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Modern approaches to safety

04 September 2017

Industrial control systems have evolved rapidly in recent years and at times it has seemed that the options available for implementing safety functions have failed to keep up with these rapid developments.

Click here to read this article in the digital issue

That’s no longer the case, however, as Peter Croucher, Product Manager at Weidmüller UK explains.

For many years, electromechanical components such as relays with positively driven contacts were at the heart of most safety systems used in machine and process control. There was a good reason for this. These relays, which are specially constructed so that their contacts will always work in synchrony, even if one or more of the contacts weld, are well-proven devices offering very high reliability. Advancements in technology mean more complex safety relays are still widely used today, but they are not necessarily the best choice for new designs.

This is because not only are relay-based safety systems inflexible – even small changes involve costly and inconvenient rewiring – those relays also take up a lot of space, which is always at a premium in today’s control panels. More modern methods of implementing safety functions eliminate these problems and are, therefore, much to be preferred.

Since many of today’s control systems make extensive use of distributed I/O (which is sometimes known as slice I/O), a particularly attractive approach to safety function implementation is to use safety modules that are designed to fit within the distributed I/O assemblies. Just one of the benefits of these modules is that they are remarkably compact – some offer up to eight channels and are only 11.5mm wide – which makes them easy to accommodate even when there are severe space restrictions.

The modules are available in two basic versions: those that are designed to operate independently without a safety PLC, and those that are designed to be used in conjunction with a safety PLC and fieldbus system. Having these two options available means that control system designers can choose the approach that will meet the requirements of the application in the most cost-effective way.

Safety modules that are intended to be used without a safety PLC are often called safe power feed modules. The principle on which they operate is straightforward: they monitor the safety inputs from devices like emergency stop buttons and guard switches, and, when any of these are activated, they remove the power from selected circuits within the distributed I/O system.

Modules of this type are available in versions that are compatible with input devices that have conventional contact outputs, and also those that have solid-state OSSD outputs. The best of these modules have a safety level of SIL 3 CL in line with DIN EN IEC 62061 and Category 4, PL e, in line with DIN EN ISO 13849-1, which means they can be used in even the most demanding applications.

Some applications need a more complex response to an emergency condition than simply cutting the power to selected outputs. In these cases, the second type of safety module, which is designed to work in conjunction with a safety PLC and safety fieldbus, will provide the necessary flexibility and functionality.

Like their safe power off counterparts, these modules typically feature space-saving compact construction. They are available in versions to match the most widely used PLC/fieldbus options, including Fail Safe over EtherCAT (FSoE) for EtherCAT networks, and PROFIsafe for use with the PROFIBUS and PROFINET fieldbus systems.

Conventional electromechanical devices have certainly played an important role in safety systems, but changes in control system technology mean that, for all but the very simplest of applications, it’s now time to move on. Safety modules for use with distributed I/O are an ideal choice and Weidmüller products in particular are backed by freely available expert technical advice that will help users to select the best and most cost-effective solution whatever their application.


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