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A mains filter (or line filter) is an electrical circuit which reduces both electrical disturbances of electronic equipment into the public mains supply (radio interference suppression) and the electromagnetic compatibility EMC of electrical equipment against disturbances from the power supply system (rise of the interference immunity).
Mains filters consist of resistors, chokes and capacitors. Current compensated chokes (suppress well asymmetrical or common-mode currents) are combined with uncompensated chokes (suppress symmetrical currents).
Special interference suppressing capacitors are used in mains filters. X capacitors suppress push-pull interfering voltages, Y capacitors suppress common mode interference. Both X and Y capacitors are self-healing, i.e. they do not cause any short circuit at inner electrical breakdowns.
The two Y capacitors at the lines of single-phase equipment of the safety class I (equipment with protective earth terminal connection or protection line) cause an increased leakage current: the full mains voltage against earth or protection line is connected to one of the capacitors, an idle current may flow of 0.5 mA (medical equipment) or 3.5 mA (other equipment of the safety class I) or 5 mA (industry assemblies) against ground.
If several equipment are operated with mains filter at a Residual Current Protective Device (RCD), the leakage currents possibly cumulates to a value above the turn off threshold (typ. 30 mA) and it triggers. This difference current is particularly high switching on in the tension maximum of the mains AC voltage, RCDs are therefore offered with a delayed response behavior.