Both contact and air discharge are applied with magnitude dependent on the class of the device. Surge test simulates the effect of lightning and destructive line transients to electronic devices and is perceived to be the most stressful test among the three. Application of surge voltage up to several kilovolts to the device under test is not uncommon. EFT, which simulates line switching, switch bouncing and inductive line transients, is another stressful test applied in a very fast rate. Attacking straight to the source itself is the most effective and economical one.
Electromagnetic interference takes into two forms, Conducted and Radiated.
The former is transmitted through the input and output cables and the latter one through space. Conducted noise is further divided into Differential Noise and Common Mode Noise. Differential noise are those that flows in both directions along the line and neutral wires and common mode noise are characterize as those that flows between one line and the ground. Typically, conducted emission problems below 2 to 3Mhz are differential mode and above 3Mhz, they are common mode noise. Common mode noise is not as dominant as the differential mode at low frequency for several reasons.