EN 1088 - requirements relating to fasteners for interlocks

Machine guards play a vital role in ensuring that machinery is safe to operate, hence they figure prominently in safety legislation and regulations. Not surprisingly, the new Machinery Directive 2006/42/EC, which applies from 29 December 2009, contains specific provisions relating to machine guarding, and these have to be observed by anyone supplying a machine in the European Economic Area after that date.

In the Essential Health and Safety Requirements (EHSRs) that form Annex I of the new directive, Section 1.4.2.1 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, what is new is that 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 fasteners to be held captive when the guard is removed.

One of the other requirements of Section 1.4.2.1 is potentially even more challenging for machine designers to satisfy. It states that, wherever possible, guards should be incapable of remaining in position when their fixings have been removed. This means that some careful thinking will be required to ensure, for example, 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.

Interlocking devices

Underpinning the new Machinery Directive is a whole raft of standards and, when it comes to guarding, one of the most important and relevant of these is BS EN 1088 Safety of machinery - Interlocking devices associated with guards - Principles for design and selection.

Originally issued in 1995, this standard has since been amended twice. The second and most recent version (BS EN 1088:1995+A2:2008) includes two notes about the relationship between the standard and the Machinery Directive. The first amendment is, however, more interesting. This affects Section 5.7 of the standard and it deals with design to minimise the possibilities of defeating guard interlocks.

One of the key statements in Section 5.7 is '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.'

What is 'reasonably foreseeable?'

The standard goes on to explain that the extent to which these measures are applied should be decided on the basis of a risk assessment. Exactly what constitutes 'a reasonably foreseeable action' that could defeat the interlock could have been open to conjecture, but fortunately the standard provides further guidance by explaining that it means an intentional attempt to defeat the interlock either manually or with the aid of some readily available object.

In addition, the standard lists a selection of objects that could be considered as readily available. These include screws, needles and sheet metal pieces; objects in daily use such as keys, coins, adhesive tape, string and wire; spare actuators or spare keys for trapped key interlocking devices; tools needed for the intended use of the machine; and readily available tools such as screwdrivers, hex keys, wrenches and pliers.

It is clear that, to meet this stipulation, interlock systems need to be very carefully designed. And the standard goes even further by stating that 'defeat in a reasonably foreseeable manner' includes the removal of actuators and switches with help of the tools already mentioned, with the intention of disabling an interlocking device.

Implications for fixings

A little thought will show that this requirement has interesting ramifications. How is it possible to fix switches and actuators in such a way that they cannot be removed with normal tools? The options are limited to methods such as high-strength adhesives, welding and the use of special anti-tamper fixing devices such as screws with one-way heads.

This is all well and good, but what happens when it is necessary to remove one of these devices for legitimate reasons? The answer is almost certain to be that you cannot; the only available methods of removal are very likely to result in the destruction of the device. This may be an unintended consequence of the standard, but destroying a few interlocking devices could be considered a small price to pay for enhancing operator safety.

It is only fair to state that the use of sophisticated interlocking devices welded or glued into place is not necessarily put forward by BS EN 1088 as the best option. The standard suggests that it is often better to adopt alternative measures to guard against interlocks being defeated. These could include, for example, regular cyclic checking of the interlock device by some form of safety control system. When measures like this are used, it is permissible to relax the requirements for ensuring that the interlocking device itself cannot be defeated.

Design-out temptation

The approach most strongly supported by BS EN 1088, however, is to recognise the reasons why operators attempt to defeat interlocks and to address these in the machine design. It may be, for example, that operators try to defeat an interlock because using a guard reduces productivity. The real question here is whether the machine design can be improved so that the guard no longer inhibits productivity, which, in turn, means that the operator will no longer be tempted to defeat the interlock.

There is no doubt that guards play an important role in machine safety and that compliance with the latest standards will help to ensure that they are effective in this role. It is not always easy, however, to devise practical ways of meeting the requirements of the regulations. In these instances, advice and support from a third party with extensive and proven experience of machine safety issues is likely to prove invaluable.

Contact Laidler Associates for more information about complying with EN 1088 or the new Machinery Directive 2006/42/EC.