When an electric current flows through a conductor, it generates a magnetic field. If the current is alternating at a very high frequency, it will act like a radio signal and may cause interference with other nearby electronics. This effect is called EMI (Electromagnetic Interference). At radio frequencies it may be called RFI (Radio Frequency Interference).
In the United States, the FCC (Federal Communications Commission) regulates all sources of electromagnetic radiation. The FCC sets EMI standards to which electronic devices must comply. If you are manufacturing a new electronic device, it must be tested to assure it complies with FCC regulations.
EMC (Electromagnetic Compatibility) is the other side of EMI. It is the ability of an electronic device to operate as specified when in an environment of EMI that is within specified levels. EMS (Electromagnetic Susceptibility) is the degree to which an electronic device malfunctions when subjected to varying levels of EMI, sometimes referred to as the device's immunity from EMI.
Correcting EMI or EMC problems can be an art and a science. You need to determine the method of interference, it can be radiated, conducted, or coupled emissions. High frequency signals may ride on top of 120/240 volt power and interfere with other devices on the same circuit. High frequency signals may be transferred though a control of communication cable between two devices. Suppressing it may require a circuit board layout redesign, or redesign of the circuit electronics.
