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Slip Hazards

Published: 10th Dec 2011 in OSA Magazine

In the workplace, at home or at leisure, slipping on a surface can take just a fraction of a second, while the consequences could last for a lifetime. Broken bones, lost reputation and litigation under health and safety legislation are some of the possible outcomes. Thousands of slipping accidents and injuries that occur each year are caused by a lack of friction, or slip resistance, between footwear and floor and often can be prevented (or at least much reduced) with some forethought. 

Incidents of slipping leading to an accident are at the top of workplace health and safety statistics, and are often the initiators of accidents that are attributed to other causes. For instance, some accidents involving injury by machinery, scalding and possibly more than one-third of reported falls from height are likely to have been initially caused by a slip. Further to this, it is reported that half of injuries to members of the public resulting in hospital attendance are related to slips and trips which, in today’s litigation culture, can lead to costly settlements.

Many organisations have long recognised that management of health and safety issues is an integral part of sound overall business practice, and that the careful selection and use of appropriate footwear and flooring can substantially reduce the risk of slipping accidents.

In many parts of the world – including Europe and the USA – there are specific legal obligations requiring suppliers to place only safe and fit-for-purpose products on the market. Slip resistance is one of the most important safety-critical tests for both footwear and floorings, so manufacturers of both types of products need to take this into account during the design stage. Distributers and retailers also need to be clear as to the appropriate end use of the products and market accordingly, ensuring that they have sufficient evidence of good performance in order to prove that ‘due diligence’ has been applied to best assess any potential slipping hazards associated with their products. Testing can form part of this process and the most up to date and technically relevant test methods should be used.

Footwear versus floorings

The first point of contention when investigating a slip accident is often the identification of the real cause of the accident. Is the footwear or flooring at fault? In reality, the answer is rarely ‘black and white’, as there can be many contributory factors.
Clean, dry floors are rarely a cause of concern. The majority of slip accidents occur on wet or contaminated surfaces. Therefore, housekeeping is often a major factor, along with the type and surface roughness of the underfoot flooring.

Footwear alone will not eliminate slip on contaminated surfaces and, indeed, some footwear may be slippery on smooth dry surfaces. It is impossible to make footwear ‘non-slip’ or resistant to slip under all conditions which may be encountered in wear. Careful selection of footwear that meets basic tread pattern design guidelines and surpasses minimum friction guideline levels, however, will help to minimise the risk of slipping accidents.

Risk assessment

When considering flooring and slip resistance, the most important thing to remember is that all floors are suitable, providing they are installed in the correct application and care is taken to ensure that the surface is maintained while other contributing factors are accounted for. The key to understanding the suitability of existing floor surfaces and, indeed, selecting surfaces for a given application, is to assess a number of important factors. The first consideration is to undertake a slip risk assessment.

Risk assessment is the systematic and careful examination of factors (hazards) that could cause harm to people, and the evaluation of whether the controls in place are sufficient to prevent harm. There are five key points to consider in risk assessment:
• Identifying the hazards
• Deciding who may be harmed
• Evaluating the risks and deciding if the existing precautions are adequate
• Recording the significant finding
• Reviewing the assessment periodically

From the results, it is essential that remedial action is taken when the controls are insufficient to prevent harm. It is recognised that a number of issues contribute to the potential for pedestrian slip accidents – it is not sufficient to consider one or two in isolation.

By understanding the interrelationship and the relevance of each component in a particular circumstance, an holistic assessment of the slip potential may be made. Factors which are controllable and those which are predictable need to be identified.
‘Controllable’ factors are those that can usually be changed or influenced by a direct or indirect action: floor type, environment, use, footwear (at least for employees) and the contamination.

Factors that are ‘predictable’ are assessed with an understanding of the final use of the surface in question. The users, their footwear (in public access) and the expected behaviour are all significant.

In any site investigation, the intended application of the floor within the building and associated factors in the area need to be considered. These include:
• Lighting conditions – making sure that such aspects as poor lighting or glare do not prevent people seeing where they are walking
• Entrance systems – matting for dirt removal or water absorption and overhangs at entrances can significantly reduce walking water into the building
• Public and/or employee access – can footwear be controlled?
• Expected contamination – what dry and wet contaminates can be predicted? Are the control measures to deal with them adequate?
• Maintenance regime – using the correct cleaning method to ensure that the floor’s grip is maintained
• Obstacles, stairs, ramps – considering whether these are necessary and, if so, whether there are adequate handrails and visibility
• Changes in surface – moving from a surface with high coefficient of friction (CoF) to a surface with a low CoF can result in slips and trips. This is especially so where the users are unable to see the change, such as busy walkways
• Distractions – possible distractions, such as loud noises, posters, signs, or moving screens
• Behaviour – the expected behaviour in the final application, and are there control measures, such as barriers?
• Expected user groups – different user groups such as children, infirm and the elderly will act differently
Once the application is fully understood, the actions that are required can be identified.

Human factors and location influence slip risk

Some people will be more able to avoid slip hazards than others. Being fit, alert and having good eyesight are major advantages. Carrying boxes or equipment; pushing, pulling or lifting loads; wearing loose or heavy clothing and being distracted by the task, noise or lights can all increase the risk of slip accidents happening.

The workplace may be full of potential hazards, particularly relating to the underfoot conditions. Poor lighting can make it difficult to see spillages. Entrances and doorways often have different types of adjacent surface – perhaps wet outdoor paving meeting a polished reception hall, which introduces a surprise element. Running, turning corners, climbing up or down ladders, steps and sloping surfaces change the biomechanics of gait (the forces between shoe and ground change increasing the risk of slip).

Flooring factors

The type of flooring material, such as concrete, resin, ceramic, steel, wood, glass, rubber and vinyl, affects the achievable level of slip resistance. The surface roughness of the flooring has a significant influence on slip resistance, especially under contaminated conditions.

Smooth surfaces have particularly low roughness and, consequently, can be very slippery when wet. Rough surfaces may become smooth with age and wear or through layers of polish or other contaminants applied to the surface.

Measurement of the flooring surface roughness is also to be included in any assessment of slip potential. Roughness is a principle factor in determining the friction properties of floorings. The benefit of a rough surface is that in wet or oily conditions, any peaks will project through the thin fluid film trapped between the shoe sole and flooring, biting into the soling and reducing the risk of hydroplaning.

Roughness is measured using a fine stylus that traverses the surface, mapping the microscopic profile. The most commonly quoted parameter for roughness is Rz or Rtm – the average of all peak and valley heights in a measured length.

A glazed tile will have an Rtm of, perhaps, 6 microns – compared with a safety resilient floor of 40 microns. When measuring surface roughness, the hardness of the flooring material under test should be considered, as softer floors will demonstrate better coefficient of friction with lower Rtm measurements.

Slip resistant footwear design

Attributes to look for:
• Fit – good fit means that the footwear is secure on the foot. Shoes that are too big or are loose on the foot have potential to cause instability and loss of balance
• Comfort – a comfortable shoe is less likely to be discarded for an alternative product which may have poorer slip resistance. Comfort also promotes normal gait and reduces fatigue
• Flexible sole – gives the wearer a good feel (proprioception) for the underfoot conditions, sensing slippery or loose, gravelly surfaces
• Flat sole – maximises contact area between shoe and ground
• Low heel height – moulded soles on ‘sensible’ shoes are ideal. Women’s fashion shoes with separate heels become increasingly unstable as heel height increases and top-piece size decreases
• High friction materials – a diverse range of rubber and plastic types is used, each in a range of formulations and hardness. Industrial footwear can only be made with a limited range of materials, due to the high performance and durability requirements demanded
• Good tread pattern – on clean, dry surfaces, a tread pattern is not necessary. On lubricated surfaces, however, an effective tread pattern is required to sweep aside lubricant in much the same way as a car tyre tread

Test methods

SATRA has been in the forefront of assessing slip resistance properties of footwear and underfoot surfaces for many decades and originally published its slip resistance test method SATRA TM 144, in 1992. Since then, it has been regularly updated to take into account developments in slip technology and science over the last two decades.

This test, and the STM 603 test equipment associated with it, has become the international standard for dynamic friction measurements on footwear, and is widely used to determine the coefficient of friction between footwear and floorings under conditions simulating a typical walking step. It realistically produces the conditions at the three moments during a normal walking step when slip is most likely to commence.

Unlike many other methods, it uses loading forces representing an adult walking gait, realistic floor contact angles, static contact time and sliding speed. It is based on biomechanical studies of humans walking normally or slipping, and the method is true to normal confident gait on a level surface.

Being a machine-based method it is, therefore, definable and consistent every time, and does not require able-bodied and willing, trained wearers of a particular shoe size.

It is important to remember that the SATRA TM144 test method is designed to assess slip behaviour under conditions simulating those experienced in the phases of a typical walking step when slip is most likely to occur. In the?case of footwear that causes wearers to adopt an abnormal mode of walking, different test conditions derived from biomechanical conditions may be required.

Derivatives of TM 144 have also been incorporated into European, Canadian and American footwear slip resistance standards.
How the test works
SATRA TM144:2011 (utilising the SATRA STM 603 slip rig) measures the slip resistance of both footwear and flooring in the laboratory, and is based on biomechanical studies of slips. The test defines a dynamic coefficient of friction measurement at a constant speed under a vertical load representative of human body weight.

Slip is a complex process, and an effective slip test machine needs to control and measure a number of key criteria reproducing the critical factors relating to slip when walking. The SATRA STM 603 Slip Resistance Testing machine enables testing of footwear to be conducted to both SATRA TM144 and its derivative test methods. Users can also programme their own slip test parameters into the machine; for example, if they have their own internal development standards.

The machine accepts a wide range of ground surfaces against which footwear can be tested for its resistance to slip. It is common to test footwear against a dry floor and a wet floor, or, in the case of European standard EN ISO 13287, a lubricated surface. The standard tests specify closely controlled, calibrated standard flooring surfaces, designed to give reproducible footwear slip test results. Likewise, when testing flooring a standard shoe is used.

The output from the test is a determination of the coefficient of friction between the shoe and the test surface. A low coefficient of friction indicates that only a low horizontal load would be needed to cause the shoe to slip against the flooring type used in the test.

By using a standard test surface, and with reference to acceptable required minimum and maximum coefficients of friction, the footwear can be declared to have passed or failed the test. Similarly, test floorings can also be assessed with reference to the ‘standard’ shoe, or shoe of choice for more specialist applications.

In addition to the reported coefficient of friction result, the loads, speeds and horizontal distance covered, and a more detailed continuous calculation of coefficient of friction, are recorded graphically for the duration of the test.

Pendulum test

The pendulum test is a quick and easy method used to assess floorings, not footwear, and was originally developed to simulate the action of a slipping foot.

The method is based on a swinging dummy heel (pendulum), which sweeps over a specified area of flooring, just making contact with the surface in a controlled manner.

The slipperiness of the flooring has a direct influence on the pendulum value obtained, known as the ‘Slip Resistance Value SRV’, with high CoF values slowing down the pendulum and reducing the distance it travels after contact with the test floor.

The test is simple to operate, and is said to demonstrate a good correlation to real life conditions. Its portability makes it useful in the evaluation of on-site slip potential.

Ramp test

Other test methods used to evaluate floor coverings include DIN 51130, DIN 51097 (a barefoot test), EN 13845 Annex C and BS 8445 for bath mats (involving inclined ramp slip testing).

A human subject is asked to walk backwards and forwards with a certain gait on a floor surface attached to an inclined plane, after a lubricant has been introduced onto the surface of the flooring sample to reduce the friction. This lubricant can either be a steady stream of potable water or soap solution from a reservoir associated with the test machine, or a coating of engine oil spread evenly by hand before the test.

The angle of the plane is slowly increased by the operator until the subject slips or feels unsafe. The angle at which the subject slips – or halts the test – is then recorded, compared against performance on a standard calibration surface, and then used to calculate a single Index which classifies that flooring by category. The ramp test has also been used to compare and rank footwear performance.

The ramp test has limited usefulness, however, in terms of demonstrating due diligence, as its subjectivity and methodology is far removed from a typical human walking gait and normal modes of slip. It lacks the elegant mechanical accuracy of the Pendulum Test, or the controlled repeatability of the SATRA STM 603 Slip Rig.

Summary

Slip resistance testing will only grow in importance as governments introduce ever more stringent product safety legislation and a ‘compensation culture’ spreads across the world.

The key to slip accident prevention is a systematic and careful examination of things that could cause harm to people, and an evaluation of whether the controls in place are sufficient to prevent harm. The characteristics of footwear and underfoot surfaces are important considerations to be taken into account.

Footwear analysis involves:
• Looking at tread patterns – no one sole design will be best on all different types of surface and contaminants
• Examining material selection
• Looking at test results on surfaces that relate directly to end use conditions – for example, concrete, resin, wood, metal, rubber or plastic floors – and likely contaminants
• Asking for more information specific to end use
• Trialling samples before buying

When analysing floorings:
• Select the correct flooring for the application
• Conduct a risk assessment
• Control contamination
• Monitor cleaning and maintenance
• Look at test results
• Monitor performance – changes occur post installation

In terms of flooring and footwear, a thorough understanding of the factors that influence slip resistance will help to reduce potentially hazardous products before they reach the market. It is impractical to prevent all accidents, and good design and reliable testing will identify poorly performing products and help to minimise their occurrence.

Readers interested in having their materials or products assessed for slip resistance should, in the first instance, contact footwear@satra.co.uk

To view a video on SATRA slip testing, visit www.satra.co.uk/portal/special_view.php?id=3
For more general information on PPE testing at SATRA, visit www.satrappeguide.com

About SATRA:

SATRA tests and certifies protective footwear, gloves, clothing (including high visibility, motorcycle protective clothing and gloves, protection against mechanical hazards, cutting, chemicals, impact, heat), fall protection, helmets, hearing and eye protection.

SATRA Notified Body scope includes:
• EC Type examination (Article 10)
• Final product Quality control (Article 11A)
• Quality of production - by monitoring (Article 11B)

These PPE facilities are available to all manufacturers and distributors who are interested in PPE certification and CE marking, regardless of membership status (SATRA is well known as a research and technology centre of excellence), and our team of PPE specialists looks forward to being able to offer you an unparalleled service testing and certifying PPE.

SATRA has established an enviable reputation for PPE testing and certification, and has become a byword for quality and integrity. Indeed, many companies insist on seeing a copy of a SATRA certificate before purchasing PPE. SATRA is also a truly global player, working with major manufacturers and distributors worldwide. We also have offices in Taiwan and China to help Far East manufacturers with their problems locally.

The future holds many challenges but you can be sure that SATRA will remain at the forefront of PPE test and certification and our highly respected experts will continue to provide a high level of quality and customer service backed up by state of the art facilities and equipment.

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Published: 10th Dec 2011 in OSA Magazine

Author


SATRA


SATRA is one of Europe’s foremost Notified Bodies and test houses for fall protection products and Peter Doughty MSc BSc CEng MIEE has worked at SATRA for over 20 years. He sits on various technical committees for PPE including BSI/PH5 and VG11 which cover standards for PPE against falls from a height. 


SATRA

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