The purpose of IS420YDOAS1B is to provide a brief overview of the security lifecycle used to design the SIS, as well as general information to help perform the tasks defined in the security lifecycle. This includes data tables, lists of definitions and acronyms, equations, and instructions for using these resources. is an instrumentation and control system that detects uncontrolled process conditions and automatically returns the process to a safe state.

The IS420YDOAS1B is the last line of defence (or close to it) against chemical process hazards, and it is not part of the basic process control system. The last line of defence is what distinguishes a safety instrumented system from a basic process control system used for normal supervisory process control.

IS420YDOAS1B demonstrates how a safety instrumented system differs from a basic process control system (BPCS). In this example, pressure is a potential hazard. Pressure is normally controlled by a regulatory control loop, as shown in the basic process control system, which regulates a pressure control valve. A separate high pressure shutdown function is implemented in the safety instrumented system.