I believe that you are using an incorrect problem description.
What I believe you are describing, based on your description, is an input transient voltage caused the supply to produce a brief output pulse.
But I have to ask: how did you apply the 2Kv transient to the Lambda supply? What waveform (8X20, 1.2X50, 1000Khz ringwave)? What location service category, A, B, C?
BTW, you have to define also what you mean by immunity. The lowest immunity level *only* means that the device will not suffer permanent damage, but can misbehave and require user intervention to restore functionality.
An enhanced immunity level the device can still misbehave but will recover its functionality automatically, and for the highest level the device not only will not suffer any damage, but functionality will not be impaired at all.
Depending on the answers above, there will be different levels of upstream or downstream protection requirements. It could be as simple as an enhanced physical ground (earth), or as complex as a motor generator set which completely isolates the device from the grid. There is a whole spectrum of protection levels.