Annex S – Software Integrity Levels in safety-critical industries
Software integrity level schemes are often required in safety-critical industries such as nuclear power,
medical devices, space exploration, etc. Safety-critical industries often require compliance with industry
regulations and use of industry-specific standards. Each industry has its own unique terms for defining
risk and integrity levels as illustrated in Table S-1 below.
An integrity level scheme is a set of discrete values used to define the level of rigor (applied by Software
Development and SQA) to be applied to portions of the system – often from lowest to highest – as
determined by some critical project attribute, such as consequences associated with system failures.
Alternate definitions of integrity level schemes can be developed based on critical project attributes in
addition or instead of consequences. Examples of integrity level scheme are defined in IEEE-1012.
Table S-1 – Summary of Terms Used for Safety-critical Software
Industry
Standard or Reference
Terms Used for Safety Critical Software
Avionics
DO-178B
DO-178B defines five software levels.
Each level is defined by the failure condition
that can result from anomalous software
behavior.
Software Considerations in Airborne
Systems and Equipment
Certification
Failure Condition
Nuclear
Power
Medical
Devices
IEEE-Std 7-4.3.2-2010
IEEE Standard Criteria for Digital
Computers in Safety Systems of
Nuclear Power Generating Stations
Software Level
Catastrophic
Level A
Hazardous/Severe - Major
Level B
Major
Level C
Minor
Level D
No Effect
Level E
Software Integrity Levels.
IEC 61513 provides requirements
and recommendations for the
instrumentation and control for
systems important to safety of
nuclear power plants.
“The software V&V effort shall be performed
in accordance with IEEE Std 1012-2004.
The IEEE Std 1012-2004 V&V requirements
for the highest integrity level (software
integrity level 4) apply to systems
developed using this standard (i.e., IEEE
Std 7-4.3.2). See IEEE Std 1012-2004
Annex B for a definition of integrity level 4.”
IEC Standard 62304:2006
Software Safety Classification:
Medical Device Software - Software
Lifecycle Processes
IEC Standard 62304:2006 software safety
classification are based on severity as
follows:
Class A
No injury or damage to health is
possible.
Industry
Space
Exploration
Transportation
Standard or Reference
Terms Used for Safety Critical Software
Class B
Non-serious injury is possible.
Class C
Death or serious injury is
possible.
NASA Technical Standard 8739.8
2004
Software Class:
Class A
Human Rated Software Systems
Software Assurance Standard
Class B
Non-Human Space Rated
Class C
Mission Support Software
Class D
Analysis and Distribution
Software
Class E
Development Support Software
IEC 61508
Safety Integrity Level
Functional Safety of
Electrical/Electronic/Programmable
Electronic Safety-related Systems
The Safety Integrity Level (SIL) is
determined from the probability of failure.
For systems that operate continuously the
allowable frequency of failure must be
determined. For systems that operate
intermittently the probability of failure is
specified as the probability that the system
will fail to respond on demand.
Rail
EN 50128 provides a specific
interpretation of IEC 61508 for
railway applications.
SIL
Low demand
mode:
average
probability of
failure on
demand
High demand or
continuous
mode:
probability of
dangerous
failure per hour
1
≥ 10-2 to < 10-1
≥ 10-6 to < 10-5
2
≥ 10-3 to < 10-2
≥ 10-7 to < 10-6
3
≥ 10-4 to < 10-3
≥ 10-8 to < 10-7
4
≥ 10-5 to < 10-4
≥ 10-9 to < 10-8
Automotive Software
The development of software for
safety related automotive systems
is predominantly covered by the
Motor Industry Software Reliability
Association guidelines. The MISRA
project was conceived to develop
guidelines for the creation of
embedded software in road vehicle
electronic systems. In November
1994 Development guidelines for
vehicle based software was
published. This document provides
the first automotive industry
interpretation of the principles of the
emerging standard IEC 61508.
Additional information on the definition and use of integrity levels can be found in ISO-15026-Part 3.