SIL comprises four safety requirement levels, where Level 4 represents the highest level of safety integrity and Level 1, the lowest. PL comprises Performance Levels a to e, where e is the highest level.

Equipment manufacturers design their products according to the stipulated safety levels. From Level 3 upwards, these products must be certified by a testing body. For this, Elster Kromschröder works in collaboration with the German inspection authorities TÜV Rheinland and TÜV Süd, and with KIWA Gastec in the Netherlands.

Manufacturers of systems with safety-relevant functions determine the Safety Integrity Levels and Performance Levels for the relevant safety function as part of a risk assessment. On the basis of this definition, the suitable devices are selected and assembled to form a system.

The devices and assemblies are divided into sensor, logic and actuator units. The combination of sensor + logic + actuator forms the safety function.