This is a really interesting question and I’ve love your insight on it. One of the disadvantages of electromagnetic locks is that when power is cut to the mag-lock the door is not locked, which creates an obvious security problem during a power failure. Imagine an existing door with a mag-lock that is released by a sensor when a building occupant approaches the door. The mag-lock is also released by an auxiliary push button, activation of the fire alarm, and power failure, as required by the model codes. Is it code-compliant to add a deadbolt to a door with a mag-lock, that can be used to lock the door during a power failure?
My initial reaction was that you can’t add a deadbolt because it would be a second lock, and the codes require the door to unlatch with one operation. BUT – mag-locks are often installed on doors with latching hardware – like a lockset/latchset or panic hardware, and the sensor unlocks the mag-lock without any releasing operations performed by the building occupant. The IBC specifically says that you can have a mag-lock and panic hardware on the same door as long as the mag-lock is released either by a sensor or by a switch in the panic, so that sets a precedent for allowing the mag-lock and another piece of locking/latching hardware.
Even if the deadbolt was engaged while the mag-lock was powered, a building occupant would only have to perform one releasing operation to unlatch the door (turn the thumbturn). If the sensor was not working and the auxiliary push button had to be used, it still wouldn’t be any different from someone pushing the auxiliary push button and then turning a lever or pushing on the touchpad of a panic device. One disadvantage is that the deadbolt could mechanically lock the door and override the access control system, but that’s not a code issue.