These tests relate to the immunity requirements for equipment to unidirectional surges caused by overvoltages from switching and lightning transients. Several test levels are defined which relate to different environment and installation conditions. These requirements are developed for and are applicable to electrical and electronic equipment.
 

System switching transients can be separated into transients associated with :
	Major power system switching disturbances, such as capacitor bank switching;
	Minor switching activity near the instrumentation or load changes in the power distribution system;
	Resonating circuits associated with switching devices, such as thyristors;
	Various system faults, such as short circuits and arcing faults to the earthing system of the installation.
The major mechanisms by which lightning produced surge voltages are the following :
	A direct lightning stroke to an external circuit (outdoor) injecting high currents producing voltages by either flowing through earth resistance of flowing through the impedance of the external circuit;
	An indirect lightning stroke (i.e. a stroke between or within clouds or to nearby objects which produces electromagnetic fields) that induces voltages/currents on the conductors outside and/or inside a building;
	Lightning earth current flow resulting from nearby direct-to-earth discharges coupling into the common earth paths of the earthing system of the installation.

 

The rapid change of voltage and flow of current which may occur when a protector is excited may couple into internal circuits.

Test voltages of up to 4 kV are applied synchronized to the voltage phase at zero-crossing and peak value of the A.C. voltage wave (positive and negative). The surges are applied line to line and line to earth. When testing line to earth the test voltage is applied successively between each of the lines and earth. All lower levels including the selected test level is tested.

The test levels shall be selected according to the installation conditions.

All voltages of the lower test levels shall be satisfied.
 
The preferential range of test levels is given below.