Electrical contractor dropped a piece of 3/4 rigid conduit pipe approx. 4' long from a walk platform above a fuel silo. The pipe slid down the roof of silo and fell approx. 20' and hit the EE point down between his back and shoulder blade. The EE was working in a boom lift appr. 70' from ground level. EEG was diag.with fractured ribs. EE was released for work upon leaving doctor.