The article ‘Selecting the right mains filter for your application’ by Keith Armstrong (EW Sep. 2018) provides extremely useful guidance. It describes the construction of several types of filter, identifies their characteristics and indicates how the manufacturer’s data can be used during the process of selection.
It includes a section on the design considerations involved in installing such filters in the equipment under review. This is especially useful in that it illustrates the fact that skin effect ‘encourages’ noise currents to remain outside the conducting enclosure and that internal interference is contained within the enclosure. Signals within the enclosure are ‘clean’ whilst those outside the box are ‘dirty’.
In each of the five circuit diagrams, the bottom conductor is identified by the earth symbol. This conveys the impression that unwanted current flows through this conductor back to the source where it flows into the soil via grounding grids; rather like rainwater flowing through a pipe into a soakaway. Such a depiction ignores the basic lesson of Transmission Line Theory. At any instant, current is flowing in both directions along any conductor.
Capacitive coupling between the live, neutral and earth conductors of the supply cable ensures that current flows to and fro across the gaps between them. Inductive coupling ensures that electromagnetic energy flows along the path defined by the routing of the cable. When it arrives at the discontinuity presented by the mains supply distribution box, much of the transient energy is reflected back to the filter. This energy is reflected straight back to the distribution box, then it bounces back and forth between the two units.
This process does not last long, since the cable also acts as an antenna. Current which is not captured by the other two conductors escapes out into the environment. This can be defined as ‘antenna-mode current’ and can be measured with a current transformer clamped round the cable.
The process is not restricted to the cable between distribution box and filter. That energy which is not reflected at the distribution box continues outward into the power distribution wiring where the process continues. When a switch opens, the transient energy stored in the supply line disappears into the environment as a brief burst of radio-frequency radiation.
Mains filters may well protect the unit-under-review from interference, but they do this by dumping the unwanted electromagnetic energy into the environment.
The only way of minimising this form of pollution is to include resistors to absorb the unwanted energy. This could be a resistor in parallel with each selected inductor, or a resistance in series with each selected capacitor. The sections on Differential-Mode Damping and Common-Mode Damping in the article www.designemc.info/16DesignGuidelines.pdf describe the technique.
16 October 2018