Can a unit with a Null Rod cast psychic powers providing they don affect there own unit?
Yes, if a unit with a Psycher wants to cast a psychic power and a member of the same unit (or potentially the psycher himself) is armed with a Null Rod they can cast psychic powers so long as they have no affect on the unit itself as the rules for the Null Rod state in the wargear section of the Grey Knights Codex.
