EFT/B simulators are test equipment used to evaluate the immunity from switching transients originating from inductive load interruption, relay contact bounce, etc through power and external interface lines.  Take an example of a relay opening.  The first spark occurs when the contacts have only just started to open.  As the contacts move further apart, the arc shuts off and the resultant high rate of change of current causes another spark. 

This process generates sparks at higher repetitions even of a few MHz with lower amplitudes and ones at lower repetitions of tens of kHz with higher amplitudes. However, for the standardization purpose, the IEC61000-4-4 standard calls for single spikes with a 5ns rise time and 50ns duration, constantly spaced between spikes, at a rate of 5kHz and 100kHz (200μs/10μs pulse period）.