As well as safety, reliability and serviceability, electrical appliances must meet the further requirement of interference-free operation in the presence of other equipment in their intended operating environments. Interference-free operation of appliances intended for use in the same operating environment is guaranteed by electromagnetic compatibility (EMC). An electrical appliance must not generate unreasonable interference in its environment and must also tolerate reasonable levels of outside interference.
Any electromagnetic phenomena created by an appliance other that those intended for its practical use are regarded as interference. Interference spreads from one appliance to another via cables or as radiation. Phenomena such as stripes on a TV screen, the crackling of a radio or malfunctioning of a computer are often manifestations of interference caused by other electrical appliances.