adequately with technical uncertainties
& Quality Management
Safety vs. Reliability
IEC 61508 vs. ISO 13849
61508 plays a vital role in functional safety. IEC 61508 applies
for suppliers, manufacturers and distributors of safety related
systems. Safety certificates are a popular means for demonstrating
compliance with safety standards, however, such certificates very often
useless because there are no official rules regarding safety
certificates. Consequently, the most important information may be
hidden behind those topics that are not mentioned
on the certificate.
For plant manufacturer, IEC 61511 (very similar to IEC 61508) applies.
Like other process oriented management standards, IEC 61508 is very
generic. It does not state how safety related products should look
like. Instead, IEC 61508 states requirements for the development
process of safety related
products. (The most popular process oriented management standard is ISO
61508 is not only the first safety standard with remarkably
international recognition, it is also the most comprehensive one.
Furthermore, IEC 61508 is still of high importance because all today's
safety related standards are either derivatives from IEC 61508, or have
at least been inspired by IEC 61508.
While being generic on the one
hand, the official name of IEC 61508 proves that this standard is
formally restricted to electronics:
of electrical / electronic / programmable electronic safety related
However, from a practitioner's
viewpoint, this formal restriction is not factual because all basic
elements of IEC 61508 can be applied in a similar way for nonelectronic
system components like hydraulics, pneumatics, mechanics, etc.
The actual reasons for focussing on
electronics may be these:
IEC 61508 has been released in 1998,
however, its roots come from older standards like the German DIN 19250
(withdrawn long ago).
- Electronics offer way more freedom in design than other system
types. As a result, there are way more failure modes and therefore way
more potential dangers that have to be addressed.
- Most safety related concepts do need electronics.
The principal motivation for
developing this generic functional safety standard can be seen in the
consequences of man-made catastrophes like Piper Alpha 1989.
IEC 61508 is a so called umbrella standard. All today's safety
standards (at least those with some international recognition) are
derivatives of IEC 61508, however very often with industry specific
add-ons. All derivatives have the following things in common:
The table below lists some
safety related standards derived from IEC 61508.
- They are less comprehensive than IEC 61508.
- They focus more on practical applicability.
Applies automatically if no industry-specific standard exists, or if no
specific safety related standard is required.
|Modified IEC 61508 for plant
manufacturers and process industry.
More pragmatic, e.g. allows poor component quality to be compensated
||Relatively small standard.
Applicable for all industries.
"Safety of machinery - safety-related parts of control systems"
Very popular standard due to its compactness while still addressing all
basic safety aspects. Easy to apply.
Special feature: Two key performance indices (instead of one): Category
and Performance Level.
specific standards for hardware and software.
|Simplified but more specific
variant of ISO 13849 (see above). Tied to the needs of agricultural
standard. Similar to IEC 61508 at first glance, but a closer look
reveals huge differences. ISO 26262 uses Automotive SIL (ASIL) instead
of SIL (IEC 61508).