- the systems integrator, the panel builder, the sub-assembly manufacturer
- knaves all three?
Chris Marshman, Managing Director, York EMC Services Ltd, University of York
Electromagnetic Compatibility (EMC) has always been a concern for panel builders as illustrated by questions frequently asked and comments posed by attendees at a seminars and meetings:
Control panels are usually designed for specific purposes and in general comprise contactors, relays and a variety of electronic equipment ranging in complexity from programmable logic controllers (PLCs) and inverter drives to instruments and time delay relays. This equipment is usually installed in ‘wardrobe’ style steel enclosures and wired together to suit the application. There may also be CRT, LED or LCD displays, with keypads or keyboards accessible from outside the enclosure.
According to the EMC Directive 2004/108/EC and the UK legislation, a panel is defined as a ‘component’ or ‘sub-assembly’ since it forms part of a larger installed system and as such does not need to comply with the regulations on its own. Components intended for incorporation into equipment or systems do not need to comply in their own right. Hence the panel builder appears to be excluded from having to comply with the regulations.
The EMCD also considers sub-assemblies to be equivalent to apparatus when supplied to an ‘end user’. In this instance a panel will need to carry the CE marking. The user will need to be provided with information as to how the product is to be installed, operated and maintained to ensure its EMC. The panel builder will need to prepare Technical Documentation.
For guidance on this issue we need to consider as an example low power inverter drives which are very often incorporated into a panel.
Low power inverter drives are components/sub-assemblies, and are a known source of conducted and radiated emissions. Whilst drives are components for integration into a system they may be marketed by distributors, can be procured and used by an end user, therefore carry the CE marking and manufacturers publish installation instructions for them. So the fundamental difference between the drive and the panel is the way the product is actually sold. The drive may be procured in effect ‘off the shelf’, whilst the panel is custom built and ‘taken into service’ after integration into the equipment or system.
Hence EMC for the panel builder will depend on whether the panel is supplied to a specification from the system integrator; when it is not a legal issue or whether the panel is supplied directly to the end user. The market is driving EMC compliance and not the enforcement agencies. Many systems integrators are insisting that their suppliers, including the panel builder, supply compliant equipment and this will in effect be enforced by the contract between the panel builder and system integrator. Often there is an insistence on procuring CE marked equipment. What is actually required is that the systems integrator should specify his EMC requirements as a part of the panel specification.
The panel builder is therefore likely, in some way, to have to demonstrate that a panel is compliant even if legally not required to do so! At its simplest this may involve demanding that all the components and sub-assemblies carry the CE marking for EMC (they may also have to comply with the LVD as well, but this is another issue.), ie CE + CE = CE. This is anathema to the EMC engineer as both emissions and immunity of the panel will be dependent to some extent on the panel wiring. Indeed the cable lengths are likely to be the dominant factors as far as the EMC performance of the panel is concerned. However it would be commercially impractical to test every panel for emissions and immunity.
One method for the panel builder is to put together generic technical documentation. The intention of the file would be to demonstrate that the design and installation practices of the panel builder ensure that any panel he builds is likely to be compliant. In other words sufficient testing is carried out on a representative panel to verify for example that the equipment segregation and wiring practices adopted achieve good EMC. The file would also contain information describing the procurement policy for components and sub-assemblies such that these should carry the CE marking or that they have a known EMC performance. The additional burden for the panel builder may then be limited to ensuring that cabling follows the company’s EMC wiring practice, the cable routing intended by the design engineer and it is not left to the wireman on the shop floor to do his own thing!
The panel builder will also need to consider EMC as part of the design review procedure, such that any significant variations from the sample tested can be identified, the EMC implications considered and a decision taken as to whether any retesting will be necessary. Since the majority of panels will be one-offs the panel builder may need some pre-compliance test instrumentation so that he can make comparative measurements against his previously assessed ‘golden product’. This however will depend on the contractual requirements placed upon him by the system integrator.
The system integrator will ultimately be responsible for demonstrating that a system complies with the regulations. To minimise his risks and costs he will want to ensure that all the constituent sub-assemblies, including panels, meet the overall EMC requirements for the system. Emission and immunity levels for industrial applications are defined by the generic standards EN 61000-6-2 and EN 61000-6-4. However for a particular application the environment in which the system is being used must be considered, as the immunity threat may be far greater than that specified by the generic standards. If in doubt then a Notified Body should be consulted for an opinion.
‘The Electrical Industry Guide to Electromagnetic Compatibility Part 1: Introduction and Enclosure Techniques’, Electrical Review, sponsored by Groupe Schneider
‘Guide to Electromagnetic Compatibility Part 2: Products and Standards’, Electrical Equipment, sponsored by Groupe Schneider
‘Electromagnetic Compatibility EMC Practical Installation Guidelines’, Telemechanique, Groupe Schneider
The EMC Directive 2004/108/EC
The EMC Regulations 2006 SI No. 3418, HMSO
Last updated: 2008-Feb-13