IEC TS 61508-3-1:2016

IEC TS 61508-3-1 ®
Edition 1.0 2016-07
TECHNICAL
SPECIFICATION
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 3-1: Software requirements – Reuse of pre-existing software elements to
implement all or part of a safety function
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IEC TS 61508-3-1 ®
Edition 1.0 2016-07
TECHNICAL
SPECIFICATION
Functional safety of electrical/electronic/programmable electronic safety-related

systems –
Part 3-1: Software requirements – Reuse of pre-existing software elements to

implement all or part of a safety function

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.240.50 ISBN 978-2-8322-3516-4

– 2 – IEC TS 61508-3-1:2016 © IEC 2016
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative References . 6
3 Terms and definitions . 6
4 Requirements . 6
Bibliography . 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/PROGRAMMABLE
ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 3-1: Software requirements –
Reuse of pre-existing software elements
to implement all or part of a safety function

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.

– 4 – IEC TS 61508-3-1:2016 © IEC 2016
IEC TS 61508-3-1, which is a technical specification, has been prepared by subcommittee
65A: System aspects, of IEC technical committee 65: Industrial-process measurement, control
and automation.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
65A/780/DTS 65A/802/RVC
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61508 series, published under the general title Functional safety
of electrical/electronic/programmable electronic safety-related systems, can be found on the
IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

INTRODUCTION
The requirements set out in this technical specification deal with the reuse of software
elements when they are intended to form part of a safety function.
In many fields of automation, software elements are used today in support of safety functions.
Such applications will certainly be further developed and extended. Software engineers,
however, do not always wish to write the software for these applications from scratch, but will
in many cases use already existing software and integrate it with the new application which
might be slightly different from the one for which the software was originally specified.
In IEC 61508-3:2010, a requirement is given in 7.4.2.12. It offers three routes to the
achievement of the necessary integrity for the pre-existing software element. The
requirements to comply with the second route, Route 2 , are defined in IEC 61508-2:2010,
s
7.4.10.
This entails that IEC 61508-3:2010 –dealing solely with software –refers to requirements in
IEC 61508-2:2010 which concerns complete systems including hardware but excluding
software (see IEC 61508-2:2010, 1.1 enumeration “e“).
This technical specification defines the requirements for software elements explicitly, because
IEC 61508-2:2010 excludes software, and is intended to replace the text of the second bullet
(“route 2 ”) of a), 7.4.2.12 in IEC 61508-3:2010 in a future revision of IEC 61508-3.
s
– 6 – IEC TS 61508-3-1:2016 © IEC 2016
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/PROGRAMMABLE
ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 3-1: Software requirements –
Reuse of pre-existing software elements
to implement all or part of a safety function

1 Scope
This Technical Specification presents requirements by the application of which pre-existing
software elements may be claimed to be proven-in-use for all or a part of safety function(s) of
SIL1 or SIL 2.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 61508-3:2010, Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 3: Software requirements
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Requirements
4.1 Notes 1 to 4 below apply to the entire Clause 4 (4.2 to 4.9).
NOTE 1 Any documentation required by a clause in this document could either be available with the pre-existing
software or could be included as part of the documentation of the safety related function.
NOTE 2 A reused software function in this document means a function specified on the level of the requirements
specification (see IEC 61508-3:2010, 7.2). A reused software function does not refer to a programming language
construct.
NOTE 3 Conditions are set for the data on the history of the pre-existing software in 4.2 b) and c). The fulfilment
of these conditions does not entail that the software is deterministic: hidden internal states of the software can
affect its execution even when the required combination as specified in 4.2 b) and c) is exactly the same. The use
of pre-existing software is thus restricted by 4.7.
NOTE 4 In some cases (e.g. input data are analogue data or a clock signal) the demonstration of proven-in-use
for software could be difficult.

4.2 An element shall only be regarded as proven-in-use when:
a) its description:
1) exists and is available;
2) fulfils the requirements of IEC 61508-3:2010, 7.2;
3) describes the previous use,

and
b) the execution of the software with all combinations of all claimed
– combinations of input data,
– sequences of execution of the reused software function(s),
– timing relations within sequences of execution of the reused software function(s)
which will occur in the intended use are documented;
and
c) comb
...