More and more devices on the power grid must function smoothly without interfering with each other. Not so easy, since switching power supplies and highly clocked integrated circuits are used in almost all electronic devices. EMC filters can help here. Where necessary in multi-stage design.
In practice, it is quite possible that the attenuation is not yet sufficient despite the use of a filter. In such cases, several filter stages optimized for the specific interference signals are then connected in series. Which in interaction significantly improve attenuation in both differential and common-mode interference. Usually you get very good results with a doublestage filter. However, triple-stage filters are also conceivable, feasible and available.