I don’t like the Risk Assessment process because it is highly subjective and
not very repeatable.
When I was with Hewlett Packard, three of us developed “Hazard Based Safety
Engineering,” HBSE. The basis for HBSE was James J. Gibson’s (Cornell
University) research into child injury from auto accidents. Gibson said:
“Injuries to a living organism can be produced only by some energy interchange.
Consequently, a most effective way of classifying sources of injury is
according to the forms of physical energy involved. The analysis can thus be
exhaustive and conceptually clear. Physical energy is either mechanical,
thermal, radiant, chemical, or electrical.”
In a moving automobile, the automobile and its passengers have kinetic
(mechanical) energy. In an accident, the kinetic energy of the automobile is
dissipated in crumpling parts. The kinetic energy of the passengers is
dissipated in injuries to the body. Seat belts transfer the passenger kinetic
energy to the automobile. Air bags slow the rate of kinetic energy transfer to
the automobile.
HBSE identified the magnitudes each kind of physical energy necessary to cause
injury. We called this “hazardous” energy. Then, HBSE went on to specify
safeguards that would attenuate or prohibit hazardous energy interchange.
When I evaluate a product, I look for the physical energy sources, and then
determine if the energy sources are hazardous or not. Unlike Risk Assessment,
this is easy and repeatable and not subjective. For example, all primary
circuits are hazardous energy circuits that can cause injury (electric shock,
thermal, fire, and maybe more) and safeguards must be provided.
Best regards,
Rich
From: Douglas E Powell <[email protected]>
Sent: Friday, February 11, 2022 11:37 AM
To: [email protected]
Subject: Re: [PSES] EN 62368-1 : 2020 Ed 3
In my view, the Risk Assessment should never be treated as a 'get out of jail'
card or panacea. Instead, it is only a starting point for a safe design and
should be done near the beginning of a project, not the end. I agree with what
Rich says, I've seen a lot of subjective assessments by cross-functional teams,
with variability based on personal risk tolerance or risk aversion. There are
any number of articles pointing to why humans are not very good at assessing
risk (Google search
<https://www.google.com/search?q=humans+are+not+very+good+at+assessing+risk> ).
When using FMEA for risk assessment, I always stress that the RPN factors of
probability of occurrence, severity, and detection be quantified separately
without regard to the other factors, not an easy task. There is also the
problem of RPN vs Criticality (severity x occurrence). If using the RPN, there
is the possibility that Detection can dilute the RPN number to a point below
the threshold for action. So in my view, Criticality alone should be used to
trigger action.
Kenneth Ross wrote a very good article last month on Navigating the Safety
Hierarchy; for me, it was an excellent refresher on how I should use risk
assessment more effectively
(https://incompliancemag.com/article/navigating-the-safety-hierarchy/).
-Doug
Douglas E Powell
Laporte, Colorado USA
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