Single-point fault

The Single-Point Fault Metric (SPFM) is defined as follows ... 

This metric reflects the robustness of the item to single-point and residual faults either by coverage from safety mechanisms or by design (primarily safe faults). A high single-point fault metric implies that the proportion of singlepoint faults and residual faults in the hardware of the item is low. 

C.2.1 This requirement applies to ASIL (B), C, and D of the safety goal. The definition given by the following equation shall be used when calculating the single-point fault metric ... 

The failure rate assigned to residual faults can be determined using the diagnostic coverage of safety mechanisms that avoid single-point faults of the hardware element. The following equation gives a conservative estimation of the failure rate associated with the residual faults ... 

NOTE 3 If the above estimations are considered too conservative, then a detailed analysis of the failure modes of the hardware element can classify each failure mode into one of the fault classes (single-point faults, residual faults, latent, detected or perceived multiple-point faults or safe faults) wifli respect to the specified safety goal and determine the failure rates apportioned to the failure modes. Annex B describes a flow diagram that can be used to make the fault classification. 

For all residual elements or parts, which have somehow the potential to violate given safety goals, it should be identified, if their fault-modes could propagate direct to a safety goal violation, than these fault-modes have to be considered as single-point faults, if only indirect or by order higher than 2 they are multiple-point fault. 

NOTE 2 The transient faults are considered when shown to be relevant due, for instance, to the technology used. They can be addressed either by spec-ifying and verifying a dedicated target single-point fault metric value to them (as explained in NOTE 1) or by a qualitative rationale based on the verification of the effectiveness of the internal safety mechanisms implemented to cover these transient faults. 

NOTE 4 Some or all of the applicable safety goals can be considered together for the determination of the single-point fault metric but in this case the metric s target to be considered is that of the scffety goal with the highest ASIL. 

The architecture metrics (single-point fault metrics (SPFM) and latent failure metric (LFM)) would result from the safety architecture and would be a mathematical function of the failure rates (MFxx) and the implemented safety mechanisms (DCxx). 

Constraining architectural metrics, to cover faults, are defined as single point fault metrics and latent fault metrics, their definition being ... 

The single point fault metrics can be seen as the capacity of a system to be non-dangerous or to control dangerous failures ... 

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