Susceptibility

Immunity is the ability of equipment or systems to perform without degradation in the presence of an electromagnetic disturbance. Susceptibility is the lack of immunity or that equipment or system is not susceptible to the electromagnetic disturbances. Susceptibility thought of as a temporary reduction of performance or degradation of quality. Susceptible equipment may directly or indirectly be a safety issue or a performance issue.

It is difficult to take into account all the possible electromagnetic occurrences. Testing for susceptibility does not represent all the possible electromagnetic occurrences and therefore should not be seen as any kind of guarantee that a tested and approved equipment will never be affected by an electromagnetic phenomenon.

Immunity standards describe typical electromagnetic phenomena, severity levels, and test methods. It is important to know that immunity standards try to achieve reproducibility. For this reason immunity standards have to be followed very closely. Creating an electromagnetic filed for checking the susceptibility of an electronic circuit need not be very complex, but immunity electromagnetic radio frequency fields requires some extensive facilities.

Immunity test methods are described in detail in basic standards. Most time commonly used immunity test methods are spelled out in the basic standards to which generic and product specific standards refer. Some commonly used immunity standards to test for susceptibility are as follows.

Electrostatic discharges (ESD)
IEC/EN 61000-4-2

Radiated RF Electromagnetic Fields
IEC/EN 61000-4-3

Electrical Fast Transients/Burst (EFT)
IEC/EN 61000-4-4

Surges
IEC/EN 61000-4-5

Conducted Disturbances Induced By RF Fields
IEC/EN-4-6

Voltages Dips, Short Interruptions, And Voltages Variations
IEC/EN 61000-4-11

Testing needs to be conducted in a well-planned and reproducible manner. The equipment under test should be it its normal mode and connected to any auxiliary equipment. The most susceptible modes of operation should be selected and the configuration varied to achieve the maximum susceptibility. The required severity testing level/class for an apparatus or system depends on the environment and installation conditions where it will be used.