Electromagnetic Emissions & Immunity

EM Environment  |  Emissions & Immunity  |  Disturbance Types  |  Achieving EMC  |  Functional Safety

So there are two sides to the EMC equation:

• Source equipment whose controllable emissions must be limited.
• Equipment that needs to have adequate immunity to those disturbances in its environment to which it is exposed.

Typical sources include:

• Power lines
• Electronic circuits
• Electric motors
• Radio and radar transmitters.

Equipment that is disturbed, often called "victim" equipment by EMC specialists, can include virtually anything that uses or can detect EM energy, such as:

• Radio receivers
• Domestic appliances
• Electronic circuits of any kind.

Electronic circuits are everywhere, from the smallest hand-held device or modem to the control circuits at an electricity generating plant serving an entire region.

EM disturbances may work in more than one direction, disrupting more than one device, or multiple sources may have a cumulative effect on a single piece of equipment. Thus an air traffic control radar may affect the display of a laptop computer being used in an aircraft as well as other vital devices in use on the ground. At the same time, emissions from the laptop computer may combine with those from a mobile phone to disturb systems in the aircraft.

On the emissions side of the equation, therefore, the aim of EMC is to ensure that equipment does not disturb other equipment, radio services, power or other networks. On the immunity side, the aim is to ensure that equipment is not affected by, e.g., radio transmissions, mains-borne disturbances, electrostatic fields and other phenomena.

Types of disturbance
Solving EMC problems, by limiting controllable emissions and by improving the immunity of susceptible devices or systems, first of all involves trying to identify not only the victim equipment but also the source and the path of the disturbance between them. In practice, clear identification of the source or the path is often difficult and sometimes even impossible, but the way in which source and victim are coupled may be described in general terms as being through:

• An electric current or voltage
• An electric field
• A magnetic field
• An electromagnetic field
• Some complex combination of the above.

The IEC has classified these principal EM disturbance phenomena in six categories so that its technical committees can take a systematic approach in preparing EMC standards for all kinds of products and systems. Of the six, two are conducted, two are radiated, one is electrostatic discharge (ESD) and one covers special high-power EM transients including nuclear high-altitude electromagnetic pulse (HEMP).

Achieving EMC
Once the potential problem areas are identified, EMC can be achieved in a number of ways. For example, the source or the victim equipment may in effect be removed from an area by rule or regulation (e.g., no cellular phones permitted). Alternatively, if some equipment emits more EM fields than intended or desired, there is the possibility of reducing those emissions and therefore the level of disturbance for other equipment occupying the same area.

Since avoidance techniques may not always be successful, however, it may be more effective to mitigate the effect of a disturbance by filtering or shielding, thereby increasing the immunity, or by some degree of redesign that ensures the problem is no longer significant to the exposed equipment.

Whatever the method, the aim always remains the same: to secure the proper functioning of equipment in the role for which it is designed.

Which is where the IEC makes a major contribution. IEC standards and other technical publications, for example:

• Describe all the basic elements of the EMC problem
• Specify emission measurement methods
• Set emission limits
• Detail immunity testing techniques and recommend protection methods either in general or for specific products.

Functional Safety
Although the normal procedures developed by the IEC will produce EMC, they are intended to be cost-effective for industry and do not prevent the possibility of electromagnetic interference under worst-case conditions. Consequently in cases where EMI could produce a safety hazard to humans, special procedures need to be investigated.

The basic approach to achieving this functional safety is to evaluate the margins between the expected levels of emissions that create the EM environment and the levels of immunity that equipment possesses. In many cases, the most appropriate approach is to raise the immunity level of the equipment to ensure that unsafe conditions do not result during normal operation.

Source: International Electrotechnical Commission (IEC).