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.


James Davis said...

I agree it can be challenge to provide customers with accurate reliability data for their specific application.

Rohan Pandit said...

What is the differance between MTTFd and B10d?

Craig Imrie said...

MTTFd (Mean Time To Dangerous Failure) is a reliability measure based on time. The product will have a MTTFd in terms of years, safety components such as safety relays, safety PLCs, safety I/O will be supplied with a MTTFd from the manufacturer.

B10d is a reliability measure based on number of cycles, this figure is the number of cycles at which 10% of the tested product fail to a dangerous state. This measure is supplied for products that reliability is dictated by the number of operations such as Interlock switches, limit switches, contactors. There is a simple equation in Annex C of ISO 13849-1, that the designer can use calculate a MTTFd from the product’s B10d.

Alistair Keenan said...

B10d is the number of operations that a device makes before 10% of the test sample fails.
MTTFd is the calculated value in years that represents the mean time before a failure to a dangerous or undetected unsafe state. B10d value is used to calculate this value along with expected number of operations.

Machine safety Blog said...

Great Contribution to Safety,

Published: 1 October 2012