Last edit: 03/03/2023
The idea of Functional Safety was formalized with IEC 61508 in 1997.
Hereafter is an example to explain what functional safety is: we need to protect a tank from cracking caused by high pressure. We can design the tank in order to stand to the maximum possible pressure related to any process dysfunction, or we can realize a safety loop (input/pressure sensor, safety logic, output/valve). The loop acts by closing the valve in cae of high pressure. Only the latter case can be considered Functional Safety.
Considering that it is not possible to have zero risk, the aim of IEC 61508 is to provide a method to realize safety systems with a well defined reliability. That is achieved through the evaluation of a risk related to an hazardous event and its reduction to a level considered socially acceptable.
The risk is a function of harm severity and its probability of occurence. For the risk estimation it is always considered the death of a person as harm severity and an occurence of 10-6/years in order to get a socially acceptable risk.
In other words, each process or machine introduces a risk normally much higher than the socially acceptable level. That risk must be reduced through functional safety systems whose reliability is as high as the risk that the safety system is going to reduce.
From IEC 61508, several standards are derived. They contain methods to design safety loops whose reliability is suitable for the level of danger that they protect from. Two of the standards derived from EN 61508 are EN 61511 for processes and EN 62061 for machineries.
EN 61511 is for safety systems in "Low Demand Mode", which means that the system acts only in case of process failures.
EN 62061 is for safety systems in "High Demand Mode", in this case the system acts every time a protection device is activated for the normal operations of the machine. In this case, despite the first case, the safety system operation is requested with a high frequency.
GT Engineering perform consulting about industrial safety systems applying EN 61511, including in the Risk Assessment the LOPA method (Layer of Protective Analisys).