Electromagnetic compatibility: the safety issues
18 December 2012
While electromagnetic compatibility (EMC) is a machinery safety issue that many machine builders find complex, there can be no doubt about the need for it. For example, if the control system of a machine experiences electromagnetic interference it may malfunction and create a dangerous situation. Conversely, if the electrical and electronic systems fitted to a machine generate a high level of interference, they may also cause other equipment nearby to malfunction.
Many machine builders assume that if each individual electrical or electronic component in a machine carries the CE marking, then the entire machine will also meet EMC requirements. However, that is not necessarily the case, says Paul Laidler.
Machine builders have a legal obligation to ensure that products offered for sale in the European Union carry the CE marking. The CE marking shows compliance with any of the New Approach Directives such as the Machinery Directive and in order to comply with this particular directive, it is also necessary to show that the machine meets the requirements of all other applicable directives. For any machine that has electrical or electronic components, that includes the EMC Directive (2004/108/EC). In the UK, this directive is implemented by the Electromagnetic Compatibility Regulations 2006.
To understand the implications of these regulations, a good starting point is to look at the ‘Guide to the Electromagnetic Compatibility (EMC) Regulations 2006’, which can be viewed on the Department for Business Innovation and Skills website – www.bis.gov.uk.
Section Two of this guide includes statements that can be summarised as saying that equipment must be designed and manufactured so that the electromagnetic disturbance it creates is not excessive, and that it has a reasonable level of immunity to electromagnetic disturbances. In addition, a fixed installation, which includes the majority of machines, must be installed by applying good engineering practices and respecting the intended use of its components.
But how can machine builders be sure that their products really do have satisfactory EMC performance? It is very tempting to think that the answer is to use only components that are themselves compliant with the EMC Regulations. Surely, if all of the components used in a machine satisfy the regulations, it is reasonable to conclude that the whole machine must also meet them?
Unfortunately, that’s not how it works. Consider a variable speed drive that produces a level of electromagnetic interference about half of that acceptable under the regulations. Clearly, there is no problem in stating that this drive complies with the regulations. However, if you put four of those drives on a machine, is it reasonable to assume that the machine complies with the regulations, simply because each of the drives is compliant? The machine may be compliant, especially if measures to control EMC have been incorporated in its design, but the point is that it cannot be assumed to be compliant. This problem can be neatly summed up with – ‘CE plus CE doesn’t equal CE’.
But how should machine builders demonstrate compliance with the EMC Directive? The directive specifies that they should compile technical documentation to show that basic requirements have been met and then complete a Declaration of Conformity. Nowhere in the EMC Directive does it state that testing is mandatory, but since there is no proven way of calculating or modelling the EMC performance of a machine, the only way compliance can be effectively verified is by testing. This opinion may be considered by some as rather controversial, but when the Health & Safety Executive was asked to comment on this issue, it provided the following statement:
“Section Six of the Health & Safety at Work Act (HSW) places a duty on manufacturers to carry out or arrange for the carrying out of such testing and examination as may be necessary to ensure that the article is so designed and constructed that it will, as far as is reasonably practicable, be safe and without risks to health. In the context of EMC, in most applications it is the electromagnetic immunity of equipment that is of interest in relation to Section 6 of the HSW. If it is reasonably practicable to carry out testing for immunity to electromagnetic disturbances, the HSW requires this to be carried out.”
This statement leaves no room for doubt about the necessity for EMC testing of machines in the vast majority of cases. Unfortunately, there is also no doubt that EMC testing can be complex and time consuming, especially for the majority of machine builders that lack in-house expertise in this specialist area.
So those machine builders that assume that all that they have to do to meet the requirements of the EMC Directive is to use components that carry CE marking are following a dangerous route. As there is no shortcut to achieving compliance with the EMC Directive in order to meet legislative requirements and ensure safety, machines must be thoroughly tested.
Paul Laidler is director, machinery safety at TUV SUD Product Service
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