A: Whoever is in the chair (chairman, vice-chairman, principal, or other), in order to preside well, must give the appearance of impartiality. This generally means that he does not enter into debate and does not vote. (RONR pp. 389-390.) There are major exceptions.

Exceptions in debate.
If the chairman feels he must participate, he may step down after appointing someone to take the chair (usually the vice-chairman). That person must not have already spoken on the issue. After stepping down, the ex-chair becomes like any other member and may debate, vote, etc. When the issue is decided, he returns to the chair.

Exceptions in voting.
The chair may always vote, of course, whenever the vote is by secret method, e.g., by ballot.
He may also vote when his vote would change the outcome. For example, if a majority is needed and the vote is 10 to 10, the chair could vote in the affirmative, making the vote 11 in favor to 10 opposed, causing the motion to be adopted when it would have otherwise failed. Or, if a majority is needed and the vote is 11 in favor to 10 against, the chair could vote in the negative, making the vote 11 to 11 and causing the motion to fail when it would have otherwise been adopted. If, however, the chair's vote would not make a difference in the outcome, he may not vote. (RONR pp. 400-401.)