Surge immunity - 1.2/50 µs voltage and 8/20 µs current combination wave, levels 0.5–4 kV.
The standard specifies a method for testing equipment immunity to unidirectional surges caused by switching events and lightning. In practice, surges arise from sudden load changes and short circuits in the power network and from voltages induced in power and signal lines by indirect lightning strikes.
The standard does not cover direct injection of lightning currents or dielectric withstand testing.
The test demonstrates that equipment withstands a surge impulse of specified energy and voltage with defined functional behavior. The method number is not a declaration of conformity; test voltage, coupling method and performance criterion are selected by the product or generic EMC standard according to installation conditions.
The combination-wave generator produces an open-circuit voltage waveform of 1.2/50 µs and a short-circuit current waveform of 8/20 µs with an effective output impedance of 2 Ω; for symmetrical telecommunications lines, a 10/700 µs generator (5/320 µs current) is used.
For AC/DC power ports, the impulse is coupled through a coupling/decoupling network: capacitively through 18 µF between conductors (L-L) and 9 µF in series with a resistor to earth (L-PE). At least five positive and five negative impulses are applied for each coupling mode and point, at a rate not exceeding one impulse per minute; for AC mains the test plan records the phase angle at which the impulse is coupled.
Assessment: the equipment must operate as intended after the test without degradation below the level specified by the manufacturer; classification uses four categories from normal operation to irreversible loss of function.
Check test voltage separately for L-L and L-PE (typical entry: 1 kV L-L / 2 kV L-PE) and the performance criterion. EN 50121-3-2:2016 references this method for rolling-stock power ports with ±2 kV L-PE and ±1 kV L-L, criterion B.
