Slips Potential Model

The Slips Potential Model forms an effective risk assessment framework and is the basis of our own comprehensive slip risk assessments.

The Slips Potential Model is the result of research conducted by the Health and Safety Laboratory on behalf of the HSE. It highlights the need for a holistic approach to slip risk assessment, identifying the various factors, not limited to flooring alone, affecting the risk of a slip.

Overview

The Slips Potential Model was devised by the HSE on the basis of research conducted by the HSL into the causes of slips. The model takes into account the range of contributing factors in a slip incident. The factors are as follows, falling generally into controllable or predictable issues;
     Controllable - Floor material, environment and contamination
     Predictable - Use, behaviour and footwear

The Slips Potential Model aims to demonstrate the inter-relationship between factors affecting slip risk. For an accurate risk assessment all factors must be considered, a single factor in isolation is meaningless. This holistic approach to slip risk assessment is "considered to be the definitive method within the current state of understanding" and forms the basis of our assessment.

Floor material

The vast majority of slips occur on surfaces that are contaminated in some way. Largely, clean and dry floor surfaces provide safe levels of grip on a level surface. In our extensive experience we have tested only a single clean and dry floor that presented an unsafe level of grip. It follows then, that wherever and whenever possible flooring surfaces accessible to pedestrians should be kept clean and dry.


Flooring should be specified to ensure that it has sufficient slip resistance to cope with its expected use. The pendulum test (BS 7976-2) accurately measures the coefficient of dynamic friction of a surface and has an extensive history of correlation to slip incidents. It remains the only method expressly recommended by the HSE.

The construction of the floor surface can have a significant and sometimes unexpected impact on slip resistance. Heavily profiled floors with smoothed edges serve to reduce contact area and increase slip risk significantly, contrary to expectations of those using the surface.

The micro roughness of a floor surface is an important property. Generally a greater roughness means more viscous contaminants can be dispersed, securing sole/floor contact and ensuring safe grip levels. We are not of the opinion that the Rz surface roughness parameter should be used as the basis of a slip risk assessment however.

The surface roughness properties of a floor product can also give an indication of slip resistance, though they are by no means as accurate as the direct friction measurement taken by the pendulum. Generally to provide adequate grip a floor will require a higher surface roughness to cope with more viscous contaminants. As a guide, a roughness of 20µm is likely to be sufficient to provide a low risk of slip in water contaminated conditions.

​Floor surfaces change over time with the effects of wear. The roughness of a floor surface, the property aiding slip resistance in contaminated conditions, is usually (though not always) reduced over time. Surface roughness measurement can give an indication that wear is occurring whilst pendulum testing will directly measure effects of the wear on slip resistance. It should be noted that even newly installed products are likely to have a different slip resistance to 'factory value' due to installation processes, contamination and any sealing/cleaning processes.

Where floor surfaces of different slip resistance join it should be visually apparent. A pedestrian going from a high to low friction surface is more likely to slip if they do not adjust their gait to take the new surface into account. Likewise a pedestrian may stumble and fall going from a low to high friction surface unexpectedly, especially the elderly with low toe clearance, impaired vision, or reduced agility. A surface of uniform appearance is expected to be of uniform slip resistance by those using it.

Further reading  -  Slip Test Services  /  The BS 7976 Pendulum Test  /  Surface Roughness (Rz)

Environmental factors

The Slips Potential Model highlights environmental factors contributing to slips. One of the greatest environmental contributors is lighting, either an excess or inadequate levels to safely see. Excessive glare on polished, reflective surfaces can obscure the view of contaminants exposing the user to a significant reduction in the available friction without warning. This is likely to result in a slip. Conversely, insufficient lighting can hide contamination and also prevent satisfactory cleaning, increasing the risk of a slip

Environmental factors not directly related to the floor but distracting the users of the floor can increase slip risk. If a pedestrian's attention is concentrated elsewhere, they are not paying as much attention to their surroundings and may fail to notice contaminated, sloped or low friction surfaces and act accordingly. Loud noises, visual distractions and activity that the user is involved in can contribute to a slip in this way. If the pedestrian is rushing in crowded and unfamiliar surroundings, such as a hospital or transport terminal, they are more likely to be concentrating attention on overhead signage at the expense of the floor surface, increasing slip risk.

Condensation forms a less obvious source of contamination stemming from environmental factors. In humid conditions condensation is likely to form on cooler surfaces such as walls, windows or ceilings and drip onto the floor, or even form on the floor directly. Glazing, pipe work and metal surfaces should be given specific attention.

It is a statutory requirement that floor surfaces provide a safe level of grip. Specification should take into account likely use and contamination to ensure pedestrians are not put at risk.

Low lighting is likely to hide surface contamination, denying pedestrians a chance to adjust their gait and significantly increasing chances of a slip.

Contamination

Of all the factors considered in the Slips Potential Model, contamination is likely to be the greatest contributor to slip incidents. The vast majority of recorded slips occur in contaminated conditions with very few incidents occurring on clean and dry floors. "Contamination" includes water but is not limited to liquids. Dust or loose particulate can cause a similar loss of grip through a different mechanism. Pendulum testing is generally conducted with water as it forms the most common contaminant, however it is recommended that testing is conducted with contaminants most likely to be found on that particular surface. We have conducted testing with a wide range of contaminants including water, motor oil, tomato sauce, mayonnaise, chocolate, MDF dust, rape seed oil and meat juice. More viscous contaminants are likely to reduce friction further than water based contaminants.

Effective cleaning regimes are pivotal in keeping slip risk to a minimum. Cleaning should be structured to restrict access to wet areas. Effective anti-slip footwear may be required for cleaners.

Contamination has a detrimental effect on the available grip due to the effects of the hydrodynamic squeeze film, causing a hydraulic uplift and preventing or reducing sole/floor contact. Surface roughness, though not necessarily the Rz value, of a floor has significant importance in its ability to puncture the hydrodynamic squeeze film and restore sole/floor contact and acceptable levels of grip. Further discussion of the role of surface roughness in slip risk assessment can be found here.

The likelihood of contamination causing a slip can be reduced through a variety of controls. Firstly, wherever possible, contamination should be prevented from ever reaching the floor where it can pose a risk. Working practices and effective doorway management can serve to reduce the chances of the floor becoming contaminated. Cleaning effectively is vital to remove contamination that cannot be avoided. It should also be noted that a wet cleaning process in itself will contaminate the floor for a period and during that time controls should be put in place to ensure access to the slippery floor is restricted.

Where contamination cannot be reliably prevented the surface should be capable of providing safe levels of grip in expected conditions. This is especially important in high risk areas such as stairways and ramps.

Where a floor surface is known to offer insufficient grip in contaminated conditions management regimes should be effective at removing contamination, or restricting access to the contaminated area. This must occur within an period of time such that users of the surface will not be exposed to an undue risk of slipping.

The HSE offer the following "Top Tips" on effective cleaning;

• Use the right amount of cleaning product
• Detergent needs time to work on greasy floors
• Cleaning equipment will only be effective if it is well maintained
• A dry mop or squeegee will reduce floor-drying time but whilst the floor is damp there will be a slip risk
• A well-wrung mop will leave a thin film of water sufficient enough to create a slip risk on a smooth floor
• Spot clean where possible

In our experience around 50% of floors tested that presented an unsafe level of grip could be significantly improved with an effective cleaning regime and products.

Further reading  -  Slip Test Services  /  Surface Roughness (Rz)  /  Slip Prevention Tips

Surface usage

Ergonomic and biomechanic characteristics of a pedestrian affect their gait, how they interact with a walking surface and consequently their susceptibility to slip and fall. Whilst it is impossible to cater for all users based on their specific gait, a surface should offer sufficient slip resistance for likely users performing likely activities with likely contamination present.

The use of a surface should ideally be considered at the design stage, though remedying actions can and should be carried out on installed surfaces if they are found to be unsatisfactory.

Surface usage is central to the assessment of slip risk. User groups, loads, speed and direction all affect frictional demands on the surface.

Floors likely to be used by 'high risk' groups, including the very young, the very old, disabled or infirm, should offer a greater slip resistance. This is due to the increased frictional demand of a high risk user and greater likelihood of serious injury following a fall. Elderly users in particular are at greatest risk for a fall, due to the increased reaction time taken to respond to a heel slip, the weaker muscles reducing chance of recovery from the initial slip, more brittle bones increasing chances of a serious injury and greater recovery time after sustaining injury.

Ramped floors will also require an increased slip resistance given increased horizontal forces. The UKSRG guidance states that target pendulum test values on ramped surfaces should show an increase over horizontal values of 100*tan(x), where x is the angle of the ramp above the horizontal.

The likelihood of transportation of significant masses across the floor surface should be considered. Aside from the increased wear, users pushing, pulling or carrying heavy items will have a significantly higher frictional demand. The likelihood of injury following a slip is also increased.

Hospitals feature an unavoidable combination of high frequency cleaning regimes and high risk user groups. Floor specification, cleaning methods and proper maintenance are instrumental in keeping slip risk to acceptable levels in this environment.

Pedestrian behaviour

As with all aspects of health and safety, the commitment to safe working practices must start at a managerial level if it is to succeed in influencing the behaviour of staff throughout the organisation. A safe working culture should be established, encompassing risk management strategies for slip risks.

Slip risk assessments should not only identity flooring performance but consider behavioural influences. Inappropriate behaviour such as running or horseplay can be a major contributing factor in a slip on an otherwise safe floor. The users of a floor can be a valuable resource for information regarding the floor's real use as opposed to its intended use.

Signs warning of a slippery floor are commonly misconstrued as a 'get out of jail' for accidents occurring on the slippery surface. Whilst it may be the case that less serious slips go unreported, signage does not grant immunity from responsibility. However, installed at eye height in prominent places signage may be used to modify the behaviour and gait of pedestrians, consequently reducing the risk of a slip. Signage is a relatively inexpensive short term risk reduction method, however if floors are known to be slippery a more permanent remedy should be undertaken.

Signage warning of a slippery floor should be removed as soon as the floor is no longer slippery. An obviously incorrect sign reduces the response to signage in future cases. This can prove disastrous if the sign is located permanently in a place that is infrequently contaminated.

It is a common misconception that the erection of "slippery floor" signage is sufficient mitigation of slip risk.

A 'see it, sort it' attitude is conducive to a safer working environment. Kitchens in particular with their busy, distracting atmosphere, high likelihood of contamination and high risk of post slip injury benefit from a workforce who take slip risk seriously.

Transportation of heavy masses across the surface as described in Surface Usage above increases slip risk and should be avoided if possible. Analysis of working practices should be undertaken to ascertain whether transportation of masses is absolutely necessary or whether mechanical aids or different working methods can be employed to reduce risk in this area. Consultation with the users of the surface is vital in order to ensure long term adherence to a safer method.

Footwear

A common misconception that the Slips Potential Model aims to dispel is that all "safety footwear" is slip resistant. EN ISO 20344:2004 (A1:2007) is the set of European test standards which describes properties of safety footwear, including slip resistance. If the shoe is being sold as slip resistant the manufacturer must prove its performance, typically through BS EN ISO 13287:2007.

Shoe soles have a significant impact on the likelihood of a pedestrian slipping. In areas accessible to the public it is largely impossible to control footwear, however in production environments an intelligently specified anti-slip sole can significantly increase available friction on an otherwise slippery floor. It is worth noting that anti-slip footwear forms part of PPE and as such should be considered a last defence if the risk cannot be otherwise avoided.

This sole shows some good design features aiding anti-slip properties. Leading edges in all directions and sufficient space between cleats of sufficient size and roughness all increase the chance of puncturing the hydrodynamic squeeze film and securing good sole/floor contact.

This common sole design provides excellent dry grip due to the large surface contact area. It provides extremely low contaminated grip however due to its hard smooth surface. For an area commonly accessed in these soles, with a possibility of contamination, the floor surface would have to provide the necessary profile to puncture a liquid film.

Well designed anti-slip soles will feature;

• a flat, soft and flexible construction to maximise surface contact
• deep cleats of at least 2mm with a width between 3mm and 20mm
• spacing between cleats of at least 2mm
• profiling with defined leading edges in all directions
• profiling that pushes contamination away from cleats

When specifying footwear to be used on slippery surfaces the material hardness and roughness should be considered. Excessively hard soles will not mould to the surface profile of the floor, reducing contact area and friction. Excessively smooth soles will not puncture the hydrodynamic squeeze film, again reducing effective contact area and reducing friction.

Soles should be easily cleaned to avoid a build up of contamination smoothing the surface and reducing slip resistance. Effective working practices should be employed to ensure that soles are cleaned regularly. Wearers expecting the usual high grip from their anti-slip shoe are likely to slip if grip is unexpectedly and significantly reduced.

The wear rate of anti-slip soles should be given consideration. Over time the sole profile will be worn to a smooth surface. Shoes should be replaced before this occurs for obvious reasons.

Care should be taken to ensure that controlled footwear fits the wearer. Not only are slips more likely to occur in an ill-fitting shoe, but staff will be less inclined to wear a shoe if it is uncomfortable.

Where 'sensible footwear' policies can be put in place, footwear should be flat, maximising sole/floor contact and friction. This is especially important with regards to the heel, where the initial slip often occurs. Research has shown that slip incidents can be considerably reduced with the implementation of a sensible shoe policy.

Further information

Slip Test Services
Slip Test Methods
FAQs

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