Drives and Controls Magazine

ISO votes to replace EN 954-1

15 December, 2006

Standards-makers have voted to replace the well-known EN 954-1 safety standard with a new norm, EN (ISO) 13849-1: Safety of machinery, safety-related parts of control systems. Part 1, General principles for design.

There had been some doubts about whether the new standard would be accepted but, in the event, only three nations — the UK, the US and Japan — voted against its adoption.

The most significant change in ISO 13849-1 is its emphasis on a probabilistic method for determining safety integrity and the control of systematic faults and errors. It introduces "performance levels" (PLs) as a measure of safety integrity. These are specified in terms of the average probability of a dangerous failure per hour.

Although the new standard had been seen as an opportunity to remove the heavily criticised "risk graph" used to select safety categories in EN 954-1, a similar graph is used in EN (ISO) 13949-1 to select PLs (which are similar to safety categories). Guidance is included to help with system design, minimising the amount of maths needed, in most applications.

The new standard takes a four-stage approach to designing safety-related control systems:

• perform a risk assessment;

• allocate a safety measure (PL) for the identified risks;

• devise a suitable system architecture for that level; and

• validate the design to ensure that it meets the requirements of the initial risk assessment, using manufacturers` data for the reliability of their components and how they are configured in the architecture.

The results are compared with charts contained in the standard to produce the required parameters for cross-checking against the original assessment. To assist in this, a second part of the standard is planned. This is expected to be the same as the old prEN 954-2 that was never ratified.

The newly-approved standard will be issued soon and is expected to be harmonised with the Machinery Directive in the first half of 2007. There will be a three-year transition period, during which EN 954-1 can still be used.