See also my posting on 21 Dec 2001, giving a link to the IEC FAQ:
http://www.iec.ch/61508/
EXTRACT: "What is meant by a SILn system, subsystem or component?
"A safety integrity level (SIL) is not the property of a system,
subsystem or component. However, the above phrase is often used to denote
that the system, subsystem or component is capable of providing safety
functions with a safety integrity level up to n." [end of quote]
It is indeed appropriate to evaluate the safety function(s) of a PLC (or
other electronic sub-assembly of a safety-related system) to determine
their SIL rating. This facilitates their selection and integration into a
complete safety system.
Of course, a standard PLC is not suitable for a safety application. To
achieve the SIL rating you need to meet specific requirements for failure
rate, architecture, and (most especially) diagnostic coverage. In practice,
a safety PLC will almost certainly be either dual- or triple-redundant.
Also, IEC 61508 takes a "life-cycle" approach, so it is supposed to be
considered at the design stage of the PLC. (If you have good design and
production control processes, you may be able to assess compliance
retrospectively.)
You may also be interested in an FAQ that I wrote a while back.
[Disclaimer: has a small amount of commercial content.]
http://www.tuvasi.com/faq-fs.htm
See http://www.tuv-fs.com/plclist.htm for some certified examples.
[Disclaimer: This is on a website operated jointly by two test agencies].
As to what standard your enquirer referred to, the answer probably depends
on the industry and the region:
-IEC 61508 is international and generic (all industries). Published in 7
parts, total 700 pages approx.
-ANSI/ISA-84.01-1996 "Application of Safety Instrumented Systems for the
Process Industries"
is for the (USA/Canada) PROCESS industry. It was published in 1996
when IEC 61508 was still at draft stage.
-EN 61508 is identical to IEC 61508
-A process-industry-specific functional-safety standard is at draft stage,
IEC 61511
-A machinery-industry-specific functional-safety standard is at draft
stage, IEC 62061
(The sector-specific standards will apply the relevant principles and
methods of IEC 61508 to a specific industry.)
-Some other key differences:
-ANSI/ISA S84.01 uses only three SIL levels (SIL 1 to 3)
-IEC 61508 uses all four SIL levels (1 to 4)
-S84.01 does not address the complete product life-cycle, from design
to decommissioning, IEC 61508 does.
-For a complete analysis, see clause 12 of S84.01. (Clause 12 was written
in 1996, hence is somewhat out-of-date.)
Per ISA: "This standard [S84.01] is process industry specific within the
framework of the International Electrotechnical Commission (IEC) draft
Publication 1508."
"There are significant differences in S84.01 from IEC draft
Publication 1508-1995, as described in Clause 12. However, IEC draft
Publication 1508 was still being developed at the time that S84.01 was
published. As a result, ISA SP84 will continue to support and monitor IEC
draft Publication 1508 development, and will modify S84.01 as needed when
IEC draft Publication 1508 is published."
Per IEC: "The standard [IEC 61508] is generic and can be used directly by
industry (as a 'standalone' standard) and also by international standards
organisations as a basis for the development of sector standards (e.g. for
the machinery sector, for the process sector or for the nuclear sector).
The standard will therefore influence the development of electrical,
electronic and programmable electronic (E/E/PE) safety-related systems
across all sectors."
best regards, glyn
----------------------------------------------
TUV Rheinland of North America, Inc.
Product Safety & Quality
Industrial Machinery Division (Chicago Office)
Glyn R. Garside
Senior Engineer
1945 Techny Rd, Unit 4
NORTHBROOK, IL 60062-5357, USA
Tel (847)562-9888 ext 25
email [email protected]
http://www.us.tuv.com
"Dan Kinney (A)"
<[email protected]> To:
"'[email protected]'" <[email protected]>
Sent by: cc:
owner-emc-pstc@majordom Subject: SIL Ratings
o.ieee.org
05/04/2002 07:45
Please respond to "Dan
Kinney (A)"
A customer called this week and asked what our SIL rating is on one of our
products (Programmable Logic Controller). I had never heard of a SIL
rating. I went to the web and discovered SIL stands for Safety Integrity
Level and is addressed in IEC61508. My impression, from the web search, is
that a SIL analysis would be accomplished on a large manufacturing process
and the outcome would be based on the attributes of all of the components
of
the process, i.e.. sensors, controllers, output devices. I haven't seen
anything on the web that indicates the individual components have a SIL
rating. Does anyone have any experience with either SIL ratings or
IEC61508
who might be able to shed some light?
Dan Kinney
Lead Qualification Engineer
Horner APG, LLC
Indianapolis, IN
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