Why would one need a SPD rated for 400 kA when most transients are well under 10 kA?
The rating of the unit doesn’t dictate the size of a surge it can take, but often gives some indication of how robust it is. It could be argued with a substantial certainty that anything within a 400 kA lightning strike probably would not survive even if there were a SPD rated at 400 kA. However, a more common 40 kA lightning strike may occur. In this case the strike hits the power pole the energy begins to disperse its various ways and by the time it gets down to your service entrance there will be well under 10 kA. With a 200 kA unit the service entrance could be protected with the same results as a 400 kA unit and for less expense. This same philosophy is true at the branch panel. A 120 kA unit could do the job of a 200 kA unit for less cost. So no, bigger is not always better. What is better is determining if you are in an area prone to transients and how sensitive is your equipment to these transients. Then using this information to determine the size of kA rating you need for you
