that of applying the original STPA, which demonstrate the effectiveness of the tailored STPA in identifying causes of abnormal system behaviors and the LGLD documentation in representing the relations among the results. The contributions of this paper are as follows. * We discussed the characteristics of the smart home in the viewpoint of occupants and the safety of the home environment. * The concept of the Performers System, which emphasizes the behaviors performed by various home appliances, is proposed. * We propose the STAMP-PP model for understanding accident formation, i.e., abnormal system behaviors that result in abnormal changes in physical processes and cause hazards. * We tailored the STPA and applied it to a smart home system to identify inappropriate and unsafe control actions that cause abnormal system behaviors, which result in abnormal changes in physical processes, and hazards. * An LGLD approach is proposed for documenting the STPA analytical results. This paper extends a conference paper [13] that introduces the STAMP-PP model while considering physical processes. The application of the tailored STPA and the new way of documenting the results in this paper are novel. The rest of this paper is organized as follows. Section 2 discusses some knowledge about smart home and home environment safety. Section 3 introduces the proposed accident model STAMP-PP. Then, the hazard analysis technique STPA is tailored and applied to a smart home system for indoor temperature adjustment in Section 4. In Section 5, a discussion is given.