From UFStarfleet LCARS
On Federation starships, a bridge separation mechanism is a rarity. This is mainly due to the nature in which such a feature is often used(e.g. the bridge/command crew escaping or abandoning ship against proper morality codes and general orders). However, sometimes it is installed as a pre-caution for ships in highly dangerous assignments when the command level staff may need to be evacuated, or in situations where the main command center is compromised and needs to be jettisoned.
Such a separation procedure requires the authorization by atleast 4 command level or senior staff officers. Usually this comprises of the Captain, the XO, the Second Officer, and another senior ranking officer on-board.
So far, a bridge separation has never been performed in real field operations, however, a limited selection of ships have this function such as the USS Redeemer-A.