Writing Good Technical Safety Requirements

One of the key premises of the ISO 26262 functional safety standard is the development of an appropriate Technical Safety Concept for the item under development. This is specified in detail in Part 4 of the standard - Product development at the system level. The Technical safety requirements and the technical safety concept form the basis for deriving the hardware and software safety requirements that are then used by engineering teams for developing a safe product. Just like any other form of product development, making multiple revisions of the requirements are highly undesirable. This is primarily due to cost increases, chances of having inconsistencies within work products and its impact on the overall project schedule. Good technical safety requirements are in fact the foundation for an effective functional safety implementation. Presently the ISO 26262 standard does not provide any direct guidance on any specific method to derive technical safety requirements for a given safety goal for an item. This paper provides guidelines to come up with a comprehensive and concise set of Technical Safety Requirements using safety analyses techniques like FTA or FMEA. The paper is intended to support those safety engineers tasked with developing the technical safety concept. Additionally, the paper recognizes that in practice projects face challenges such as lack of stakeholder interest, multi-party development and missing or incomplete upstream work products. The paper captures these real world challenges and provides proposed solutions. The paper concludes by citing a few methods for Fault tolerant Time Interval (FTTI) determination at the ECU level; a key parameter that is critical for the effectiveness of the technical safety concept.