The measuring electronics of field transmitters like flowmeters are constantly becoming more efficient and more compact. Digital data processing with its microprocessors has opened up new avenues for new communications, diagnostic functions, and short response times. The integral mount design also requires new ways in terms of the power supply. Advanced measuring electronics are usually provided with switch-mode power supplies. In order to ensure reliable functionality and a high immunity to interference, the measuring electronics are subjected to various tests. Electromagnetic compatibility (EMC) plays an essential role.
With the advent of the common European market, the European Union has adopted various directives. It is stipulated that all devices released to the European market must comply with the valid directives. This is visible to the customer through the CE mark that has to be attached to the devices, and the CE declaration of conformity delivered with the devices.
Over the years, the measuring electronics and, thus, the requirements on the test procedures have considerably changed. The EMC Directive has been revised to meet these new requirements. Since July 20, 2007, the new EMC Directive 2004/108/EC has been in force and applied by the manufacturers.
At national level, the new EMC Directive 2004/108/EC has been implemented, among others, through the generic standards EN 61000-6-2:2005 (Immunity standard for industrial environments) and EN 61000-6-4:2007 (Emission standard for industrial environments). EN 61326 is the product standard for Electrical equipment for measurement, control and laboratory use.
Besides the requirements stipulated in EMC Directive 2004/108/EC, there are additional requirements, for example for the chemical industry, which are defined in the NAMUR Recommendations. Here, more stringent test requirements are stipulated than in the EMC Directive, for example a higher immunity test level.
Moreover, there exist special requirements from the Organisation Internationale deMétrologie Légale (OIML) (English name: International Organization of Legal Metrology) regarding the measuring accuracy under the influence of electromagnetic interference.
What is EMC?
EMC is an abbreviation of Electro-Magnetic Compatibility. It is the term used to describe how well a device or a system is able to function in an electromagnetic environment without introducing electromagnetic disturbances that interfere with the operation of other electrical products in the environment.
Types of Interference
In the context of electromagnetic compatibility (EMC) a distinction is made between two different types of interference:
Interference Causes
An interference signal may be caused by a voltage variations in time or by currents, e.g. resulting from a switching operation. These produce periodic voltage or current changes. The currents and voltages of the interference source that are variable in time cause magnetic or electrical fields which may generate an interference signal at the load.Interference sources may have the following technical or natural causes:
Interference signals are often generated by electrical switches, relays, contactors, fluorescent lamps, solenoid valves, electric motors, radio transceivers or atmospheric disturbances like lightning.
Evaluation of the Interference Response
The interference response of a device is a means of evaluating how a device reacts in the event of a disturbance or interference. There are three evaluation criteria:
1.No reduction in functionality.
Mainly analog devices may be affected within their error limits. Purely digital devices must not be affected at all.
2.Reduction in functionality.
A certain reduction of the functionality while the device is subjected to the interference is permissible under the proviso that the device is capable of automatically resuming normal operation once the interference is over. No permanent damage may be caused.
3.Loss of functionality.
Evaluated is primarily the functional failure at the onset of an interference until an automatic or manual restart occurs. When the tolerances are exceeded or underscored, the devices must be capable of restarting automatically or must remain, ready for restart, in their defined fail-safe position.
Limitation of Interferences
Interferences can be limited by EMI/RFI shielding of the interference source or by providing a sufficient interference immunity of the load.
In order to achieve an EMC compliant circuit layout, the following measures are taken:
Regarding the overall EMC protection independent of the individual devices, the following is aspired:
Electromagnetic compatibility of plants and systems can be reliably realized by introducing an interference protection concept with defined conditions for the devices to be used.
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