Fields, Circuits and Relevant Voltmeters

Electric and magnetic fields play a central role in Electromagnetic Compatibility. It is therefore important to decide what model to use for the description of difficult EMC-problems: Field Theory (FT), based on Maxwell's laws or Circuit Theory (CT), based on Kirchhoffs laws. Voltages between A and B, measured in the 3D-world or calculated with the FT model differ in two respects from what CT can offer. Firstly the points A and B that seem well defined in the equivalent circuit diagram are sometimes fuzzy in the real world. Secondly the voltage depends on the path, when the path from A to B runs through a region with changing magnetic flux. Indeed, a large variety of voltages can be measured in the real world, but many of those are meaningless. We therefore restrict ourselves to important voltages, as seen by “relevant voltmeters”: People, as “wandering” voltmeters, should not be exposed to too high “step” and “touch” voltages. Electric shock is dangerous at low frequency and at relatively large currents. The characteristic length between the “voltmeter terminals” is 1 m. Electronic units are vulnerable to interference voltages across their input terminals. Risks show up at all frequencies and often already at low voltages. The characteristic length is 1 mm. Analysis of the differences between FT and CT leads to an emphasis on currents, in particular on Common Mode currents. Secondly, we restrict ourselves to the relevant voltmeters. This results in a systematic and practical EMC-approach.