There is an older thread that discusses emergency doors, see: http://www.antique-locks.com/showthread.php/1884-Emergency-Doors?highlight=emergency
In a more general way of thinking about them, an emergency door should be just as tough to get through as the main door. In practice this seems to mean it’s just as thick, has the same number and style of locks, and the same number of time locks. They also seem to have a kind of “day gate” as well. Most of the time.
There are some variations. Many emergency doors have “lug” style boltwork in contrast to the main door’s projecting boltwork. There are other minor variations. But equal thickness seems to be common.
