The latest Machine Safety Directive offers a number of benefits, over and above those of improved safety. Philip Shardlow of Wieland Electric explores the key features
For quite a while the whole area of machine safety was in a state of flux, thanks to a transition period between the previous Standards (EN 954-1) and the new Standards (as defined in either EN ISO 13849-1 or the alternative EN (IEC) 62061. During this time, anyone involved in machine safety could choose to utilise either the old or the new.
Since December 2009 though, EN-954-1 has been obsolete and so designers are now no longer able to use this standard and have to instead refer to either of the replacements. But rather than seeing this as a inconvenience, as many people view changes to regulations, I would argue that this is provides opportunities for electrical designers, installers and maintenance contractors. Not least of these is the ability to make better use of new safety system technologies to improve performance while saving time on design, installation and commissioning.
To fully understand the benefits, it’s important to be aware of the reasons for introducing the new Standards. While some argued that EN 954-1 was doing a perfectly good job, the reality was that it hadn’t kept pace with the changes in technology that have been applied increasingly to ensuring and managing machine safety. In particular, EN 954-1 focused on calculated risk using a simple category system, whereby system behaviours were set against categories.
However, in recent years the industry has seen much wider implementation of programmable electronics in safety systems. Consequently, the relative simplicity of EN 954-1 was no longer appropriate. So essentially the new Standards brings the regulations into line with what is already current practice.
Given the general recognition that EN 954-1 is no longer suitable for many applications, there is clearly a health and safety issue to be taken into account. This, in itself, was a good reason for adopting the new standard as safety must be of paramount concern to all companies.
There are also other commercial reasons for taking on the new standards. In the past where European regulations have been phased in, different EC members have responded in different ways, so adopting the new regulations has increased the likelihood of acceptance throughout Europe. Ultimately, this could also have a bearing on CE marking.
In fact, CE marking is an important consideration as any alterations to an EN 954-1 compliant system will require the system to be CE marked again, to ensure compliance with the new Directive. This will enable a Declaration of Conformity to be issued, as modifying an existing machine is treated in the same way as putting a new machine on the market.
Looking beyond Europe, it’s also important to bear in mind that EN ISO 13849-1 and EN (IEC) 62061 are both international standards – in contrast to EN 954-1. Thus, for end users with global facilities that want to standardise across their estate, this could be an important consideration.
Returning to the important issue of safety, this is where I feel most of the benefits come from adopting the new standards. It is accepted within the new Machinery Directive that zero risk is not achievable in the real world, but that arriving at an acceptable residual risk is feasible. In practical terms, this means that safety control systems must either be designed to ensure the probability of functional errors is acceptably low – or that any errors should not bring about a loss of the safety function if the former cannot be achieved. And that’s where the harmonised standards come in.
EN ISO 13849-1 takes its core from the familiar categories in EN 954-1:1996 by examining complete safety functions, including all the components involved in their design. However, it goes beyond this qualitative approach to include a quantitative assessment of the safety functions, based on a performance level (PL) that builds on the category approach.
The components and devices that make up the system require the following safety parameters:
- Category (structural requirement)
- PL: Performance level
- MTTFd: Mean time to dangerous failure
- B10d: Number of cycles by which 10% of a random sample of wearing components have failed dangerously
- DC: Diagnostic coverage
- CCF: Common cause failure
- TM: Mission time
The standard also describes how to calculate the PL that can be achieved when several safety-related parts are combined into one overall system. Any deviations from EN ISO 13849-1 are referred to IEC 61508.
As noted above, EN ISO 13849-1 will be available as well as EN 62061, which is a sector-specific standard under IEC 61508. Based on quantitative and qualitative examinations of the safety-related control functions, it describes the implementation of safety-related electrical and electronic control systems on machinery. It also examines the overall life cycle from the concept phase through to decommissioning.
In EN 62061, the performance level is described through the safety integrity level (SIL) and the safety functions identified from the risk analysis are divided into safety subfunctions. As a safety-related control system is made up of several subsystems, these safety subfunctions are assigned to the actual devices (hardware or software) that are the subsystems or subsystem elements. The safety-related characteristics of these subsystems are described through the SIL and Probability of Dangerous Failure Per Hour (PFHD) parameters.
Cost effective compliance
There can be no doubt, therefore, that the new regulations will make a significant contribution to improving safety in the workplace, in line with modern systems and working practices. At the same time, it’s just as clear that they bring with them a higher level of complexity. However, as mentioned above, there is an opportunity to deploy newer safety system technologies to ease this burden without compromising on safety.
For example, in ensuring that safety systems are operating properly at every level, higher efficiencies can be introduced by ensuring that all levels, or sub-functions, can be addressed through the same system. This is also more convenient.
Programmable safety relays
A major benefit of the new Standards is that they enable the wider use of programmable safety relays. And while this was once seen as an expensive option it’s worth noting that the cost of these relays has fallen considerably in recent year. We calculate that a system using just three or four stand-alone relays can be replaced by a programmable system for about the same cost. With larger systems, the savings are even greater.
Once programmable safety relays are deployed, there are additional time-savings at all levels. For example, a flexible logic editor can be used to test the safety relays in the software before any installation work begins. Potential problems are therefore recognised, and avoided, at the design stage so there is less time devoted to ironing out problems once the installation and commissioning have begun.
Installation times are also potentially reduced, as programmable safety relays are wired back to a central I/O point, rather than requiring feedback loops and interconnecting terminals. Also, as the software highlights any errors, commissioning time is reduced as any adjustments made during commissioning can be quickly reversed if they don’t work.
Thanks to the fault diagnosis that is integrated into the system, it also makes it easier and quicker to trace and fix any faults that occur during everyday operations, compared to manually tracing them. The result is reduced downtime for the end user, while also helping the maintenance contractor to remain within Service Level Agreements.
A further benefit is in the generation of reports, as a full reporting structure is linked to the technical file, so that reports can be produced quickly without manual intervention. This is a useful tool for both end users and maintenance contractors as it provides greater visibility of maintenance patterns.
All of which boils down to a smarter way of doing things that not only ensures legislative compliance but also offers time and cost savings for all parties. So it makes a lot of sense to take a fresh look at the technologies available and how they can be implemented to best effect.