Hello all,
I am keenly interested in this discussion on energy vs energy/time with
respect to causing injury, especially from a physical contact
perspective. A couple of other considerations that are worthy to be
included in such a discussion, which would add another layer of
complexity would be:
* The geometry of the object that comes into contact with the body.
Take a needle as an example, or a knife edge. Let's say the needle
is very light (a gram or two) and moving very slowly (1 mm/hour), at
some point severe injury could take place through piercing of skin.
Real world scenario aside, this illustrates a different kind of
potential harm that can be inflicted. I don't know if there is a
practical way to incorporate such a factor into an equation (such as
the Energy Transfer equation), but certainly it needs to be included
as part of a thorough analysis.
* The kind of "victim" person being considered, for example a child vs
an adult. A child may not be able to withstand the same level of
energy transfer as an adult, or may be subject to a different kind
of injury as the result of an energy transfer, that is, a secondary
injury, such as being more easily pushed down and hitting their head
on the floor. In fact, I am interested to know if there are others
out there who are aware of any data or studies regarding impact with
children.
Thanks,
Mark
Hi Doug:
Thanks for your comments.
What bothers me about risk assessment is that the committees that have
written the standards requiring risk assessment have not critically
evaluated the risk assessment process. If they had done so, we would
not have the process as we know it today.
Actually, I do not fully agree with the Gibson finding that energy
causes injury. I can show that the injury parameter is energy per
unit time, e.g., joules/second. The body can absorb energy slowly
without injury, but not quickly. Consider that a car with people in
it can brake or stop without causing injury to the passengers, but
cannot “crash” to a stop where injury is likely. In both cases, the
kinetic energy to stop is the same, but the kinetic energy per time to
stop is low in braking, but high in crashing.
The attached picture is that of catching three objects and assumes the
deceleration time is the same for each object. Note that when we
catch an object, we can catch it “slowly” and distribute the energy
over a longer time than catching it “directly.” I submit this as
proof that energy per unit time is the parameter that causes injury.
HBSE does indeed have (or can have) energy criteria for each form of
energy. However, I agree that some energy data is not readily
available and must be researched. And, using the energy model can be
quite complex. For example, injury from thermal energy is often
simply taken as accessible temperature, sometimes including a time of
contact. Using a single parameter, temperature, or including time of
contact parameter, does not address the difference between an aluminum
block and aluminum foil (which is the issue some members of IEC
TC108/HBSDT are addressing). Or the difference between an aluminum
block and a plastic block.
Best regards,
Rich
*From:* Douglas Nix <d...@mac.com>
*Sent:* Monday, February 14, 2022 12:46 PM
*To:* Richard Nute <ri...@ieee.org>
*Cc:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* Re: [PSES] Risk assessment versus HBSE
Hi Rich,
I have to admit that I’ve been thinking about your reply all weekend.
As you know, I teach machinery risk assessment and consult in this
area regularly. I want to stipulate that there are some significant
issues with risk assessment the way it is most commonly applied in
industry, see my list of references on this topic at the end of my
message.
The inherent subjectivity of risk assessments that are performed
without empirical data is unquestioned. The difficulty is that for
many areas of human endeavour we have no empirical data, and try as we
might we cannot calculate without numeric data. Nevertheless, we must
be able to make risk-based decisions when designing products and
equipment, and so we muddle along with the best tools that we have,
hopefully while recognizing their flaws.
The HBSE model is a good one, and it fits machinery applications as
readily as does risk assessment, however, the risk assessment methods
that are used today have a history that stretches back to the 1960s,
while the HBSE model is much younger. This doesn’t take away from HBSE
in any way for me, but it does have an impact on the broader
acceptance of the method since it is not yet as widely known as
“conventional” risk assessment. None of the the standards in the
machinery safety sector recognize the method as yet, so getting
regulators and users to consider the method is a challenge.
HBSE also suffers from issues with lack of data when it comes to
characterizing some hazards, leaving the user to estimate the
characteristics. This brings in the biases of the person(s) doing the
estimating just as surely as conventional risk assessment methods.
The absence of a probability parameter in the HBSE model is an
interesting one, since the probability aspect is the one most subject
to error in conventional risk assessment. Humans are notoriously bad
at estimating probability. It appears to me that the absence of that
parameter implies that the presence of a hazard will inevitably lead
to harm, which I don’t disagree with. CSA Z1002, OHS risk assessment,
actually states that this is the case, and recommends that hazards are
eliminated on this basis whenever possible.
So we’re left with this situation, I think:
1) Risk assessment, when done quantitatively using sound statistical
techniques and valid data is a useful and relatively objective method
to provide data to decision makers,
2) Conventional risk assessment using subjective opinions and risk
matrices or decision trees are unrepeatable and therefore
unscientific, however. despite their flaws, they provide a means to
help guide decision makers,
3) HBSE improves on some aspects of conventional risk assessment by
eliminating the probability parameters, but is still subject to some
subjectivity, and is still not widely accepted enough for some
decision makers.
I wish there was a more utopian perspective to take on the topic, but
I have yet to find my way to it.
*References*
[1] E. S. Levine, “Improving risk matrices: The advantages of
logarithmically scaled axes,” /J. Risk Res./, vol. 15, no. 2, pp.
209–222, 2012.
[2] R. Long, “Calculators , Matrices and Mumbo Jumbo Risk Assessment,”
/Safetyrisk.net/ <http://Safetyrisk.net>, 2016. [Online]. Available:
http://www.safetyrisk.net/calculators-matrices-and-mumbo-jumbo-risk-assessment/.
[Accessed: 03-Feb-2016].
[3] D. J. Ball and J. Watt, “Further Thoughts on the Utility of Risk
Matrices,” /Risk Anal./, vol. 33, no. 11, pp. 2068–2078, 2013.
[4] C. Bao, D. Wu, J. Wan, J. Li, and J. Chen, “Comparison of
Different Methods to Design Risk Matrices from the Perspective of
Applicability,” /Procedia Comput. Sci./, vol. 122, pp. 455–462, 2017.
[5] C. Peace, “The risk matrix : uncertain results?,” /Policy Pract.
Heal. Saf./, vol. 0, no. 0, pp. 1–14, 2017.
[6] B. Ale and D. Slater, “Risk Matrix Basics,” 2012.
[7] P. Gardoni and C. Murphy, “A Scale of Risk,” /Risk Anal./, vol.
34, no. 7, pp. 1208–1227, 2014.
[8] P. Baybutt, “Guidelines for Designing Risk Matrices,” /Process
Saf. Prog./, vol. 00, no. 0, p. 7, 2017.
[9] H. J. Pasman, W. J. Rogers, and M. S. Mannan, “Risk assessment:
What is it worth? Shall we just do away with it, or can it do a better
job?,” /Saf. Sci./, vol. 99, pp. 140–155, 2017.
[10] X. Ruan, Z. Yin, and D. M. Frangopol, “Risk Matrix Integrating
Risk Attitudes Based on Utility Theory,” /Risk Anal./, vol. 35, no. 8,
pp. 1437–1447, 2015.
[11] S. Albery, D. Borys, and S. Tepe, “Advantages for risk
assessment: Evaluating learnings from question sets inspired by the
FRAM and the risk matrix in a manufacturing environment,” /Saf. Sci./,
vol. 89, pp. 180–189, 2016.
[12] P. Thomas, R. B. Bratvold, and J. E. Bickel, “The Risk of Using
Risk Matrices,” /SPE Annu. Tech. Conf. Exhib./, no. April 2015, 2013.
[13] F. Gauthier, Y. Chinniah, D. Burlet-Vienney, B. Aucourt, and S.
Larouche, “Risk assessment in safety of machinery: Impact of
construction flaws in risk estimation parameters,” /Saf. Sci./, vol.
109, no. June, pp. 421–433, 2018.
[14] O. Amundrud and T. Aven, “On how to understand and acknowledge
risk,” /Reliab. Eng. Syst. Saf./, vol. 142, pp. 42–47, 2015.
[15] S. O. Hansson and T. Aven, “Is Risk Analysis Scientific?,” /Risk
Anal./, vol. 34, no. 7, pp. 1173–1183, 2014.
[16] J. Li, C. Bao, and D. Wu, “How to Design Rating Schemes of Risk
Matrices: A Sequential Updating Approach,” /Risk Anal./, 2018.
[17] L. A. Cox, D. Babayev, and W. Huber, “Some limitations of
qualitative risk rating systems,” /Risk Analysis/, vol. 25. pp.
651–662, 2005.
[18] L. A. Cox, “What’s wrong with risk matrices?,” /Risk Anal./, vol.
28, no. 2, pp. 497–512, Apr. 2008.
[19] A. Quintino, “What’s Wrong with Risk Matrices? Decoding a Louis
Anthony Cox paper Reshaping dowsntream configuration View project An
integrated risk management model for an oil and gas company View
project,” no. March 2011, 2016.
--
Doug Nix
d...@mac.com <mailto:d...@mac.com>
“If there are no dogs in Heaven, then when I die I want to go where
they went.” -Will Rogers
On 12-Feb-22, at 16:59, Richard Nute <ri...@ieee.org> wrote:
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 <doug...@gmail.com>
*Sent:*Friday, February 11, 2022 11:37 AM
*To:*EMC-PSTC@LISTSERV.IEEE.ORG
*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/
<https://incompliancemag.com/article/navigating-the-safety-hierarchy/>).
-Doug
Douglas E Powell
Laporte, Colorado USA
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Brain Corp <https://www.braincorp.com>
Mark Ortlieb
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