Clarification of HARA ratings
ISO 26262:2018 requires a HARA to be produced for every item that is within its scope. This includes rating the hazardous events that can occur. There are often mistakes in dealing with two of the six ratings. This article provides an overview to what the six possible ratings are and how to use them.
One of the many confusing parts of ISO 26262:2018 are the ratings. Here we provide a guide to the ratings and clarify many points that have led to confusion in the past.
Four of the possible HARA ratings are the automotive safety integrity level (ASIL) ratings A to D.
ISO 26262-1:2018 3.6 gives the following definition of ASIL.
one of four levels to specify the item's or element's necessary ISO 26262 requirements and safety measures to apply for avoiding an unreasonable risk, with D representing the most stringent and A the least stringent level
Another possible rating is Quality Management (QM).
ISO 26262-1:2018 3.117 gives the following definition of QM.
coordinated activities to direct and control an organization with regard to quality
These ratings are determined using the three criteria of severity (S0 to S3), exposure (E0 to E4) and controllability (C0 to C3) in the risk assessment part of the hazard analysis and risk assessment (HARA) and then using the following tables.
ISO 26262-3:2018 Table 4 ASIL Determination
ISO 26262-12:2018 Table 5 MSIL Determination
ISO 26262-12:2018 Table 5 is used for motorcycles. The MSIL is converted to an ASIL rating in ISO 26262-12:2018 Table 6. Which reduces the rating by one level.
For example:
MSIL D = ASIL C,
MSIL A = QM,
QM stays as QM.
When a hazardous event has a ASIL rating - A to D - then it is necessary to apply ISO 26262:2018 to reduce the risk of the hazardous event occurring to be below an acceptable level.
When a hazardous event is QM rated then it is not necessary to apply ISO 26262:2018.
ISO 26262-12:2018 clause 8.4.3.11 Note 1 states :
The class QM (quality management) denotes no requirement to comply with ISO 26262. Nevertheless, the corresponding hazardous event can have consequences with regards to safety and safety requirements can be formulated in this case. The classification QM indicates that quality processes are sufficient to manage the identified risk.
QM still means hazardous and QM rated hazards should be treated as safety critical - but according to the organisations quality processes. Rather than having to meet the requirements of ISO 26262:2018.
But there are no S0, E0 or C0 ratings in the above tables, because this is the sixth possible 'rating' which is 'No ASIL / MSIL assignment is required'.
S0 = No harm = No ASIL / MSIL assignment
E0 = Incredible = No ASIL / MSIL assignment
C0 = Controllable in general = No ASIL / MSIL assignment
No ASIL / MSIL assignment means do not rate the other metrics of risk. e.g. if S0 then do not rate controllability or exposure. Do not consider further in the HARA.
For S0, it is still worth passing information about the related malfunction to be considered in accordance with the quality management system. As a S0 rated hazardous event, is still resulting from a malfunction that although it does not result in harm, can dissatisfy the customer and so should be prevented as part of quality management.
C0 is not rated because it is considered controllable in general by the persons at risk of harm. But it has the potential to cause harm if not controlled, so it could be considered safety relevant and be dealt with by the organisations quality processes.
E0 is not rated because the operating situation itself is considered so rare as to be incredible. So ISO 26262 states they do not need further analysis. Be careful when using E0 to really only exclude situations that do not need further attention.
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