The Three Elements of an EMC Problem

There are three essential elements to any EMC problem. There must be a source of an electromagnetic phenomenon, a receptor (or victim) that cannot function properly due to the electromagnetic phenomenon, and a path between them that allows the source to interfere with the receptor. Each of these three elements must be present although they may not be readily identified in every situation. Electromagnetic compatibility problems are generally solved by identifying at least two of these elements and eliminating (or attenuating) one of them.

For example, in the case of the nuclear power plant that was mentioned in the previous section, the receptor was readily identified. The turbine control valves were malfunctioning. The source and the coupling path were originally unknown, however an investigation revealed that walkie talkies used by the plant employees were the source. Although at this point the coupling path was not known, the problem could be solved by eliminating the source (e.g. restricting the use of low-power radio transmitters in certain areas). A more thorough and perhaps more secure approach would be to identify the coupling path and take steps to eliminate it. For example, suppose it was determined that radiated emissions from a walkie talkie were inducing currents on a cable that was connected to a printed circuit card that contained a circuit that controlled the turbine valves. If the operation of the circuit was found to be adversely affected by these induced currents, a possible coupling path would be identified. Shielding, filtering, or rerouting the cable, and filtering or redesigning the circuit would then be possible methods of attenuating the coupling path to the point where the problem is non-existent.

The source of the tramway problem was thought to be transients on the tramway's power. The coupling path was presumably through the power supply to the speed control circuit, although investigators were unable to reproduce the failure so the source and coupling path were never identified conclusively. The receptor, on the other hand, was clearly shown to be the speed control circuit and this circuit was modified to keep it from becoming confused by unintentional random inputs. In other words, the solution was to eliminate the receptor by making the speed control circuit immune to the electromagnetic phenomenon produced by the source.

Potential sources of electromagnetic compatibility problems include radio transmitters, power lines, electronic circuits, lightning, lamp dimmers, electric motors, arc welders, solar flares and just about anything that utilizes or creates electromagnetic energy. Potential receptors include radio receivers, electronic circuits, appliances, people, and just about anything that utilizes or can detect electromagnetic energy.

Methods of coupling electromagnetic energy from a source to a receptor fall into one of four categories.

1. Conducted (electric current)

2. Inductively coupled (magnetic field)

3. Capacitively coupled (electric field)

4. Radiated (electromagnetic field)

Coupling paths often utilize a complex combination of these methods making the path difficult to identify even when the source and receptor are known. There may be multiple coupling paths and steps taken to attenuate one path may enhance another.