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Functional Safety: IEC 61508

Functional Safety, Introduction

Reliability vs. Safety

→ IEC 61508

ISO 13849

IEC 61508 vs. ISO 13849

Summary

IEC 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 are very often useless because there are no binding rules associated with safety certificates. Consequently, the most important information may be hidden behind 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 9001).

IEC 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:

Functional safety of electrical / electronic / programmable electronic safety related systems

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:

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.

IEC 61508 has been released in 1998, however, its roots come from older standards like the German DIN 19250 (withdrawn long ago).
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:

They are less comprehensive than IEC 61508.
They focus more on practical applicability.

The table below lists some safety related standards derived from IEC 61508.

Standard	Description
IEC 61508	Umbrella standard. Applies automatically if no industry-specific standard exists, or if no specific safety related standard is required.
IEC 61511	Modified IEC 61508 for plant manufacturers and process industry. More pragmatic, e.g. allows poor component quality to be compensated with redundancy.
ISO 13849	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.
EN 50128 and EN 50129	Railway specific standards for hardware and software.
ISO 25119	Simplified but more specific variant of ISO 13849 (see above). Tied to the needs of agricultural machinery.
ISO 26262	Automobile 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).

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