We want our system to work. We're going to do everything we can to make it work properly. If it doesn't work, we want it to fail in a predictable and hopefully safe manner. If the failure modes are predictable, we can design the equipment to be fail safe. That's the whole thing. The IEC 61508 standard was created to achieve this goal. 

There are two key fundamental concepts:

• Perform the intended function correctly -  fundamental reliability Engineering 
• Fail in a predictable manner - fundamental safety engineering 

Many countries were involved early on with the development of IEC 61508. I found the strongest influence from the UK and the Germans. The US was also very active in the original standard. IEC 61508 was written from the bespoke system perspective. It's written as if you are going to be building a system. What's the problem with that? If you're trying to use it for a product, like a pressure transmitter, some of the requirements are not applicable. This bespoke system perspective creates some confusion.

61508 is called a basic safety standard. It established the fundamental principles which are used in many different industry specific standards. Remember our goal? Make it work right or know exactly how it's going to fail and try to make it fail in a safe way. We do that by setting up detailed engineering processes, primarily software processes. Evaluate all the hardware based on probability of failure. There's two fundamental concepts that are meant to take care of systematic failures, including design, documentation, faults, everything and random failures. 

If you would like to learn more about IEC 61508, check out our self-paced FSE 211 - IEC 61508 - Functional Safety for Design & Development (Electrical, Mechanical, Software) Course