Letters

An EMC technical construction file is just one route to EMC compliance

Dear Editor,

I hope that you can correct an error in an otherwise accurate `EMC jungle’ feature in the April issue of C&I (page 151).

The offending paragraph suggests that users should ask manufacturers for a technical construction file (TCF) certifying that a competent body has tested and approved the product. This is incorrect as:

* The TCF is only one of several routes to compliance with the EMC Directive, and self-certification against appropriate standards is more common.

* The TCF has only to be endorsed by a competent body, which will normally require tests of verified quality to have been done. There is no requirement for the body itself to carry out the tests.

* Where a TCF exists, as well as being a lengthy document, it will almost certainly contain confidential information relating to the product design. So it is not reasonable to expect this to be revealed.

What I would suggest that users should require from a supplier of a C&I product are:

* A clear statement of which EMC standards the product meets, and under what test conditions.

* Clear installation instructions showing how to ensure that the installed products meets the standards.

* Sample EMC test results and/or certificates where EMC performance is particularly crucial to a project.

* And advice and guidance on the choice of appropriate standards and installation techniques.

A reliable basic requirement would be compliance with the generic standards for industrial environments EN50081-2/82-2, but there will be cases where more or less stringent standards are appropriate.

It is interesting that in your publication, and the world of control and instrumentation generally, EMC and the EMC Directive seem to receive relatively little attention. When they do, as in this case, it is in the context of electronic motor drives.

By their nature, drives are potentially a source of quite high levels of electrical noise, so they have tended to attract attention, and their manufacturers have been obliged to take measures to control their EMC characteristics.

In my experience, a good many of the transducers and subsystems installed on process plant have rather mediocre EMC qualities, especially in respect of immunity.

The applicability of the EMC Directive to subsystems is rather ambiguous (the component versus apparatus dispute). Realistically, in the UK at least, it is quite unlikely that it will be seriously enforced on such equipment.

However, immunity problems are not uncommon, and I think your readers may find a survey of C&I suppliers to EMC compliance interesting.

{{Dr C Hargis,Control Techniques,Newtown,Powys.}}