Fail safe and fail secure describe what a lock or device does when it loses power. A fail safe lock unlocks when power is cut, prioritizing free movement and evacuation. A fail secure lock stays locked when power is cut, prioritizing protection of whatever is behind the door. The distinction comes down to a single question: when something goes wrong, should the default state favor safety or security?
How Fail Safe Works
A fail safe device needs continuous electrical power to stay locked. The moment power is interrupted, whether from an outage, a fire alarm, or a system malfunction, the lock releases and the door can be opened freely. The logic is simple: if the system fails, people can get out.
Electromagnetic locks (often called mag locks) are the most common fail safe hardware. They work by running electricity through a coil to generate a magnetic field that holds an armature plate on the door. The magnetic field creates a strong hold, but it depends entirely on power. Cut the electricity and the field collapses instantly, releasing the door. There is no mechanical latch involved, so without power, nothing physically keeps the door closed.
This makes mag locks inherently fail safe by design. You cannot configure a mag lock to be fail secure because the locking mechanism itself is the electromagnetic field. No power, no field, no lock. If you need a mag lock to remain secure during brief outages, a battery backup or uninterruptible power supply (UPS) is required to keep electricity flowing to the coil.
How Fail Secure Works
A fail secure device takes the opposite approach. It stays locked when power is removed, and power is applied to unlock it. If the building loses electricity, the door remains locked from the outside, keeping unauthorized people out.
Electric strikes are the most common fail secure hardware. An electric strike replaces the standard strike plate in a door frame. When the system sends power to the strike, it releases and allows the door to open. When power is cut, the strike stays in its default locked position, and the door’s latchbolt holds firm against the frame.
The important detail most people miss: fail secure does not mean people are trapped inside. The lock or panic hardware on the inside of the door still works mechanically. You can turn the lever or push the panic bar to retract the latchbolt and exit, regardless of whether the electric strike has power. Fail secure controls entry from the outside, not exit from the inside. In most cases, a physical key can also override the lock from the secure side if manual access is needed.
When Each Type Is Used
The choice between fail safe and fail secure depends on what matters more for a given doorway: letting people through quickly in an emergency, or keeping a space locked down no matter what.
Fail safe locks are used where rapid, unobstructed evacuation is the top concern. Emergency exits, hospital corridors, and public assembly spaces often use electromagnetic locks because they guarantee the door opens the instant a fire alarm triggers or power drops. The trade-off is obvious: every power failure is also a security breach unless backup batteries are in place.
Fail secure locks are the default recommendation for most doors, especially those protecting high-value or hazardous areas. Research labs, biocontainment facilities, data centers, server rooms, and any space where unauthorized access could be dangerous all call for fail secure hardware. According to NIH guidelines, all doors serving high-value or hazardous areas should be fail secure while still meeting egress requirements. The only exception is doors where quick entrance (not just exit) is needed during emergencies.
Fire-rated doors add another layer. NFPA 80, the standard for fire doors, requires fail secure electric strikes on fire-rated openings. Fail safe strikes don’t provide the positive latching that fire doors need to function as barriers during a fire. This is why stairwell re-entry doors in commercial buildings use fail secure strikes rather than mag locks.
Fail Safe and Fail Secure Beyond Door Locks
These concepts extend well beyond building access control. In industrial settings, valves use the same logic. A valve can be designed to fail open (releasing pressure when control signals are lost), fail closed (sealing off flow during a malfunction), or fail in place (holding its last position). The actuator’s design and mounting determine which behavior occurs. You’ll sometimes see these marked “FO,” “FC,” or “FIP” directly on the valve.
The principle is the same everywhere it appears: you decide in advance what the safest or most secure default state is, and you design the hardware so that losing power or signal automatically puts the system into that state. In a chemical plant, a fail-closed valve on a toxic gas line prevents a leak. In a building, a fail safe lock on an emergency exit prevents people from being trapped. The “right” choice always depends on what the worst-case scenario looks like for that specific application.
The Role of Backup Power
Because fail safe locks unlock during any power interruption, facilities that use them almost always pair them with battery backup systems. A UPS or dedicated backup battery keeps the electromagnetic field energized through short outages, preventing the security gap that would otherwise occur every time the lights flicker.
Fail secure locks are less vulnerable to power issues since their default state is locked. But they still need power to grant authorized access, so a prolonged outage means no one gets in through the electronic system without a physical key override. Access control power supplies often include slots for multiple backup batteries to keep the system responsive during extended outages.
Choosing Between the Two
The decision comes down to one question: what is the greater risk for this specific door? If a power failure locks people in a dangerous situation, fail safe is the answer. If a power failure lets unauthorized people into a dangerous or valuable space, fail secure is the answer.
Most commercial buildings use a mix. Exterior doors and restricted interior spaces are typically fail secure. Emergency exits and areas requiring immediate evacuation access use fail safe electromagnetic locks paired with release devices like panic bars with built-in switches, request-to-exit buttons, or fire alarm triggers. Specialty environments like laboratories, clean rooms, and containment areas need individual assessment to balance safety, security, and containment requirements, since a single building can have doors where the priorities directly conflict.