Success and failure: human as hero -- human as hazard

Human Factors are often cited as the cause of hazards within safety-related systems (human as hazard); yet system safety cases often contain no mention of them. Conversely, system operators often provide substantial mitigation between hazards and their associated accidents (human as hero); yet this is also often overlooked. If the human factors risks are not considered a system will not achieve the required level of integrity. If the human factors mitigations are not considered the technical components may be over engineered at additional cost to achieve a safety integrity target. This paper explores the positive and negative contributions that humans make to system safety. This paper deals with problems associated with Human Factors throughout the development of safety-related programmable systems that typically rely on people, procedures and equipment to function safely within a specific operational environment. Typical examples of such systems are found in Air Traffic Control and Railway Control Rooms. The paper begins by highlighting the problematic relationship between Human Factors and System Safety Engineering before briefly examining several reasons for this difficult relationship. The paper then examines some important safety-related concepts related to risk and barriers before introducing the concept of success and failure cases. Finally, the paper describes a Human Factors process, based upon Critical Task Analysis and Human Error Analysis, used to generate evidence to support human success and failure cases.