I don't know for certain, but it sounds feasible to me..........a microwave is designed to contain the electromagnetic radiation that a microwave produces, so I would assume the reverse would be applicable and it would block possible all or most of the 4 different forms of EMPs ( Electric field, Magnetic field, Electromagnetic radiation & Electrical conduction).........that's about all I can remember about EMPs.......I guess I should add that to the list of things to study up on.......it's not if, but when we get hit by one (again).......Google Carrington Event if you are not already familiar.