Designers' guide to guarding standards

This guide directs readers towards the main machinery guarding standards, as well as providing some advice on how the standards can be applied.

There is no hiding from the fact that guarding is a crucial element of a machine or production line, but designing the guarding is often left until last and is not given as much thought as the parts that are seen to relate directly to productivity.

However, guards usually control the interaction between man and machine, so it should be remembered that their design can often be a significant factor in optimising a machine's performance.

One of the perceived difficulties with designing guards is that there are numerous regulations and standards that must be complied with.

In fact, as the designer becomes familiar with the 'rules', the 'game' becomes easier.

Alternatively, specialist guard manufacturers such as Procter Brothers can be employed to undertake the complete design, fabrication and installation.

This guide seeks to direct the reader towards the main standards, as well as providing some advice on how the standards can be applied.

Most employers today have at least a basic understanding of safety issues and accept that their employees are one of the company's biggest assets.

But many still see machine guarding expenditure as a necessary evil rather than a key investment that can really help deliver improved productivity.

If guards are well designed they will not interfere with efficient operation; ill-considered guards invariably do.

Worse than that, poorly designed guards encourage operators, maintenance staff and management to bypass them, leading to poor production and high risks for all concerned.

Machinery safety in the UK is driven by two sets of regulations: The Supply of Machinery Safety Regulations 1992 (which requires all machines sold in the EU to carry a CE mark) and The Provision and Use of Work Equipment Regulations 1998.

To comply with the requirements of the Supply of Machinery Safety Regulations 1992, a machine needs to pass six tests: the machine must comply with the EHSRs (Essential Health and Safety Requirements) of the Supply of Machinery Safety Regulations; the machine must have been assessed as complying with the EHSRs; there must be a declaration of conformity; the supplier must be able to assemble a technical file; and the machine must carry a CE mark.

The machine must 'in fact' be safe.

It is a common misconception that machine guards should carry a CE mark.

This is incorrect, because CE marking covers many aspects of safety (such as control systems) other than guards.

Only machines carry CE marks; guards supplied for new machines should be provided with a declaration of conformity as evidence that they comply with the harmonised standards.

In practice, the way suppliers and users of machines can most easily meet their legal obligations is to ensure that their machines, guards and other safety devices are designed to conform with harmonised European standards (Euronorms).

These standards have been developed over the last 10 years to ensure an equally high standard of machine safety across the EU.

The good news for users of machines in the UK is that these standards incorporate most of the principles of BS5304:1975 and 1988 (the code of practice for safety of machinery), which served British industry well for a long period and, although no longer current, is now available as a published document, PD5304:2000.

The main EN machinery safety standards applying to machine guarding are as follows: BS EN294:1992, Safety of machinery, Safety distances to prevent danger zones being reached by the upper limbs; BS EN811:1997, Safety of machinery, Safety distances to prevent danger zones being reached by the lower limbs; BS EN953:1998, Safety of machinery, Guards - General requirements for the design and construction of fixed and movable guards; BS EN954-1:1997, Safety of machinery, Safety related parts of control systems - General principles for design; BS EN999:1999, Safety of machinery, The positioning of protective equipment in respect of approach speeds of parts of the human body; BS EN1050:1997, Safety of machinery, Principles for risk assessment; and BS EN1088:1996, Safety of machinery, Interlocking devices associated with guards - Principles for design and selection.

In addition to the general standards there are also many applicable to particular classes of machine, such as conveyors or packaging machines.

Furthermore, some extremely useful information is available from the HSE in the form of a series of blue guides, collectively referred to as the 'Six pack'; these cover legislation such as the Supply of Machinery Regulations and PUWER.

Today the accepted approach to the design of any machine guarding system is based on risk assessment.

BS EN1050 sets out different methods, which must take account of the probability and degree of possible harm relating to any foreseeable injury.

More guidance on risk assessment can be found the in the HSE's leaflet 'Five steps to risk assessment' INDG163.

Once a machine has been assessed, if the resultant risk is considered unacceptable, measures must be applied to reduce the risk rating - which often includes guarding.

This process is repeated until the measures applied reduce the risk to an acceptable level.

The main guarding standard BS EN953 covers all machines from simple drive couplings to very complex installations involving robots, conveyors and processing machinery.

The standard lists those aspects of machinery, people and the design and construction of guards that need to be considered.

Machine aspects to be considered are, of course, the function of the machine and the hazards that arise from these.

These include the obvious ones, such as entanglement or impact from moving parts, as well as the less obvious ones, such as the potential for ejection of broken tools, hazardous materials and invisible emissions such as noise and radiation.

Guards should be designed to minimise exposure to each of these hazards by the selection of appropriate materials, construction methods and correct safety distances.

The successful design of machine guards needs a clear understanding of all the ways in which people interact with machines at all phases of the machine's life including commissioning, production and maintenance.

Before designing any guarding, the designer should talk to the operators and maintenance staff that will use the machine; if the machine operates for more than one shift per day, the designer needs to talk to the operators and maintenance staff from all shifts because the working practices may not always be the same.

Well-designed guards should permit machines to be loaded, unloaded, cleaned and maintained efficiently without exposing people to hazards (remember that half of all accidents occur while maintenance is being carried out).

Machinery guarding can be constructed from a variety of materials and the skill of the designer lies in creating a system that will be fully compliant with the regulations, yet be user-friendly, cost-effective and, more-so today, aesthetically pleasing.

The main choice of infill materials is between sheet steel, welded wire mesh and clear polycarbonate - although sheet steel guards can also be provided with mesh or polycarbonate windows to allow process viewing.

If ejected parts are an issue, sheet steel or polycarbonate must be chosen, while welded wire mesh offers a cost-effective solution for many other applications.

For corrosive environments or where frequent washdowns are required, stainless steel is usually specified.

Noise reduction is increasingly an issue for factories, so acoustic foam or other sound-deadening material can be added to sheet metal panelling.

Sealing around the guards can also help to reduce noise levels, and sealing is also beneficial if fluids or dust are present.

In areas where heavy use or abuse is anticipated, such as in steel manufacturing, heavy-duty materials should be selected.

When designing any guarding, whether fixed or moving, the designer should refer to BS EN294, the standard that covers safety distances to prevent personnel from reaching over or around guards, and BS EN811, the standard relating to safety distances for lower limbs.

The section of BS EN953 covering human aspects addresses human/machine interaction.

The points listed include reducing the need for frequent access and ensuring that, where the need for access cannot be eliminated, access is controlled so that the machine can only be approached when it is in a safe condition.

This is usually achieved by fitting interlocks to access gates.

The need for nonessential access can also be reduced by designing guards with good process viewing.

If an interlock is required at an access point, it is vital that a component is selected that is suitable for the relevant risk category (1, 2, 3 or 4) as laid down in BS EN954-1.

Typically the interlocks will be of the mechanical type using roller plungers or metal tongues, or noncontact using coded magnetic or electronic technologies.

Another alternative is to use switches incorporated within a hinge.

Whichever type is selected, it must be installed correctly if it is to perform its safety function properly and not start to fail (either to a safe or unsafe condition) when the guard hinges or runners begin to wear.

BS EN1088 gives more information relating to guard interlock selection.

For perimeter-type guards and others where whole-body access is possible, key exchange systems are usually the most appropriate as the person entering the guarded area can take a key inside, thereby preventing the machine from being restarted.

If frequent access is required and the machine has a short stopping time, photoelectric guards can prove very effective, though care must be taken to observe the correct distance between the guard and the hazard (see BS EN999).

Where photoelectric guards are not suitable, opening guards could be manually operated or powered.

In the case of powered guards, however, it may be necessary to install "safe edges" on the leading edge of the guards to prevent them becoming a hazard themselves.

It is hoped that this guide will point many machine designers in the right direction for designing safe, cost-effective guards that comply with the relevant regulations.

If further assistance is required, Procter Brothers can design, fabricate and install bespoke guarding to suit virtually any machine or process.

For perimeter guarding, Procter Brothers has introduced the modular Angel guarding system.

This innovative range includes sliding, hinged and lifting guards as well as fixed panels in a choice of materials.

Not only does an Angel guard provide the required level of safety at a price that is significantly lower than an equivalent bespoke guard, but the appearance is also very attractive.

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