With reference to EN 61000-4-4, the fast transient burst standard.
As with ESD, we also test from low levels upwards. As far as I can remember, we have not had a point where a failure only occurred in the middle of the test range.
I must also admit that the reasons for testing from low levels upwards is not so much to thoroughly test to meet standards, but to protect our products. In the early days of these tests, when products were not as well protected as they are now, we soon found that once a failure occured at a low level, increasing the levels to see what happened simply increased the levels of damage. Our expertise in design has since improved, and apart from areas that we suspect may be susceptible we continue increasing levels to see the effect - hence discovering the oddity with an ESD test.
From comments on this discussion point, it seems well worth continuing with this practice. Interference with the performance of our product could annoy a customer regardless of the level of the interference source.
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