Functional Safety

Functional Safety describes the fraction of the overall safety of a system that is dependent on the correct function of the safety related system. In general, Functional Safety comprises the safety-related task of a component or system which needs to be performed correctly according to the risks to be covered. This is also required when there are intern errors or failures occurring – otherwise the defined safe condition needs to be adjusted accordingly. Within the last years and decades Functional Safety as a complex field has gained increasing significance in all technical areas.

The basic safety standard IEC 61508 (also published as DIN EN 61508) describes the Functional Safety of safety-related electrical, electronic and programmable electronic systems. This standard originated from systems engineering and process industry . The term “safety-related” describes each and every programmed system, whose failure (single or in combination with other failures) may lead to damage or death of human beings, catastrophic environmental degradation or destruction of material goods. The implementation of the basic safety standard IEC 61508 created a cross-sector directive for all safety-related systems. One of the primary objectives of this application independent safety standard included to allow for a derivation of sector-specific standards. These standards permit to take full account of most important influencing variables of the particular field of application in order to meet its specific demands. In recent years, many practice-oriented derivations of the IEC 61508 have been developed for different fields of applications. Below you will find an overview over important derivatives.

 

functional safety

functional safety

 

One application-specific derivation of the IEC 61508 is the ISO 26262 (Engl.: IOS – International Organization for Standardization), which describes a safety-standard adapted to the characteristics of the automotive industry that is coordinated on an international level. As well as the basic safety standard, the ISO 26262 comprises a safety life cycle in order to detect those operations systematically that are necessary to guarantee the Functional Safety of safety related electrical/electronic-systems.

The following figure shows the safety life cycle according ISO 26262:

 

safety life cycle according ISO 26262

safety life cycle according ISO 26262

One essential stage of the safety life cycle is the risk analysis, also called hazard analysis and risk assessment (HARA), which systematically detects potential threats originating from a system – for all kinds of operation modes. This determination needs to be conducted for all reasonably foreseeable circumstances, including failure conditions and misuses. In order to classify safety functions according to their safety integrity, so called Safety Integrity Levels  can be used (SIL (SIL 1 to SIL 4) in IEC 61508, ASIL (ASIL A to ASIL D) in ISO 26262). Those can be determined through the combination of different risk parameters by employing the risk graph, for example.

The derivatives of the IEC 61508 in part place similar requirements on the safety integrity, so that the proximity to the basic standard is given to a large extent.

Depending on the classification within the safety integrity the particular sector specific body of standards places requirements on different stages of the safety life cycle, which need to be considered and fulfilled. For example, some standards define probabilistic values, which you will find in parts in the following figure:

 


Contrasting Safety Integrity Levels

Contrasting Safety Integrity Levels

 

PFD stands for “Probability of Failure on Demand”, PFHD for “Probability of Dangerous Failure per Hour” and THR for “Tolerable Hazard Rate”.

Depending on the norm, the performance of inductive and/or deductive safety analyses or the compliance with probabilistic limit values for random hardware failures can be reckoned as standard requirements.

Due to their broad-based methods portfolio and several years of experience in the field of Functional Safety, the team of the IQZ is able to support you as your competent partner. Whether you are dealing with the development of a functional safety process or the implementation of specific methods (e.g., FMEA/FMEDA/FTA/Markov) – we’ll be pleased to provide consulting!