What is Radio Frequency Interference (RFI)?
RFI, sometimes also known as Electromagnetic Interference (EMI), is unwanted electromagnetic energy in the frequency range generally used for radio communications. The frequency range of in interest are 10 Khz to 30 Mhz for conducted phenomena and 30 Mhz to 1 Ghz for radiated phenomena.

What are the modes of propogation of RFI?
RFI is propagated via radiation (electromagnetic waves in free space) and by conduction over signal lines and AC power systems.

"Radiated" :- One of the most significant contributors to radiated RFI from electronic equipment is the AC power cord. The power cord is often an efficient antenna. Since its length approaches a quarter wavelength for the RFI frequencies present in the digital equipment and switching power supplies.

"Conducted" :- RFI is conducted over AC power systems in two modes: 
Common mode (asymmetrical) RFI is present on both the line and current neutral paths with reference to the ground or earth path.  Differential mode (symmetrical) RFI is present as a voltage between the line and the neutral leads. RFI filter which is also called as Power Line Filter is used to suppress conducted RFI.

Mains Filter
Mains filter must be mounted immediately behind the mains entry into an equipment. In this position they prevent the penetration of high frequency interference energy via the leads of the cable into the equipment. They also prevent interference energy from inside such an equipment from getting out via the mains cable.

A mains filter primarily consists of a LC network and causes a mismatch of the Filter impedance to the impedance of the interference path. As a result, the interference is reflected in the direction of the interference source.
The schematic of a typical RFI line filter is given below

 The inductances L1 and L2 (MH) exhibit high impedances against common mode interference signals (asymmetrical interference currents) and also for mains frequency current.
These inductances are generally wound on toroids with two identical windings. The windings are arranged symmetrically so that the magnetic fluxes produced within the magnetic material compensate each other with the normal operating current (differential mode). This ensures that magnetic saturation is avoided. 

With symmetrical interference (Common mode), however, the two magnetic fields support each other and the total inductance is effective. The symmetrical interference components are effectively attenuated by the X capacitor between phase and neutral combined with the stray inductance of L1 and L2.

Non compensated current chokes may be used in addition to the X capacitor to achieve higher symmetrical attenuation values.
The capacitance values of the Y capacitors are limited for the type of equipment by government regulations which define the maximum leakage currents between phase / and or neutral and the protection earth.
RFI / EMI Elcom Line Filter Range