The contact discharge is the method of testing in which the electrode of the test generator is kept in contact with the EUT or coupling plane and the discharge is actuated by the discharge switch within the generator.　　

The air discharge is the method of testing in which the charged electrode of the test generator is moved towards the EUT until it touches the EUT. During this a discharge is activated by a spark.　　The IEC 61000-4-2 standard says “Contact discharge is the preferred test method.　　Air discharges shall be used where contact discharge cannot be applied.” 　

The preference of contact discharges over air discharges comes from its better reproducibility.　　Air discharges are inherently more variable than contact discharges due to factors such as ambient temperature, relative humidity and atmospheric pressure.