Failure Rate Data – Don’t fake it!
Performing a Random Hardware Failures Assessment on a Safety Instrumented Function (SIF) is a key requirement specified in IEC 61508 and IEC 61511 as part of SIL Verification. This requirement ensures that that the SIF meets the required target failure measure in terms of Probability of Failure on Demand (PFD) or Probability of Failure per hour (PFH) as identified in the SIL Determination study.
What failure rate data to use?
The most important step, before you begin any reliability modelling, is deciding on what failure rate data to use; with that, all of a sudden, there are numbers everywhere, so which do you choose!
It’s easy to pick data which fits the requirements, but is this really a true representation of YOUR system? Using incorrect data is inevitably going to lead to an unrealistic model and the danger is over-reliance on a system which is actually not as reliable in the real world than as on paper. The consequence..? Your safety critical system may not work when you need it to!
Nowadays there are endless data available from a variety of difference sources, so which one is the right one for you?
Ideally, the best source of data comes from operating experience; actual field data which has been collected on site. This reflects your device being operational in your specific application and thus, provides a true picture of how it functions and most importantly how it fails. It is down to the end-user to ensure that data is collected in a meaningful way and most often than not, it’s the analysis of this data which is deemed most useful.
Generic Device Data
There are a number of industry databases available which provide data for generic devices based on field data for various applications. All generic data should be compared with site data where available. Some failure rate databases provide a range of optimistic and conservative values which could differ by up to two orders of magnitude (102). It is therefore important to appreciate that failure rate data, especially for generic items can only be an estimate.
Data provided by Manufacturers based on field data should be used with caution as it tends to be on the optimistic side. This is because many end users will not notify the manufacturers of failures, particularly when the warranty expires, instead end-users may choose to replace the item without returning it, repair it on site, or simply may tolerate the issue due to possible disruptions in operation. In addition, it is also worth noting that many items are in store before use or may still be in the supply chain. If possible, manufacturers should carry out a Failure Modes Effective Analysis (FMEA) to determine failure rates for specific failure modes.
SIL Certified devices
The concept of using SIL Certified devices may provide a sigh of relief, but unless you have access to the full analysis (e.g. FMEA report), it’s difficult to be confident in its suitability for your application. For example if a SIL certificate for a device shows a PFD/PFH value– how can you be sure that the assumptions that have been made in the analysis are even relevant to your system? Is it the correct failure mode? What is the proof test interval? Etc.
The point is that although failure rate can only ever be an estimate, the criticality of any reliability study lies in using the most appropriate failure rate data which is suitable for the specific application.
Bottom line: Do not try and fit the failure data to what is required or what you want it be!
ESC’s consultants have the expertise and experience to carry out SIL Verification assessment as per the requirements of IEC 61508 and IEC 61511 using the most appropriate failure rate data.
ProSET Software includes the Technis FARADIP-THREE database
ESC’s own software suite ProSet®, comes complete with our IEC 61508-certified SIL Assessment module: SILComp®. The SIL Verification module incorporates the Technis FARADIP-THREE database, which is based on over 40 published data sources together with Technis’s own collection of reliability data.
ESC has extensive experience in providing Product Certification services for Products, giving Manufacturers the assurance that their device is fully compliant with the specified requirements using technical analysis such as FMEAs to ensure complete traceability and validity.