Electromagnetic Compatibility (EMC) in Action
- LAMBDA INDIA
- Aug 8, 2019
- 6 min read
Managing Electromagentic Interference For Safer, More Successful Products.
Introduction to EMC Testing
All Electric and Electronic devices emits and receive electromagnetic waves, which can interfere with other electrical or electronic equipment. To prevent harmful interferences, electric devices have to adhere to EMC guidelines.
Most countries have regulations to ensure that electrical products available in their markets are built to deliver suitable EMC performance. That means not creating interference that can affect other equipment, as well as providing some level of immunity to external disturbances. And it is up to manufacturers of electrical equipment to adhere to these regulations and make sure that their products are EMC compliant.
SECURING EMC SUCCESS
The best way to produce EMC compliant equipment is to start early, and follow EMC best practices during the design process. However, manufacturers also need to verify the effectiveness of their designs by testing prototypes for EMC.
The testing is done in an EMC laboratory.
Two types of phenomena are considered: Electromagnetic Interference (EMI) and electromagnetic susceptibility (EMS).
EMI is related to the electromagnetic radiation emitted by a product, whereas EMS describes the behavior of a product when subjected to electromagnetic disturbances coming from the environment.
Both factors are important. Together, they can mean the difference between a potentially dangerous device that cannot find a market, or a successful, highly-profitable product that takes the world by storm.
Understanding Electromagnetic Interference (EMI)
The electromagnetic emissions produced by electrical devices are propagated in two ways – radiated through the air or conducted along power lines or signal cables. While both can cause disturbances to the environment, they are normally assessed separately.
RADIATED EMISSIONS
Electromagnetic emissions radiated through the air are usually measured by an antenna located between three and ten meters from the device, which is referred to as the equipment under test (EUT). During testing, the EUT is rotated through 360 degrees while instruments record the emission levels at different turntable angles and antenna heights.
The measurements can be performed in different kinds of test sites, even outdoors. But, the most accurate results are usually obtained when testing is done in a semi-anechoic chamber.
A semi-anechoic chamber is a large, specially shielded room, where the walls and ceiling are covered by materials that absorb electromagnetic radiation.
The combination of shielding and absorbers reduce the level of environmental EM noise and prevent unwanted reflections from EM waves on the walls and ceiling.
This means that the test only records relevant radiated signals produced by the EUT, enabling a precise evaluation.
Radiated emission tests usually measure frequencies from 30 MHz to 1 GHz, and sometimes even higher.
The results are compared to the limits listed in the relevant regulation in order to verify the compliance of the product.
CONDUCTED EMISSIONS
While radiated emissions are all around, conducted emissions are different. They travel across whatever a device is connected, which means they can potentially affect any other device plugged into the same infrastructure.
Conducted emissions testing measures the level of electromagnetic noise that the EUT propagates across connected equipment, including mains electricity wiring, telephone networks or LANs.
During a test, the power and telecommunication cables of the EUT are connected to an instrument that measures the level of electromagnetic noise – typically between 150 kHz and 30 MHz – which the EUT generates. The results are compared with the specifications set out in the applicable regulation to determine the compliance of the product.
Getting to Grips with Electromagnetic Susceptibility (EMS)
Technology is everywhere. But some devices can be more sensitive to electromagnetic effects than others. That’s where electromagnetic susceptibility testing comes in. It assures market regulators and end users alike that a product can withstand a certain amount of electromagnetic disturbance without suffering unwanted consequences.
EMS testing takes a variety of forms that reflect the different types of electromagnetic disturbances. The most common EMS tests are:
ELECTROSTATIC DISCHARGES (ESD)
Have you ever felt an electric shock when touching a car door? That’s an electrostatic discharge, or ESD. And, while it is relatively harmless to human beings, the high voltages involved – sometimes up to several kilovolts – can damage sensitive electronic equipment.
The immunity of electrical equipment to ESD is verified with an ESD simulator.
The operator applies ESD pulses to all parts of a product that could be touched by a user. This checks whether such discharges will affect the proper operation of the device.
RADIATED EM SUSCEPTIBILITY
The world is filling up with equipment emitting intentionally wireless signals. From TV and radio stations to cell phones, walkie-talkies, traffic radar, RFID readers and, of course, Wi-Fi and Bluetooth transmitters. Unfortunately, the radio waves from these devices can produce interference and cause electrical and electronic devices to malfunction.
It is important to verify that equipment can withstand EM radiation before it goes on the market. This is done by exposing the product to electromagnetic signals of various frequencies – usually ranging from 80 MHz to 2.7 GHz – using an antenna connected to a signal generator.
ELECTRICAL FAST TRANSIENT BURSTS
Electrical fast transient bursts (EFT/B) are a series of fast repeat pulses that may be generated when a device switches inductive loads in the electric network. For example, when an electric motor is turned on or off, sparking will briefly occur inside the electrical switch. These sparks will cause transient disturbances to propagate through power or signal cables.
While these pulses are very short, they can have an amplitude of several kilovolts in most severe cases.
The vulnerability of electrical equipment to EFT/B is verified by connecting its power and signal cables to a pulse generator via a coupling network or a capacitive coupling clamp, and by injecting a series of test disturbances.
SURGES
Surges are unpredictable. They are often caused by lightning strikes or accidental overvoltages during switching events in the power distribution system. And they propagate through power cables that are connected to buildings – from homes and offices to schools and hospitals.
Because surge pulses can have an amplitude of several kilovolts with high energy, they can damage or destroy electrical equipment not fitted with surge protection components.
Surge resilience testing is done by connecting the equipment to a surge generator.
Test pulses are usually injected to the power ports and also to telecommunication ports that may connect to outdoor cables.
CONDUCTED LOW FREQUENCY EM SUSCEPTIBILITY
On their own, low frequency electromagnetic waves have little impact on smaller devices. But, they can be captured by long cables, which enables them to affect electrical devices through any wiring they may be attached to.
A conducted low frequency EM susceptibility test is an extension of the radiated EM susceptibility test, but it focuses on frequencies lower than 80 MHz.
Because generating radiated signals with long wavelength would require big antennas, it is easier to simulate the effects of such signals in a conducted manner.
This is done by injecting EM noise – from 150 kHz to 80 MHz – directly into the cables of the EUT with a signal generator and a coupling device.
POWER FREQUENCY MAGNETIC FIELDS
Every electrical device connected to the AC mains network operates at a specific power frequency – either 50 Hz or 60 Hz, depending on the country. At such frequencies, the electric current inside power cables generates magnetic fields that can affect sensitive equipment.
To find out whether a product performs well under such conditions, it is tested by being placed in the middle of a large inductive coil – typically 1 x 1 m or 2 x 2 m – generating a magnetic field of 50 Hz or 60 Hz.
VOLTAGE DIPS AND INTERRUPTIONS
Voltage dips and interruptions are caused by faults in the electric network.
They can range from a few percent up to 100%, and last a few milliseconds or a few seconds.
The ability of electrical equipment to withstand such unpredictable events can be verified by connecting the power cable of the EUT to a generator that produces the required voltage variations.
INFORMATION TECHNOLOGY EQUIPMNET
Information Technology Equipment (ITE) is a special category of electrical products from the EMC point of view, as it is more likely than many other device categories to be affected by, or to cause, electromagnetic interference.
Due to its complex construction and to the interdependencies of components, an EMC failure at any level of an ITE product could generate serious performance degradations, such as:
Corruption or loss of stored data
Incorrect input/output signals
Flickering, blurred, or distorted images on display
Error message, misbehaving or freezing software
Permanent damage (especially in case of ESD or surge)
An increasing number of ITE relies furthermore on wireless connectivity for its operation, making it more vulnerable to electromagnetic disturbances. Miniaturisation of ITE is an additional EMC challenge, as small ITE devices like smartphones can be easily carried around. They are therefore more likely to be located time to time close to other apparatus which may cause, or suffer from, electromagnetic interference.
An example of EM interference phenomenon that many people may observe at home is the fact that Wi-Fi devices operating at 2.4 GHz may experience degradations of their wireless performance in the vicinity of a microwave oven. Microwave ovens operate indeed at 2.4 GHz, too, with high power, which makes them likely to cause disturbances to nearby Wi-Fi equipment.
In order to be able to operate adequately in various kinds of electrognetic environments – and to comply with EMC regulations – ITE products require a great care in their conception. They are thus a good illustration of why EMC is nowadays more relevant than ever.
EMC regulations vary from country to country. The table here below gives an overview of some common EMC standards.
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