Glossary · Functional safety

SISTEMA

Also known as: Safety Integrity Software Tool for the Evaluation of Machine Applications

SISTEMA (Safety Integrity Software Tool for the Evaluation of Machine Applications) is a free software tool from the German IFA that models the safety functions of a machine and calculates their achieved Performance Level under ISO 13849-1, for comparison with the required Performance Level (PLr).

Functional safety
Verification

In one sentence

SISTEMA is the free IFA tool that calculates PFHd, MTTFd and DCavg for safety functions and checks the achieved PL against the PLr of ISO 13849-1.

Example

A designer models SF3 “guard door opens → Safe Stop 1 of the spindle drive” in SISTEMA from the vendor libraries of the door switch, the safety PLC and the drive; SISTEMA reports PL d against a PLr of d.

How it applies

Publisher: SISTEMA is developed and distributed free of charge by the IFA (Institute for Occupational Safety and Health of the German Social Accident Insurance). It is the most widely used tool in Europe for the quantitative verification step of ISO 13849-1.
Modeling: Designers rebuild each safety function virtually as a chain of subsystems, usually sensor, logic and actuator, and assign the category, component data and diagnostic measures to each block. Safety functions are commonly numbered SF1, SF2, … in the project.
Component libraries: Manufacturers publish SISTEMA libraries of their safety components in the data format of VDMA Specification 66413, so reliability values do not have to be typed in by hand.
Calculation: SISTEMA computes the probability of dangerous failure per hour (PFHd), the mean time to dangerous failure (MTTFd), the average diagnostic coverage (DCavg) and the common cause failure score, and checks whether the achieved PL is at least the PLr.
Other tools: Commercial tools and vendor selection tools (for example Pilz PAScal, SICK Safexpert or the Safety Evaluation function in the Siemens TIA Selection Tool) perform comparable calculations, some also for IEC 62061. The underlying requirements are the same; only the tool differs.
Technical documentation: Keep the SISTEMA project file and its report in the technical file, under version control together with the safety function list. A green result in the tool is not the whole verification: systematic failures, software requirements and validation under ISO 13849-2 still have to be covered.

SISTEMA vs. ISO 13849-1

ISO 13849-1 sets the requirements. SISTEMA is one implementation of its calculation method. The designer, not the tool, remains responsible for the model, the input data and the assumptions.