Sent August 10, 2007
Fundamental Safey Engineering Definitions
In the prior issue I mentioned an excellent academic article that appeared in Ergonomics, Vol 49, No. 10, 15 August 2006, 982-995. Title: “Prediction of Slips: an Evaluation of Utilized Coefficient of Friction and Available Slip Resistance,” J. M. Burnfield and C. M. Powers. It contains a figure showing the likelihood of slipping in terms of available slip resistance, as measured with an XL. It sticks in my mind clearly because when I visited Chris Powers at his lab some years ago, he invited me to witness some of the walk trials Judy was conducting, and I was given the honor of measuring the available traction on the force plate following an actual slip occurrence.
Some of the excellent feedback I got from the last issue showed me that my position on the definition of a dangerous floor had not been adequately stated, and I didn’t want my prolixity to go to waste to that extent. So here’s more.
To: William English
Sent: Wednesday, August 08, 2007 6:54 PM
Subject: Your most recent newsletter
According to the way I read the graph in the paper by Judy and Chris, at a 0.28 COF, there is about a 10% chance of slipping. I would not call that dangerously slippery. Am I reading the paper incorrectly?
Rather than just reproduce my simple answer to this loyal XL user here, I need to expand on what I wrote in the last issue, giving fuller definitions of the terms that apply. In evaluating a situation for safety/hazard (the two terms are at opposite ends of a continuum. Safety is freedom from hazard), the safety engineer uses certain defined terms:
Hazard. Any situation that can produce injury.
Risk. The mathematical probability of injury arising from the number of people exposed and the time or frequency they are in the proximity of the hazard, and the severity of injury potential presented by the exposed hazard. The more man/hours of exposure to a given hazard, the higher the risk; and the higher the injury potential, the higher the risk.
Severity. Different hazards will present varying severity potential. For example, exposure to high voltage electricity or a fall from elevation is likely to result in fatality every time. Use of a paring knife might result in a cut finger and a bandaid, but there will likely be little long-term consequence. The former category of exposure is said to have a High-Potential injury possibility, and the latter would be low-potential. The safety engineer concentrates his primary effort on high-potential exposures.
In accident analyses, severity is often inferred from incurred costs or reserves set by the risk financing function. In fall cases, however, the degree of injury (or severity) is beyond the control of the victim. Every fall case is a potential fatality, depending upon how the fallee goes down and what he hits.
Danger. The unreasonable or unacceptable combination of hazard and risk, as defined above.
The safety engineering assessment of liability in a given injury-producing accident holds that No hazard is acceptable in a given enterprise if reasonable safety engineering measures would have eliminated or reduced the hazard. Safety engineering is the application of arts and sciences to the prevention of accidents and injuries. It can involve (1) the removal of a hazard, (2) control of a hazard (by guarding or personal protective equipment), or (3) reduction of the likelihood of occurrence or severity of accidents and injuries by these means or by removal of exposure to people by automation.
Another fundamental risk management principle is that High-potential loss types are worthy of high-priority loss control management attention. And since falls are the leading non-automotive accident type producing serious injuries and deaths in the US, they are worthy of the safety practitioner’s primary attention.
Applying this process to the inquirer’s question, it can be reasoned that if one in ten people will slip on a .29 surface, and if the transaction count in the facility (say, a service restaurant) suggests an exposure of 5,000 people per week, that means that there would be 500 slip occurrences per week. And the demographics in the particular restaurant concept affects risk. If it is comprised of mostly older people (having relatively brittle bones, slow reflexes and poor muscle tone), there would be a relatively high frequency of falls reported, and severity of injury would be high. A restaurant chain having that combination of danger and exposure would have incurred losses from falls on the premises that could threaten the failure of the business after a couple of months. A medium volume coffee shop might make $1,000/day in profit, and loss from reported fall cases at that rate would exceed the store's income, if WC and general liability exposures are combined.
So an enterprise with this combination of exposure to severe hazards at such a cost to the business would be in danger itself, even if you want to argue that the people entering the store were not in danger.
As I mentioned to the questioner, there are factors affecting exposure to injury (the danger presented) beyond the available traction on the floor. Floors are seldom absolutely clean and dry in normal operations. Pedestrians’ shoes have a range of traction properties, besides the fact that they are not pristine (from walking in parking lots and restaurants, for example), and a significant safety factor is required to afford reasonable safety to the invitees of the facility. There is also the variability of the slipmeter. The ASTM F13 workshop results published in the red book on pp. 69 ff show the precision and the XL .
Aside from all of these complicating considerations, we have what I consider to be more significant indicators of floor safety. Investigators of large numbers of fall cases get to know practically what floors and conditions produce falls. Also the people working in those facilities know what is slippery. They will even tell you, if you ask them. That is more useful to me as a safety practitioner than laboratory ambulation tests conducted under artificial conditions with all physically-fit college students being attached to an overhead tram rail by a parachute harness. I’m not criticizing the USC research project. It is the best one I know of. I am just pointing out that there are considerations in most fall cases that cannot be present in laboratory experiments.
So, you won’t get a lot of respect from me if you make your living telling people that hazardous conditions are safe or that safe conditions are hazardous.
[For more background in these fundamentals of safety engineering and risk management, see the preamble to Chapter 21 on “Nonemployee Accident Prevention” in the National Safety Council’s Accident Prevention Manual for Industrial Operations and Chapter 1, “Pedestrian Slip Resistance: the Remaining Safety Frontier” in my Pedestrian Slip Resistance, How to Measure It and How to Improve It. If you testify in court, you need to know these concepts cold.
More on Definitions
Remember that a safe floor is one that meters .50 or higher with the XL.
Are We Wasting Our Time in ASTM?
The above user also asked another question:
Do you think we are wasting our time and money attending F 13 meetings?
My answer was
No, I plan to keep going to F13 meetings for several
1. I meet important people there, including important experts and my leading adversaries. It helps me keep tabs on them, and my presence might even irritate them a little.
2. I want to prevent the passage of standards that I oppose. It helps me anticipate what is coming next if I go to meetings.
3. Many of my users are there, and it is a good chance for me to network.
4. F1637 is a very useful standard, and it is of sufficient importance alone for me to be active so as to help propagandize against the incompetent defense prostitutes who don't like what it says.
5. As one who testifies in court, I need to be able to report that I am active in the relevant standards committees.
William English, Inc.
Phone 239/728-3254, FAX 239/728-2304.
Visit http://www.EnglishXL.com for the latest in Slip Resistance Technology or http://professionalsafetyengineer.com/ for safety engineering consultation. _____________________________________
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