Machine Safety
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Residual risk is alive and ever present in machine safety

Machine safety and product safety are alike when it comes to the issue of residual risk. Engineers are supposed to hold public safety as paramount and prevent bad things from happening. How?

August 03, 2010

Machine safety and product safety are alike when it comes to the issue of residual risk. From recent news headlines: Miners killed in an explosion; Workers missing after oil drilling platform burns and sinks; Cars accelerate out of control; Volcano threatens airliners; Storms destroy buildings and kill 10 people. These aren’t supposed to happen.

Engineers are supposed to hold public safety as paramount and prevent bad things from happening. Recognizing hazards and mitigating them can be complex. The dangers involving products and machines change over time; the dangers when a machine is being built are different after it is installed and are different again when it is decommissioned and disposed.

Take for example the recent sudden acceleration problems an automotive company had with their automobiles. Now, we know that the hard cable between the accelerator pedal and the carburetor has been replaced by the black box computer and software.

So – why sudden acceleration? Is it software, hardware, EMI interference, mechanical hinge binding, or some other phenomena introduce by technology innovation? And, what complications have been introduced that impact residual risk? Was this potential hazard identified in the risk assessment and what was the accepted residual risk after mitigation measures?

Who’s heard of a BOP valve? Does BP or Gulf of Mexico help? Yes, that’s a blow out preventer valve used in off shore oil drilling. This particular BOP valve has multiple shutoff valves that are designed to be capable of shutting off the flow of crude oil. The concept of multiple shut off valves is to provide several ways as redundant backup solutions thus mitigating the risk of crude oil ever polluting the Gulf of Mexico. The trillion dollar question is – why did the redundant shut off valves not function properly when the control system called for them to close? What did the risk assessment identify as hazards and how were they mitigated? What was the accepted “residual risk”?

OK, now that I have your attention, everyone should realize by now that residual risk will never equal zero. Since most things in life can somehow be explained mathematically we can now understand why traveling by commercial air is safer per passenger than traveling by automobile. It all boils down to – “per trip”! This would change on a dime if automobiles could hold 200 plus passengers. And, we can also relate this discussion to machine safety. Accepting the notion that residual risk never reached zero, then we also have to acknowledge that hazards will always exist on or around a machine. As management and engineers it is our professional responsibility (and, obligation as defined by OSHA) to identify all hazards (risk assessment) and to reduce those hazards to acceptable levels (mitigation plan). The acceptable levels are those levels you or your Company determines.

In my opinion, just as with the two examples above, there will always be residual risk which only time and continuous improvement efforts combined will push residual risk toward zero but not equal to zero. Welcome to the real world where – Residual Risk Is Alive & Ever Present!

Also read two prior blog postings:


· Machine Safety & Residual Risk

· Machine Safety & “Reasonably Foreseeable Misuse”


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For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson & Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at) and via