Understanding BS EN 953, the guarding standard

Jeremy Procter, Convenor of the European Standards Committee responsible for Machine Guards, and Managing Director of Procter Machinery Guarding, discusses BS EN 953, the machinery guarding standard.

Any machine placed on the market within the EU must be CE marked in accordance with the Machinery Directive - and this includes machinery that is built in-house for use in-house.

One of the most critical aspects of compliance with the Machinery Directive is guarding, for which several standards are applicable, the most important being BS EN 953: 1998, Safety of machinery - Guards - General requirements for the design and construction of fixed and movable guards.

Although there are no statistics, it is doubtful that every machine builder has a copy of it - or is even aware of its existence - judging by some of the machinery that appears in factories today.

Furthermore, as with any standard, it has to be applied correctly in order to achieve the aim of reducing risks to an acceptable level.

For example, moving guards, whether manually operated or powered, must not present a hazard in themselves, perhaps as a result of crushing, pinching or cutting (if a sharp edge is not protected).

Another example is that sheetmetal guards can sometimes be the cause of drumming noises if they are positioned close to machinery that vibrates.

In this respect it should be noted that while BS EN 953 is principally concerned with the protection of persons from mechanical hazards, it also refers specifically to non-mechanical hazards such as noise, fumes, radiation (typically from lasers or welding processes) or explosion.

Some engineers, especially if designing machinery for use in-house, are tempted to carry out a risk assessment assuming that the intended (experienced, conscientious) machine operator will always be the one using it.

However, BS EN 953 makes it clear that guarding design requires the 'proper consideration of foreseeable aspects of the machine and operation throughout the foreseeable life of the machine'.

This implies that the guards must be designed for use by less experienced, less conscientious operators - perhaps even at another company, as it is possible that the machinery may be sold.

The machine designer must also consider all aspects of the machinery's use beyond loading and unloading, including maintenance and unplanned events such as tool or workpiece breakages.

As far as practical, the guards (and machinery) should be designed so that maintenance can be carried out without having to remove the guards and expose the maintenance technician to hazardous parts.

As for the containment of projectile parts in the event of a tool or workpiece breakage, this is why PMMA (often called Perspex) is not usually acceptable for clear guards; PMMA can shatter, while the more expensive polycarbonate alternative does not.

Nonetheless, guards should still provide an adequate view of the process, and they should not be 'over-engineered' to such an extent that ergonomic aspects suffer.

For instance, doors should not be made so heavy or awkward to use that there is a temptation to bypass them.

If weight becomes an issue, consideration should be given to power-operating the guard, though the standard does give maximum values for force and kinetic energy, relating both to guards equipped with a 'safe edge' that retracts the guard if it is obstructed, and to those that have no 'safe edge'.

Many machine designers today are using light curtains or similar electro-sensitive protective devices for parts of machinery where frequent access is required.

Although such devices are not discussed in BS EN 953, they should be borne in mind, especially if a designer is getting bogged down trying to design a large, moving guard that meets the force and kinetic energy requirements mentioned above.

Annexes A and B of BS EN 953 provide useful flow diagrams to assist designers with selecting the right type of guard from fixed guards, distance guards, enclosing guards, movable guards with or without interlocking, self-closing guards and adjustable guards.

Again, designers should remember that electro-sensitive protective devices can be a highly effective option, especially when used in conjunction with suitable fixed, distance and/or enclosing guards.

Other useful sections in BS EN 953 refer to hygiene and cleaning requirements for machinery used in the food/beverage industry, anti-climb measures, the benefits of retained fasteners for fixed guards, and the need to verify the guard design through examination, inspection, testing or calculation.

Lastly, designers must be aware that BS EN 953 is not the only standard relating to guards.

A list of others is included within the body of BS EN 953 and while some relate to specific applications (such as: EN 1127, Explosive atmospheres; EN 1672-2, Food processing machinery), others are applicable generally (for example EN 294, Safety of machinery - Safety distances to prevent danger zones being reached by the upper limbs).

Machinery guarding can make the difference between life and death, so designers must face up to their responsibilities and work to meet the requirements of all relevant standards.

For more in-depth advice about BS EN 953, other guarding standards or, indeed, to discuss specific machinery guarding projects, please contact Procter Brothers.

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