Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are usually caused by the rapid switching action of semiconductors, relays, etc. resulting in electronic pollution. These affect the reception of broadcasts and can lead to the malfunctioning of other sensitive electrical and electronic equipment.Inverters etc. cause very severe interference over a wide frequency range, due to the very fast switching of components such as IGBTs.
Electromagnetic Interference refers to noise spread over the whole electromagnetic spectrum. Radio Frequency Interference refers to noise over the part of the spectrum used specifically for broadcasting.
EMI can be propagated in two ways; conducted interference along input and output cables; and radiated through direct transmission, capacitive and inductive coupling etc.
A device is considered to comply with EMC when:
|- both conducted and radiated interference levels are below specified limits.
|- it's immunity or susceptibility to conducted and radiated interference are above specified limits.
In the past, outside the military field, EMC was not of paramount importance to equipment manufacturers and there was a relaxed EMC regime. Power input fuses on electrical appliances was just about it for the commercial world. Where high emission levels were a potential problem (such as microwave ovens), equipment manufacturers could usually avoid emissions limits through the choice of certain unlimited Industrial, Scientific and Medical (ISM) frequency bands.
With the proliferation of the clocks that synchronize modern digital circuits, a concomitant increase in their switching speeds (increasing emissions), and lower circuit voltages (increasing susceptibility), EMC increasingly became a source of concern. Many nations became aware of EMC as a growing problem and issued directives to the manufacturers of digital electronic equipment, which set out the essential manufacturer requirements before their equipment could be marketed or sold. Organizations in each nation and across Europe were set up to draw up and safeguard these directives.
More recently, the ever-increasing use of mobile communications and broadcast media channels has put huge pressure on the available airspace. Regulatory authorities are squeezing band allocations closer and closer together, relying on increasingly sophisticated EMC design methods, especially in the digital processing arena, to keep cross-channel interference to acceptable levels. Digital systems are inherently less susceptible than the old analogue systems, and also offer far easier ways (such as software) to implement highly sophisticated protection measures.
The Directive 2004/108/EC of the European Parliament and of the Council, of 15 December 2004, on the approximation of the Laws of Member States relating to electromagnetic compatibility governs on the one hand the electromagnetic emissions of this equipment in order to ensure that, in its intended use, such equipment does not disturb radio and telecommunication as well as other equipment. In the other the Directive also governs the immunity of such equipment to interference and seeks to ensure that this equipment is not disturbed by radio emissions normally present used as intended.
The main objective of the EMC Directive is thus to regulate the compatibility of equipment regarding EMC. In order to achieve this objective, provisions have been put in place so that:
- equipment (apparatus and fixed installations) needs to comply with the requirements of the EMC Directive when it is placed on the market and/or taken into service;
- the application of good engineering practice is required for fixed installations, with the possibility for the competent authorities of Member States to impose measures if non-compliances are established.
The conformity assessment for apparatus involves Self-Declaration by the manufacturer, with the voluntary option of using a Notified Body in the assessment of the manufacturer’s Technical File.
A transition period is in place for two years from 20 July 2007 for equipment placed onto the Community market prior to that date, provided no changes are made to the apparatus or to declared specifications.
Manufacturers attest to the conformity of their equipment to the provisions of the Directive by establishing an EC declaration of conformity and affixing the CE Mark. The equipment can then be placed on the European market without further regulatory constraints in respect of the aspects covered by the Directive.
It applies to the European Economic Area (EEA) and the territory of a number of candidate countries under a pre-accession agreement called a " PECA ". In addition, via the mechanism of Mutual Recognition Agreements (MRAs), conformity assessment bodies under this Directive have been designated in a number of other trading blocks.
Guide is available to assist with the common application of the Directive 2004/108/EC which, whilst having no weight in law, deals with a number of practical issues that will be of interest to manufacturers. See the new Guide for the EMC Directive 2004/108/EC (21st May 2007) .
Summary list of titles and references of harmonised standards relating to EMC