Electric shock01 March 2008

Electrical standards governing design, build, installation, commissioning, testing and maintenance of electrical and electronic systems are changing - and the changes do impact engineering choices, methods and responsibilities. They also reach out into safety equipment involving hydraulic and pneumatic controls. So we need to be aware, and right now - because the period of grace ends next year, when the new standards will be adopted under the Machinery Directive.

The standards in question are ISO EN 13849 (ratified across Europe in December 2006) and IEC 62061 (ratified in March 2005), together superseding EN 954-1, and recognising that electrical equipment, electronics and machines, too, have moved on immensely since the old standard.

Basic or complex?
Why two standards instead of one? Quite simply, because the technology and disciplines covered are too wide for one standard and a single set of guidelines. So 13849 is an advancement of 954-1, concerned primarily with relatively basic equipment, such as safety relays and electromechanical safety equipment. Meanwhile, 62061 is concerned more with safety systems involving electronics and/or software - so safety-related programmable electronic systems (PES) - and takes its lead from the overarching IEC 61508 standard (functional safety of electrical, electronic and programmable electronic safety-related systems).

In the spirit of all safety standards, both are risk- based, with the onus on the machine builder and/or operator to carry out risk assessments and establish PLs (performance levels) for 13849 or SILs (safety integrity levels) for 62061 - see page 29. It's all about calculating risk factors for the proposed usage, and hence the scale of protection, in order to arrive at appropriate equipment choices, configurations and test regimes.

That's a fair amount of work, so there are additional cost and time implications. But the initial problem for machine builders and safety system designers is deciding which standard to select. As Paul Hingley, Siemens Automation and Drives; product manager for communications systems and safety, says: 'There is some ambiguity and the choice is left entirely to them. We would suggest that, if it's a software-orientated system looking after functional safety and interactions with safety-critical devices, PLCs and other elements of machine control, then they should consider using 62061, because it's more flexible and fit for the purpose.

'If, on the other hand, it's more about mechanical safety operations, involving, for example, guarding interlocks and inputs from light curtains - and perhaps including hydraulic, pneumatic and electromechanical equipment - then the structured approach of 13849 may be better suited. 13849 can be used for PESs, but there are restrictions to specific architectures and topologies defined in the standard, covering, for example, sensor, transfer function logic solver and actuator.'

But the new standards also impact plant engineers concerned with maintenance. 'The critical part for them is validation of the installed equipment to ensure that it adheres to the safety case,' explains Hingley. 'A competent person [engineer] needs to validate and sign-off that the system provides the right level of protection, including as the machine interacts with the wider control environment, involving other relevant plant - for example, on an automated production line.

'So engineering must validate that the machine starts and stops as it should, but also cover off its operational statuses - not only that when you hit the stop button, it creates a stop condition, but what effect that has on the rest of the line. That means also having a schedule of tests that cover other interfaces, contacts and operations - potentially right out to dump valves on pneumatics, or lock-off valves on hydraulics in other areas.'

- The IET code of practice for in-service inspection and testing covers test engineers' responsibilities under the Electricity At Work Regulations 1989
- IET is publishing its third edition to take into account technology advances and other changes in relation to in-service safety testing
- Seaward Electronic has updated its free guide to portable appliance testing (PAT) in line with the new IET code of practice
- Latest IET advice on PAT testing takes into account alternative or substitute leakage tests as well as the recommendation to perform RCD tests


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