Electromagnetic compatibility

thumb|Anechoic RF chamber used for EMC testing (radiated emissions and immunity). The furniture has to be made of wood or plastic, not metal.

thumb|[[Log-periodic antenna measurement for outdoors]]

Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage to operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment. It is also the name given to the associated branch of electrical engineering.

EMC pursues three main classes of issue. Emission is the generation of electromagnetic energy, whether deliberate or accidental, by some source and its release into the environment. EMC studies the unwanted emissions and the countermeasures which may be taken in order to reduce unwanted emissions. The second class, susceptibility, is the tendency of electrical equipment, referred to as the victim, to malfunction or break down in the presence of unwanted emissions, which are known as radio-frequency interference (RFI). Immunity is the opposite of susceptibility, being the ability of equipment to function correctly in the presence of RFI, with the discipline of "hardening" equipment being known equally as susceptibility or immunity. A third class studied is coupling, which is the mechanism by which emitted interference reaches the victim.

Interference mitigation and hence electromagnetic compatibility may be achieved by addressing any or all of these issues, i.e., quieting the sources of interference, inhibiting coupling paths and/or hardening the potential victims. In practice, many of the engineering techniques used, such as grounding and shielding, apply to all three issues.

History

Origins

The earliest EMC issue was lightning strike (lightning electromagnetic pulse, or LEMP) on ships and buildings. Lightning rods or lightning conductors began to appear in the mid-18th century. With the advent of widespread electricity generation and power supply lines from the late 19th century on, problems also arose with equipment short-circuit failure affecting the power supply, and with local fire and shock hazard when the power line was struck by lightning. Power stations were provided with output circuit breakers. Buildings and appliances would soon be provided with input fuses, and later in the 20th century miniature circuit breakers (MCB) would come into use.

Early twentieth century

It may be said that radio interference and its correction arose with the first spark-gap experiment of Marconi in the late 1800s. As radio communications developed in the first half of the 20th century, interference between broadcast radio signals began to occur and an international regulatory framework was set up to ensure interference-free communications.

Switching devices became commonplace through the middle of the 20th century, typically in petrol powered cars and motorcycles but also in domestic appliances such as thermostats and refrigerators. This caused transient interference with domestic radio and (after World War II) TV reception, and in due course laws were passed requiring the suppression of such interference sources.

Electrostatic discharge (ESD) problems first arose with accidental electric spark discharges in hazardous environments such as coal mines and when refuelling aircraft or motor cars. Safe working practices had to be developed.

Postwar period

After World War II the military became increasingly concerned with the effects of nuclear electromagnetic pulse (NEMP), lightning strike, and even high-powered radar beams, on vehicle and mobile equipment of all kinds, and especially aircraft electrical systems.

When high RF emission levels from other sources became a potential problem (such as with the advent of microwave ovens), certain frequency bands were designated for Industrial, Scientific and Medical (ISM) use, allowing emission levels limited only by thermal safety standards. Later, the International Telecommunication Union adopted a Recommendation providing limits of radiation from ISM devices in order to protect radiocommunications. A variety of issues such as sideband and harmonic emissions, broadband sources, and the ever-increasing popularity of electrical switching devices and their victims, resulted in a steady development of standards and laws.

From the late 1970s, the popularity of modern digital circuitry rapidly grew. As the technology developed, with ever-faster switching speeds (increasing emissions) and lower circuit voltages (increasing susceptibility), EMC increasingly became a source of concern. Many more 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 individual nations, across Europe and worldwide, were set up to maintain these directives and associated standards. In 1979, the American FCC published a regulation that required the electromagnetic emissions of all "digital devices" to be below certain limits. are the reference sites in most standards. They are especially useful for emissions testing of large equipment systems. However, RF testing of a physical prototype is most often carried out indoors, in a specialized EMC test chamber. Types of the chamber include anechoic, reverberation and the gigahertz transverse electromagnetic cell (GTEM cell). Sometimes computational electromagnetics simulations are used to test virtual models. Like all compliance testing, it is important that the test equipment, including the test chamber or site and any software used, be properly calibrated and maintained. Typically, a given run of tests for a particular piece of equipment will require an EMC test plan and a follow-up test report. The full test program may require the production of several such documents.

Emissions are typically measured for radiated field strength and where appropriate for conducted emissions along cables and wiring. Inductive (magnetic) and capacitive (electric) field strengths are near-field effects and are only important if the device under test (DUT) is designed for a location close to other electrical equipment. For conducted emissions, typical transducers include the LISN (line impedance stabilization network) or AMN (artificial mains network) and the RF current clamp. For radiated emission measurement, antennas are used as transducers. Typical antennas specified include dipole, biconical, log-periodic, double ridged guide and conical log-spiral designs. Radiated emissions must be measured in all directions around the DUT. Specialized EMI test receivers or EMI analyzers are used for EMC compliance testing. These incorporate bandwidths and detectors as specified by international EMC standards. An EMI receiver may be based on a spectrum analyser to measure the emission levels of the DUT across a wide band of frequencies (frequency domain), or on a tunable narrower-band device which is swept through the desired frequency range. EMI receivers along with specified transducers can often be used for both conducted and radiated emissions. Pre-selector filters may also be used to reduce the effect of strong out-of-band signals on the front-end of the receiver. Some pulse emissions are more usefully characterized using an oscilloscope to capture the pulse waveform in the time domain.

Radiated field susceptibility testing typically involves a high-powered source of RF or EM energy and a radiating antenna to direct the energy at the potential victim or device under test (DUT). Conducted voltage and current susceptibility testing typically involves a high-powered signal generator, and a current clamp or other type of transformer to inject the test signal. Transient or EMP signals are used to test the immunity of the DUT against powerline disturbances including surges, lightning strikes and switching noise. In motor vehicles, similar tests are performed on battery and signal lines. The transient pulse may be generated digitally and passed through a broadband pulse amplifier, or applied directly to the transducer from a specialized pulse generator. Electrostatic discharge testing is typically performed with a piezo spark generator called an "ESD pistol". Higher energy pulses, such as lightning or nuclear EMP simulations, can require a large current clamp or a large antenna which completely surrounds the DUT. Some antennas are so large that they are located outdoors, and care must be taken not to cause an EMP hazard to the surrounding environment.

Legislation

Several organizations, both national and international, work to promote international co-operation on standardization (harmonization), including publishing various EMC standards. Where possible, a standard developed by one organization may be adopted with little or no change by others. This helps for example to harmonize national standards across Europe.

International standards organizations include:

International Electrotechnical Commission (IEC), which has several committees working full-time on EMC issues. These are:
Technical Committee 77 (TC77), working on electromagnetic compatibility between equipment including networks.
Comité International Spécial des Perturbations Radioélectriques (CISPR), or International Special Committee on Radio Interference.
The Advisory Committee on Electromagnetic Compatibility (ACEC) co-ordinates the IEC's work on EMC between these committees.
International Organization for Standardization (ISO), which publishes standards for the automotive industry.

Among the main national organizations are:

Europe:
Comité Européen de Normalisation (CEN) or European Committee for Standardization).
Comité Européen de Normalisation Electrotechniques (CENELEC) or European Committee for Electrotechnical Standardisation.
European Telecommunications Standards Institute (ETSI).
United States:
The Federal Communications Commission (FCC).
The Society of Automotive Engineers (SAE).
The Radio Technical Commission for Aeronautics (RTCA); see DO-160
Britain: The British Standards Institution (BSI).
Germany: The Verband der Elektrotechnik, Elektronik und Informationstechnik (VDE) or Association for Electrical, Electronic and Information Technologies.

Compliance with national or international standards is usually laid down by laws passed by individual nations. Different nations can require compliance with different standards.

In European law, EU directive 2014/30/EU (previously 2004/108/EC) on EMC defines the rules for the placing on the market/putting into service of electric/electronic equipment within the European Union. The Directive applies to a vast range of equipment including electrical and electronic appliances, systems and installations. Manufacturers of electric and electronic devices are advised to run EMC tests in order to comply with compulsory CE-labeling. More are given in the list of EMC directives. Compliance with the applicable harmonised standards whose reference is listed in the OJEU under the EMC Directive gives presumption of conformity with the corresponding essential requirements of the EMC Directive.

In 2019, the USA adopted a program for the protection of critical infrastructure against an electromagnetic pulse, whether caused by a geomagnetic storm or a high-altitude nuclear weapon.

References

External links

EMC-Directive European Commission – Harmonised standards for EMC
What is EMC? YouTube video.
Introduction to EMC
Basics in EMC/EMI and Powerquality
Analog, RF and EMC Considerations in Printed Wiring Board (PWB) Design
Application Note: Design for EMC Compliance
Design for EMC - Effects of Via Slots, Split Planes, Gaps and Return Paths on Clock Signal
EMC engineering practices for panel builders
EMC Resources (Clemson University)