Reliability Data for Safety Devices

With the transition from Safety Categories to Performance Levels (PL) or Safety Integrity Levels (SIL) in international machine safety standards, product reliability has become a quantifiable requirement. Both the methods for design to PL and SIL require reliability to be calculated for devices used in the safety control system.

Guidance is given in ISO 13849-1:2006 clause 4.5.2, which states the following hierarchy of obtaining reliability values:
  1. Manufacturer’s data
  2. Use Annex C and D in the standard
  3. Choose 10 years
Most manufacturers are now supplying Mean Time To Dangerous Failure (MTTFd) or B10d for their safety components and the tables supplied in the standard offer good guidance for components where manufacturer data isn’t available. However both these data sources require the components to be used in the correct environmental conditions.

How can designers obtain reliability data for real world conditions where temperature, dust, vibration, direct sunlight, etc may adversely affect the lifetime of their components?

Searching on the internet I can find many references for process safety components and installations. Organisations such as Exida publish handbooks for reliability of different components in varying installations, based on real world results. These would be useful for machine safety but are biased towards process components and conditions. Are there such publications specific for machine safety? This would be a useful resource for designers of machine safety systems in harsh or extreme conditions.

For more information be sure to contact our team of safety specialists.

Published: 1 October 2012