The Different types of Risk

The starting point in the risk assessment is the process itself, also called Equipment Under Control (EUC). The process has a certain “Process Risk”, that is the risk without the implementation of any Risk Reduction Measure (machinery safety terminology) or Protection layer (process terminology).

The risk is then reduced thanks to the implementation of protection layers, part of which is also the Safety Instrumented System (SIS). What is left is called “Residual Risk”.

Figure illustrates the general concepts of risk reduction. The general model assumes that:

• there is a process and an associated Basic Process Control System (BPCS);
• there are associated human factor issues;

The safety protection layer includes:

• mechanical protection system;
• safety instrumented system;
• nonSIS instrumented system;
• mechanical mitigation system.