Electromagnetic Compatibility (EMC)
CE Mark Layout
Electromagnetic compatibility (EMC) is the study of coordinating electromagnetic fields give off equipment, with the withstand (compatibility) of other equipment within the vicinity. In it's simplest form, we endeavour to ensure that the magnitude of any electromagnetic fields generated are less than the withstand level of adjacent equipment.
Within Europe there is the EMC directive (2004/108/EC), which is law in all European Countries. Equipment which bears the CE mark has been certified to comply with the relevant standards.
How EMC Works
Electrical equipment which generate eclectic and magnetic fields are called sources of electromagnetic (EM) interference. Differing types of equipment generate different levels of EM; for example a large transmission line will generate a high level of EM, while a pocket calculation will not generator much.
How EMC Works
By its nature, EM can and will interfere with the correct operation of equipment. How severely it interferes with the equipment depends on its sensitivity to EM interference and the magnitude of the EM. Equipment which absorb and are affected by EM are called sinks. For most sinks there is an immunity level, which if the magnitude of the EM interference is below this it will not affect the equipment
Typical Interference Producers
- switching of main and secondary circuits
- lightning strokes and induced discharges
- operation of surge arrestors and air gaps
- radio and mobile phone transmitters
- short circuits and earth faults
- equipment electromagnetic fields
- electrostatic discharges
Common Mitigation Measures
- protection measures for lightning
- adequate earthing and bonding
- separation of power and control cables
- shielding and earthing of control cables
- avoid loops (keep send/return conductors together)
EM interference needs to moves from the source to the sink by some form of coupling. These take the form of:
Electrostatic coupling - discharge of charged bodies
Inductive coupling - via magnetic fields
Capacitive coupling - via electric fields
Conductive (galvanic) coupling - via circuit impedances
Radiation - EM propagation where structure dimensions large compared to wavelength
In system design, the aim of achieving EMC compliance is to to ensure that the EM interference generated by all the sources is less than the EM immunity levels for each of the sinks.
In general this is achieved by limiting the penetration of fields (screens for example) and ensuring a equipotential plane between equipment.
When trying to ensure or verify EMC the base international standard is IEC 6100. The standard consists of compliance requirements, collected data and guidelines:
- IEC 61000 Electromagnetic compatibility (EMC) -
- Part 1: General - 6 parts on application and methodology
- Part 2: Environment - 14 parts on compatibility levels, assessment and environments
- Part 3: Limits - 15 parts on the emissions and disturbances for various systems
- Part 4: Testing and measurement techniques - 35 parts covering emission and immunity testing
- Part 5: Installation and mitigation guidelines - 9 parts cover installation requirements
- Part 6-1: Generic standards - 6 parts on specifics for residential, industrial and commercial environments
In addition to the IEC 61000, particular items of equipment will likely have additional EMC requirements built into the product standard itself. Industries and special environments could also have their own EMC standards; for example rail systems would be required to adhere to IEC 62236 and EN 50121 "Railway applications - Electromagnetic compatibility".
Note: electromagnetic fields can also have an adverse effect on human health. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) publishes research and guidelines on recommended exposure limits for electromagnetic fields. You can visit their website at: http://www.icnirp.de.
Implementation in Installations
Determining the level of EMC for an installation essentially involves looking at all the sources of EM and comparing these to the immunity level of the sinks. For many installations this is relatively straightforward as the individual product standards, will be default guarantee For some installations it would also involve the system as a whole, its EM emissions and the effect on the neighbouring environment.
EMC Management Plan
For large installations and systems, it would be good practice to develop and EMC management plan. The EMC plan would establish the goals for the system EMC compliance and show how this will be achieved. A typical EMC plan may include some of all of the following:
- Identification of the likely sources of interference
- Listing EMC regulations, customer specifications, standards, in-house specifications
- EMC management rationale
- Responsibilities of the consultant, contractor and suppliers
- Control of suppliers - possibly with suppliers own EMC plans
- Whole system EMC management
- Deliverable documentation
- Time management - milestones, testing, etc.
- Compliance Matrix documenting each system, subsystem or equipment as both a source and sink
- Test Matrix
Earthing and Bonding
Over and above the product selection, the primary method used to achieve EMC compliance goals for most installations is the earthing and boding. Particular care should be given to this and it would be worthwhile developing guidelines to be followed for the installation as a whole. Some general earthing and bonding tactics to improve EMC include:
- earthing of the enclosures of EM generating equipment
- compliance with relevant earthing installation standards
- utilising clean earth and intrinsically safe earth systems as appropriate
- multiple earth-bonding utilising meshed earths
- create large numbers of small earth loops
- avoiding star earthing and/or the provision of separate earths for protective and clean
- provide an earth plane for each floor (mesh or reinforcing or copper)
- keep bonding connections as short as possible (and of low resistance and inductance)
- ensure the earthing and bonding is resistant to corrosion, shock and vibration
- cable screens and armour should be earth bonded at the point of entry to enclosures
Cables are both sources of EM and a distribution path. Ensuring the correct selection and installation will help minimise any EM problems. In particular it would be worthwhile ensuring:
- bonding at all ends for metallic cable trays, flexible metallic tubes and steel pipes
- bonding of cable shields at both ends
- run single core cables of the same power circuit together
- use screens or armour with multi-core cables
- apply surge arrestor and filter at the incoming to switchgear panels
- special separation of cables:
- communication/data cables > 300 mm from low voltage cables (60 V to 1 kV)
- communication/data/low voltage cables > 600 mm from high voltage cables (>1 kV)
Particular items of equipment will more prone to creating EM issues or being affected by them. Obvious examples of EM generating equipment would where large amounts of power are used, larger inverters and other large non-linear power users. Sensitive equipment examples would include rail signalling, control or communications systems or other low power devices.
Finally to ensure full compliance, it may be required to conduct field survey of the installations neighbourhood and carryout an appropriate level to testing and verification.
Electromagnetic compliance (EMC) is the design and installation of equipment and systems, such that the electromagnetic fields generated by the equipment does not affect other adjacent equipment or systems. Achieving EMC compliance is primarily governed by IEC 61000. For any large installation, the production of an EMC management plan will assist in ensuring compliance during design, installation and operation.
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