3-5-10d Diagram

3-5-10d The runner is contacted by opponents (PlayPic A). When the runner comes to the ground, his helmet comes completely off his head and an official's timeout occurs (PlayPic B). Because the helmet came off during continuing action, the player must sit out for one down (PlayPic C).

3-5-10d If any player's helmet comes completely off during the down or subsequent dead-ball action related to the down, and it is not due to a foul by an opponent. (PlayPic A), that player must leave the game for at least one down unless halftime or an overtime intermission occurs (PlayPic B). In such circumstances, an officials' timeout occurs. Note: that if the player whose helmet comes completely off has possession of the ball, the ball is dead immediately. 

3-5-10d If a players' helmet comes completely off during the down and it is directly attributable to a foul by an opponent (PlayPic A), the penalty is enforced but the player need not leave the game (PlayPic B).