BS EN 62061 and BS EN ISO 13849-1 both deal with functional safety of machine control systems, but use slightly different terms and techniques to determine their performance. At the moment, the only guidance on when to use which one comes from manufacturers and many users are confused by conflicting guidance from suppliers, who may prefer one standard or the other.
It is not ideal to have two standards that designers can choose between; it can lead to integration issues between components and can affect relationships between manufacturers, machine builders and end users. However, the International Electrotechnical Commission (IEC) and the International Standardisation Organisation (ISO) both have clear ideas of how they think functional safety in machine building should be regulated. As such, both have set out standards that can provide a presumption of conformity to the relevant essential requirements of the machinery directive.
The IEC standard has been published as BS EN 62061, and the ISO as BS EN ISO 13849-1. Both have the same objective: to take emphasis away from the architecture of individual components and refocus it on the functional safety of the machine. Both standards are intended to reduce the possibility of injury but a side benefit is that using them correctly will often reduce the likelihood of a machine failure. However, while these standards can provide similar levels of risk reduction, the way it is achieved is quite different.
There are different terms for the functional safety levels of a circuit. BS EN 62061 uses three Safety Integrity Levels (SILs), whereas BS EN ISO 13849-1 has five Performance Levels (PLs). Despite the terminology differences, it is simple to compare some of the requirements - for instance, the probability of dangerous failure per hour - but the two standards do differ in the approach taken.
Both BS EN 62061 and BS EN ISO 13849-1 have strengths and weaknesses and there is an argument for and against using each, depending on the application and individual preferences of the machine builder.
BS EN ISO 13849-1 is seen as easier to use by designers who are more familiar with the old categories of EN 954-1. Like its predecessor, the ISO standard has a simple looking ‘risk graph’ to help decide the desired PL. However, use of the risk graph is not likely to be sufficient without further judgements that need to be made by the system designer.
In applications where there is a need for more robust requirements for the management of functional safety, BS EN 62061 may be more suitable, as more guidance is given on the organisational requirements to ensure functional safety is achieved and maintained. In addition, this standard is better at considering modifications that might be made during the commissioning of new equipment or during the machine’s operating lifetime. For example, commissioning engineers need to consider the likely effects of any proposed modification, and how much the control system can be modified before it needs to be revalidated.
After years of study a joint working group of IEC and ISO has developed a comparison of the two standards, which is intended to be published by both organisations as a Technical Report – not quite the same status as a standard but quicker to publish.
The ultimate aim of the joint work is to develop a single standard but this will take a number of years. In the meantime, to help understand the two new standards Schneider Electric has published a Safe Machines Handbook, an unbiased and concise guide explaining some aspects of the new directive, which can be downloaded from Schneider's website.
Dave Collier is safety marketing manager at Schneider Electric