Safety of machinery standard set to change

Machine designers and builders are sometimes confused over the standards to which they should be working; but all that is set to change with the introduction of the new standard ISO13849-1.

For many years machine designers, builders and users have used BS EN954-1:1997 (Safety of machinery, Safety related parts of control systems, General principles for design) as the key standard with which safety related control systems must conform.

It has been suggested that some aspects of this standard may lead to misunderstandings - such as the use of the selection chart for determining system categories - and also that the standard's limitations have become increasingly evident as more machines have the safety related parts of their control systems designed to meet the requirements of IEC61508 (Functional safety of electrical/electronic/programmable electronic safety related systems).

Many aspects of BS EN954-1 are very user-friendly, but the standard may not be sophisticated enough to cover the full range of machinery requirements, especially if the hazards are severe.

For example, the existing standard is qualitative rather than quantitative, so there is little scope for evaluating statistical data such as the mean time to a dangerous failure (MTTF) or the diagnostic coverage (DC - the ratio of the probability of the detected dangerous failures to the probability of total dangerous failures).

Furthermore, BS EN954-1 does not permit the use of programmable electronic safety related systems (PES) or complex systems, both of which are now thoroughly developed, available on the market, and being implemented in line with the requirements of IEC61508.

Further limitations of BS EN954-1 are that it does not provide for evaluating common-cause system failures, the general relationship between risk reduction and system categories may be insufficient, and it does not provide for a practical implementation of IEC61508.

Now that more use is being made of IEC61508, machine designers and builders are sometimes confused over the standards to which they should be working.

In response to this situation, the relevant authorities have decided to replace EN954-1 with a new standard ISO13849-1 (Safety of machinery, Safety related parts of control systems, General principles for design), which will be largely based on EN954-1.

One of the main differences, however, will be that the selection chart will no longer show a specific category, rather it will direct the user to a required performance level (PL).

Moreover, the determination of the PL is not on the basis of qualitative choices (such as the severity of injury being slight or severe), but on a quantification for each safety function.

This approach is similar to IEC61508 but with more practicality.

Although the committee has already worked through several drafts for the new standard, the first draft for public comment is due to be published shortly.

It is then hoped that amendments can be made in time to have the new standard in place by the end of 2003.

It is not yet clear whether there will be a transition period but, in practical terms, the European standardisation process for the corresponding C-standards is likely to continue through 2004 and beyond.

Clearly the first people to be affected by the new standard are those designing and building machinery with long lead times.

Similarly, any companies purchasing machinery that will be designed prior to the likely publication date and supplied after would be wise to check that their suppliers are taking account of the forthcoming standard.

To help machine designers, builders and users to prepare for the new standard, Pilz Automation Technology will be providing introductory workshops and seminars for those responsible for machinery safety.

Meanwhile, machine designers and builders will need to consider contacting their suppliers to request the data that will be needed to calculate the required performance level.

In reality, obtaining data such as the MTTF and the diagnostic coverage values for equipment and components may be one of the more exacting aspects of conforming to the new standard.

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