by Neil Dyson, business line manager for Machinery Safety at TÜV SÜD Product Service
International Standard EN ISO 14119:2013 “Safety of machinery. Interlocking devices associated with guards. Principles for design and selection” has now been harmonised to the European Union’s Machinery Directive 2006/42/EC.
The Standard aims to tackle any previously confused areas, giving guidance on how to design and select interlocking devices associated with guards. As it has implications for the design of some guard interlocking systems, it would be advisable for both machine builders and end users to take action now.
Operator defeat
Around the same time as this new Standard was voted upon in Europe, the Health & Safety Executive (HSE) issued a report (“Identifying the human factors associated with the defeating of interlocks on Computer Numerical Control (CNC) machines”) which sought a better understanding of why operatives defeat interlocks, in order to help provide better guidance to inspectors and industry.
The defeating of interlocks has been identified by HSE as a common problem across small and medium sized engineering enterprises. This is because some anecdotal evidence suggests that despite enforcement action, operatives may continue to defeat interlocks. In fact, current research findings cited by HSE imply that defeating safety devices is widespread within the UK engineering industry.
HSE’s experience also indicates that such practices often repeat over time within the same organisations. HSE therefore hopes that its report will help to improve compliance behaviour, and possibly initiate improvements in machinery design and manufacture, as well as machinery procurement by companies.
The new EN ISO 14119 standard has a particular focus on the defeat of guarding systems, providing specific guidance on the measures that should be taken to minimise the likelihood of a successful defeat of any interlocking devices, as well as minimising the incentive of an operator to defeat them i.e. the interlocks do not interfere with, or slow down, manufacturing operations.
The Standard suggests several methods that could be considered, including the prevention of access to the interlocking device, stopping operators using substitute actuators through coding, and the integration of defeat monitoring by cyclic testing.
For example, the Standard covers coded actuators, which actuate a position switch using RFID or magnets, and assigns levels of coding to minimize defeat as follows:
The implication is that it is increasingly the designer’s responsibility to ensure that interlocked guards can’t be defeated, which in turn requires the designer to understand how the machine will be used at every stage of its life (production, maintenance, setting, cleaning and so on).
EN ISO 14119 applies to machine builders integrating interlock systems into new equipment, as well as the manufacturers of interlock devices. While machines currently in use are not directly affected, the Standard does introduce some new requirements regarding ongoing maintenance.
As the HSE is obviously concerned with interlocks, it is good to see that the new Standard is bringing machinery requirements up to date by giving guidance on more modern technologies. This includes the use of complex electronic and programmable solutions that are now used in interlocking systems. Such systems, such as non-contact, un-coded and coded interlocking types, simply weren’t available when EN ISO 1088 was first published.
In order to identify any accumulated faults for those interlocked guards that are not opened very often, the Standard also makes provision for a manual function tests by regular, manual opening and closing. The Standard therefore specifies for different performance levels. For example, those interlocking systems with a PLe require a monthly test, while PLd only necessitate an annual test.
The Standard also clarifies the previously grey area of the minimum safe distance between the guard door and the hazard from which it is affording protection for the operator. It is now very clear that interlocked guards must be installed so that a person cannot access a machine hazard before it stops. ISO 13855 “Safety of machinery – Positioning of safeguards with respect to the approach speeds of parts of the human body”, should be used in order to calculate the minimum distance.
Not only does EN ISO 14119 give the designer of safety systems increased responsibility to prevent the foreseeable and deliberate defeat of guards by operators, it also offers the possible use of a wider range of interlocking technologies for guards.
Neil Dyson is business line manager for Machinery Safety at TÜV SÜD Product Service
