Mapping to IEC 61508 the hardware safety integrity of elements developed to ISO 26262

Figures & data

Figure 1. SafeSoft innovations. Zeabuz.

Table 1. SIL equivalence table for IEC 61508-related continuous/high demand safety system in relation to EN 50129 (Okoh et al., 2022).

THR (Railway industry – EN 50129)	SIL	PFH (Generic – IEC 61508)
10^-9 to <10^-8	4	10^-9 to <10^-8
10^-8 to <10^-7	3	10^-8 to <10^-7
10^-7 to <10^-6	2	10^-7 to <10^-6
10^-6 to <10^-5	1	10^-6 to <10^-5

Table 2. Effective TFFR relationship with SIL in the railway industry [adapted from ARC (2022)].

Effective Tolerable Functional Failure Rate (TFFR) (per hour)	SIL
10^-9 to 10^-8	4
10^-8 to 10^-7	3
10^-7 to 10^-6	2
10^-6 to 10^-5	1
≥10^-5	0 / Basic Integrity

Table 3. SIL equivalence based on PFH (IEC 61508) and Effective TFFR (EN 50129) for continuous/high demand safety system [adapted from Okoh et al. (2022)].

Effective TFFR (Railway industry – EN 50129)	SIL	PFH (Generic – IEC 61508)
10^-9 to 10^-8	4	10^-9 to 10^-8
10^-8 to 10^-7	3	10^-8 to 10^-7
10^-7 to 10^-6	2	10^-7 to 10^-6
10^-6 to 10^-5	1	10^-6 to 10^-5

Table 4. Probability of Exposure Classification (ISO 26262, 2018).

Class	Description
E1	Incredible
E2	Very low probability
E3	Low probability
E4	Medium probability
E5	High probability

Table 5. Controllability classification (ISO 26262, 2018).

Class	Description
C0	Controllable in general
C1	Simply controllable
C2	Normally controllable
C3	Difficult to control or uncontrollable

Table 6. Severity classification (ISO 26262, 2018).

Class	Description
S0	No injuries
S1	Light and moderate injuries
S2	Severe and life-threatening injuries (survival probable)
S3	Life-threatening injuries (survival uncertain), fatal injuries

Table 7. ASIL Determination (ISO 26262, 2018).

		C1	C2	C3
S1	E1	QM	QM	QM
	E2	QM	QM	QM
	E3	QM	QM	ASIL A
	E4	QM	ASIL A	ASIL B
S2	E1	QM	QM	QM
	E2	QM	QM	ASIL A
	E3	QM	ASIL A	ASIL B
	E4	ASIL A	ASIL B	ASIL C
S3	E1	QM	QM	ASIL A
	E2	QM	ASIL A	ASIL B
	E3	ASIL A	ASIL B	ASIL C
	E4	ASIL B	ASIL C	ASIL D

Table 8. Comparison of ASIL and SIL using diagnostic coverage (Meany, 2019).

SIL (IEC 61508)	Min. Diagnostic Coverage	Vs.	ASIL (ISO 26262)	Min. Diagnostic Coverage
SIL 1	60%		ASIL A	No minimum
SIL 2	90%		ASIL B	90%
–	–		ASIL C	97%
SIL 3	99%		ASIL D	99%
SIL 4	99%		–	–

Table 9. Comparison of ASIL and SIL using dangerous failure rate metric (Meany, 2019).

SIL (IEC 61508)	Max. PFH (FIT)	Vs.	ASIL (ISO 26262)	Max. PMHF (FIT)
SIL 1	10000		ASIL A	No maximum
SIL 2	1000		–	–
SIL 3	100		ASIL B	100
–	–		ASIL C	100
SIL 4	10		ASIL D	10

Table 10. Comparison of failure classes for calculating FIT for PFH and PMHF.

	Failure Classes (IEC 615O8)	Failure Classes (ISO 26262)
	Safe Detected (SD)	Safe Fault (SF) Detected Multiple Point Fault (MPFD)
	Safe Undetected (SU)	Latent Multiple Point Fault (MPFL)
	Dangerous Detected (DD)	–
	Dangerous Undetected (DU)	Single Point Fault (SPF) Residual Fault (RF) Perceived Multiple Point Fault (MPFP)
	PFH	PMHF
Failure Classes for Calculating FIT	DU	SPF, RF, MPFP, MPFL and MPFD

Table 11. Comparison of SIL levels of functional-safety standards (Frigerio, 2022).

FuSa Standard	Safety Levels (lowest to highest)
IEC 61508	–	SIL 1	SIL 2	SIL 3	SIL 4
ISO26262	ASIL A	ASIL B	ASIL C	ASIL D
DO-178C	Level E	Level D	Level C	Level B	Level A
IEC 62304	Class A	Class B		Class C
EN 50128	SSIL 0	SSIL 1	SSIL 2	SSIL 3	SSIL 4

Table 12. Mapping of the safety levels of different domains (Verhulst et al., 2013).

Domain	Domain-specific Safety Levels
General (e.g. IEC 61508)	(SIL 0)	SIL 1	SIL 2	SIL 3	SIL 4
Automotive (e.g. ISO 26262)	ASIL A	ASIL B	ASIL C	ASIL D
Aviation (e.g. DO178C)	DAL E	DAL D	DAL C	DAL B	DAL A
Railway (e.g. EN 50128)	SIL 0	SIL 1	SIL 2	SIL 3	SIL 4

Table 13. SIL and ASIL comparison (Marcus & Mieslinger, 2012).

SIL (IEC 61508)	ASIL (ISO 26262)	Description
	QM	–
SIL 1	ASIL A	–
SIL 2	ASIL B	SIL 2 is not fully equivalent ASIL B
	ASIL C	SIL 2 Development requirements SIL 3 Verification requirements
SIL 3	ASIL D	SIL 3 is not fully equivalent ASIL D
SIL 4	–	–

Table 14. Comparing the least significant levels of integrity.

Domain	Domain-specific Safety Levels
General (e.g. IEC 61508)	(SIL 0)
Automotive (e.g. ISO 26262)	QM / ASIL A (conditional)
Aviation (e.g. DO178C)	DAL E
Railway (e.g. EN 50128)	SIL 0
Railway EN 5129:2018	Basic integrity
Railway EN 50126-2:2017	Basic integrity

Table 16. Proposed architectural constraints for ISO 26262 in relation to IEC 61508.

Diagnostic Coverage (ISO 26262)	Hardware architectural metric (PMHF Range in FIT)
	1000 < PMHF ≤ 10000	100 < PMHF ≤ 1000	1 ≤ PMHF ≤ 100
<90%	SIL 1	SIL 1	SIL 2
90 – 97%	SIL 1	SIL 2	SIL 2
97 – 99%	SIL 2	SIL 2	SIL 3
≥99%	SIL 3	SIL 3	SIL 3

Table 15. Proposed mapping scheme for ASIL to SIL.

Standard	Safety Levels
ISO26262	ASIL A	ASIL B	ASIL C	ASIL D	–
IEC 61508	–	SIL 1	SIL 2	SIL 3	SIL 4

Table 17. Safety goals (Copied from 55GAA, 2019).

Hazardous event and associated risk	Safety Goal (SG)	Possible ASIL ratings for selected hazardous events
Controlled Vehicle (CV) causes an accident by receiving wrong or late information from Control Centre (CC) and thus causes a severe accident	SG1: Avoid wrong control information being received by the Controlled Vehicle SG2: Avoid late control information being received by the Controlled Vehicle	If vehicle’s autonomous sensors are still functioning the incorrect information could be checked and therefore accidents due to wrong information can be avoided -> Quality Management (QM) If vehicle’s autonomous sensors are no longer functioning or they are degraded (e.g. because Control Centre commands put vehicle outside Operational Design Domain - ODD) -> ASIL D

Okoh, P., Dong, H. S., & Liu, Y. (2022). Cross-acceptance of fire safety systems based on SIL equivalence in relation to IEC 61508 and EN 50129. Safety and Reliability, 41(2), 103–120. https://doi.org/10.1080/09617353.2022.2107255

 Google Scholar

ARC. (2022). International Engineering Safety Management (IESM) Good Practice Handbook, Volume 2, Issue 1.4: Methods, Tools and Techniques for Projects. Abbot Risk Consulting Ltd. Retrieved from https://static1.squarespace.com/static/60e5bc3a5cbea5566b74ada9/t/627093c0ef8e2040b9a3def8/1651545038338/iESM_Volume_2_Issue_1.4.pdf

 Google Scholar

ISO 26262. (2018). Road vehicles – Functional safety.

 Google Scholar

Meany, T. (2019). Automotive vs industrial functional safety. ADI engineer Zone. Retrieved from https://ez.analog.com/ez-blogs/b/engineerzone-spotlight/posts/automotive-vs-industrial-functional-safety

 Google Scholar

Frigerio, A. (2022). Functional-safety analysis of ASIL decomposition for redundant automotive systems [PhD thesis 1 (Research TU/e/Graduation TU/e), Electrical Engineering]. Eindhoven University of Technology.

 Google Scholar

Verhulst, E., Vara, J., Sputh, B., & Florio, V. (2013). ARRL: A criterion for composable safety and systems engineering.

 Google Scholar

Marcus, F., & Mieslinger, J. (2012). Embedded processing marketing MCU industrial & automotive. In Functional Safety Seminar & 1-Day HerculesTM Workshop. Arrow Roadshow Silkeborg.

 Google Scholar