Unfortunately, the lack of U.S. nuclear testing since 1992 has also contributed to uncertainty about the adequacy of steps to protect conventional military gear from the effects of high-energy, electromagnetic pulse (EMP) attacks.
Military planners understand the vulnerability of the grid to upset, but have been more concerned with man-made threats such as the electromagnetic pulse that would be generated by a high-altitude nuclear detonation.
Today, the best that can be said is that the extent and duration of future EMP-induced blackouts would depend on whether the emitter generates a small or large pulse, and whether it is detonated at ground-level or from high altitudes.