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Guarding against machine safety failure

09 July 2012

Although guards play a vital role in ensuring that machines are safe, in Paul Laidler's view they are often treated surprisingly casually and continue to be the cause of many severe injuries that could have been prevented. To try and tackle this problem, the updated Machinery Safety Directive (2006/42/EC) came into force in December 2009. This must now be observed by anyone supplying a machine in the European Union.

However, many machine designers seem confused by the changes and are still not taking them into account. So what are the core elements that need consideration for guarding design and how should they be tackled?

Guarding receives specific attention in the Essential Health and Safety Requirements (EHSRs) of the Standard. EHSRs cover safety features that manufacturers must build into their machines before they can carry the CE marking that shows it conforms to EU requirements. 

Section of these EHSRs requires that all fixed guards must be removable only with the aid of tools. While a similar requirement was contained in the previous Machinery Directive, the fixing systems must now remain attached to the guards when the guards are removed. This means that ordinary machine screws and bolts can no longer be used as a means of attaching guards unless provision is made for the screws or bolts to be held captive when the guard is removed.

Section 1.4.1 now also requires that guards must protect against the ejection of falling materials and objects. For example, if a grinding wheel explodes, can the guarding contain that explosion and protect the operator?

Section also states that guards should be incapable of remaining in position when their fixings have been removed. This could be challenging for designers and means that some careful thinking will be required. For example, designers need to ensure that hinged guards open automatically when they are released and that guards on the top of machines are not retained in position by gravity when their fixings have been removed.

Fool-proofing interlocks
Interlocks prevent a user from making unsafe actions. When it comes to guarding one of the most relevant standards is BS EN 1088, which covers interlocking devices.

Section 5.7 of BS EN 1088 states that - “If the specification of the safety interlocking device is such that it is possible to defeat the safety function solely by a reasonably foreseeable action at the interlocking device itself and if the device itself is to be relied upon to prevent defeat, it should provide measures to minimise the possibility of defeat.” 

Exactly what constitutes ‘a reasonably foreseeable action’ is the operator not using machinery correctly, or choosing to take short cuts. If a manufacturer can prove they took all reasonable steps to ensure that an interlocking device could not be defeated, then it becomes a ‘deliberate misuse’ by the operator, rather than the fault of the company that supplied the machine. 

The standard also states that “defeat in a reasonably foreseeable manner” includes the removal of actuators and switches with the intention of disabling an interlocking device. For designers, this represents a problem as how can they possibly fix these so that they cannot be removed with normal tools

While solutions such as welding or gluing may seem to be the only methods to fool-proof any tamper attempts, BS EN 1088 does not recommend this. It suggests that companies recognise the reasons why operators attempt to defeat interlocks, and that these are addressed in the machine design. The real question here is whether the machine design can be modified so that the guard no longer inhibits productivity, which in turn means that the operator will not be tempted to defeat the interlock.

While spot checks by HSE hope to stop a lot of potential accidents within the workplace, it is ultimately the responsibility of machine builders to ensure compliance with the Directive and standards referring to guarding. Since the changes to the Machinery Directive the design and manufacture of guards now requires specialist expertise to achieve full standards compliance for CE marking. It’s no trivial task and the plea of confused ignorance is not acceptable as errors by machinery designers can have serious consequences for end-users.

Paul Laidler is business director, Machinery Safety at TUV SUD Product Service

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