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Slip Probability and ALARP
ALARP means reducing a risk to ‘As Low as Reasonably Practical’
For the UK HSE to arrive at a safe minimum recommended PTV value was probably not an easy journey and even today the values ‘recommended’ will, to a lesser degree, be subjective. The HSE recommend a safe PTV (Pendulum Test Value) of 36 should be aimed for, but what if the value is 33 or 34; is it still a failure?
The text below discusses the ALARP principal and how when it is applied can be used as a substantive argument where required.
Many industries, and the HSE, use the ALARP principal, which means reducing a risk to ‘As Low as Reasonably Practical’ and involves assessing the statistics and weighing the risks against the trouble, time and money needed to control them.
Statistical figures are available for topics like car and plane crashes but the science of determining what is safe in respect to slips on floors is not so straight forward; statistics are poor, people slip every day but unless they are hurt or die, then the incident goes unrecorded. Statistical estimates made have to evolve from scientific research and sometimes best judgement, using the published papers of doctors, scientists and Tribologists (the study of the science and engineering of interacting surfaces in relative motion).
The HSE has recommended that a risk figure of 1 in 1 million slips is an ALARP value that should be aimed at; and this value has been translated to a PTV value of 36.
Translating ALARP into a worked example
Take the example of an Airport where it’s determined 1 million people tread its floors every year.
The floor PTV (when Wet) is only 29, which the HSE has determined relates to a chance of slip when wet of 1 in 10,000 (See the table below).
In theory 1,000,000 divided by 10,000 means 100 people are likely to slip (and probably fall) every year. However, the floor has an excellent PTV of 80 when dry; the airport know their floor has possible issues but can’t afford the cost of new flooring but does makes every ‘reasonable and practical effort’ to prevent the floor becoming wet and clear up spills immediately thereby reducing the number of slips to a number which is ‘As Low As Reasonably Practical’ – i.e. ALARP.
However, If the floor is constantly getting wet through rain, snow, spilled drinks etc. and little effort is made to prevent this (such as entrance mats and good and prompt housekeeping) then 100 slips every year is likely to occur and would be wholly unacceptable.
Table of Slip Risk – The table below is one that has evolved over time and is accepted and used by the HSE and UKSRG as a realistic examination of the risks.
The table was devised by Pye and Harrison and googling these names will turn up a plethora of information on the subject,
This subject is also covered in a very good publication on these topics in the CIRIA (652) publication:
‘Safer Surfaces To Walk On – Reducing the Risk of Slipping’ ISBN 13: 978-
Pendulum Test Value Probability of Slip HSE categorisation of slip risk Relates to Coefficient of Friction between foot and floor of: -
36 1 in 1,000,000 Low Risk Between 0.36 and 0.40
34 1 in 100,000 Moderate Risk Between 0.34 and 0.38
29 1 in 10,000 Between 0.29 and 0.34
27 1 in 200 Between 0.27 and 0.32
24 1 in 20 High Risk Between 0.24 and 0.29
Less than 24 Less than 1 in 20 Less than 0.24
Additional information on ALARP and Slip Probability
Further info can be found at the relevant Links below – Please be aware that hyperlinks have a tendency to change on the net and not work but Googling the relevant term should find you what you seek
Google ‘HSE ALARP’
Google ‘FLOOR SLIP PROBABILITY’
Google ‘SAFER SURFACES TO WALK ON – REDUCING THE RISK OF SLIPPING’