Back to Basics: Failure Rates – λD

Apr 02, 2020


The Greek symbol  λD represents dangerous failure rates in functional safety, usually expressed in the unit of measurement of FITs, and can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λD is the number of dangerous failures per unit time for a piece of equipment. This would be a time when the failure would prevent the Safety Instrumented Function (SIF) from performing its intended job, and the SIF cannot achieve the safe state if needed.

Examples of dangerous failures could include: 

  • A valve getting stuck in the open position in a closed-on-trip application
  • Solenoid signal not alerting the actuator
  • A valve stem not able to move the ball from being sheared
  • Pressure sensor not sensing high pressure
  • A flame detector not sensing the flame
  • A PLC not transmitting a signal
  • Major Internal leaks
  • A Transmitter shorting

λD can be broken down into subclasses:  λDD and  λDU; the detectable dangerous failures, and the undetected dangerous failures, where  λD =  λDD +  λDU.

[/vc_column_text][/vc_column][/vc_row][:zh][vc_row][vc_column][vc_column_text]No! They are not Inherently Safe!

A collaborative robot is intended to work “collaboratively” with a person. i.e. share a common workspace. It is force and speed limited by design to minimize any potential hazard. Collaborative robots fit the application where the task cannot be easily or cost effectively automated. They are easy to deploy, program and repurpose. Collaborative robots are new to everyone including the standards agencies.

A hazard and risk assessment is required that assesses the robot and the environment that it is deployed in. Just as any other robot, things such as collisions, speed, type of end effector and worksite need to be evaluated. Collaborative robots have their own sorts of collisions and hazards. They may not be as severe, but they still exist.

This all comes down to risk and the amount of risk that you are willing to accept! The diagram below shows the high-level steps for doing a Hazard and Risk Assessment. When following the steps, if you assess the risk and find it to be acceptable (your companies acceptable risk norms) then you are done. No need to add any risk reduction.

The next best approach is to determine if protective measures other than a Safety Function can reduce the risk to an acceptable level. If not, then you must assign a SIL and implement a safety function that will provide the required risk reduction.

exida can effectively train your team to perform machine hazard and risk assessments to identify all possible hazards and estimate the risk for each hazard. Specifically, exida coaches you through the process of evaluating the risk, developing and implementing risk reduction options. exida can also educate your team in multiple approaches to SIL target selection. These are just some of the things exida does to ensure you are on the right path![/vc_column_text][/vc_column][/vc_row] [:]

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