Electromagnetic Interference (EMI) is generated by some power electronic devices, such as  power supplies. It has two forms: conducted and radiated interference. Conducted interference  moves to and from the device by means of input and output wiring and control cables, if any.  The same wires can also radiate the noise. Radiated interference is designated as flowing into  the environment from the device, as in a radio transmission. Nearby receivers can hear these  transmissions.

 The FCC has regulations, as well as the military and others, for controlling the allowable EMI  levels:

      Part 15, subpart J, Docket 20780 (U.S. FCC)
      MIL-STD 461 (U.S. military)
      VDE0871 (international)

 The given specs are all different, and so each must be studied separately. The spec to use  depends on where the product will be marketed: U.S. only; U.S. military only; international  only; or a combination of places.

 What to do?

 Input and output leads need r.f. filters to bypass the interference and keep the noise in the  power electronic box. Radiation is stopped or minimized by the use of shielding of the box  entirely or just those areas doing the radiating, such as transformers and power transistors.  Sparing no expense, the military product will have compartments with the offending parts  inside them and feedthrough capacitors to filter wires in and out of the compartment. Power  leads in and out with their respective filters and connectors are put in separate compartments  as well.

 Commercial and industrial devices, especially a card or open frame power supply, have less  attention given to EMI because the product they go into takes care of reducing it.

 Effect on sales

 Most products and devices that could cause EMI are sold showing the relevant EMI specs printed  on the label, to increase market acceptance and required by the governing spec organization.  For example: a product manufactured in the U.S. to FCC Part 15 could not be sold in Europe  unless it passed VDE specs. Likewise, it couldn't be sold to the military unless it passed  MIL-STD 461.

 EMI testing confirmation

 To prove capabilities, a company must buy its own EMI equipment (which is expensive) and train  employees to use them, or utilize service companies who specialize in EMI testing. The later  should be the least expensive approach, complete with the latest technology plus a final  report with all the data. If a failure occurs, often the testing engineers can suggest  solutions.