Fail-safe locks are unlocked when the power goes out. Power is applied to lock the door. Fail safe refers to the status of the secure (key) side of the door.
This type of lock is commonly used for doors where building occupants might need to make a quick exit for emergencies, such as fire exit doors or doors to stairwells. When the power is on, these doors stay locked. You can think of these locks as ones that ensure that no one’s life will be in danger if a power outage occurs. (Keep in mind that fire exit doors are required to have fail-safe locks rather than fail-secure locks.)
NOTE: When deciding whether or not you should get fail-safe or fail-secure electronic or magnetic locks for your doors, you’ll need to consider a few factors. During a power outage, would a locked door endanger lives? Would an unlocked door put equipment at risk of theft? Remember that egress doors, such as fire exits, should have fail-safe locks, while most other doors in an office or building, such as the front office door or the door to the IT room, should have fail-secure locks. Another factor to consider is energy usage. Fail-safe locks need a constant supply of power in order to remain locked, which means they can cost more money to operate.