Modes of Interference

In the general case of the diagram below, an equipment is supplied with power by lines L and N, E is the protective safety earth connected to the equipment's metal housing. Three voltages can exist and currents can flow in three loops as shown. Currents I2 and I3 will normally flow by virtue of capacitance existing in the equipment's and stray coupling caused by transformers or heatsinks or intentionally introduced filter suppression components.

Conventionally voltage V1 and current I1 are defined to be of symmetrical, differential or series mode, V2, V3, I2, I3 are of the asymmetrical, common or shunt mode.

Spurious interference voltages and currents are separated into the two modes as are the current attenuation characteristics of mains interference suppressors. In analysing interference in order to determine a suitable suppression network it is usual to consider the two modes separately and the performance curves of standard filters which are given as separate plots for the two modes