EA reduces by the stated amount any damage that hits the ship components, armor or otherwise, while it is still undestroyed itself. It will not reduce damage to shields that I am aware of. With the CA it works because every hit with CA adds to the shields so the next shot hits the small amount of shields generated then hits the armor generates more shields and is reduced by the EA amount. Quite effective.
So if you had a 31 damage weapon and hit a ship with EA of value 30 then 1 damage would be dealt to the ship.