Mortise Fail Secure Versus Fail Safe After Replace Decision Tree

Why this matters

An electric mortise lock or electric strike was replaced and now the door behaves the opposite of what the building requires. The replacement reversed the fail mode, the wiring polarity, or the lock function code, and the result is either a security exposure (door unlocked when it should be locked during a power loss) or a life-safety exposure (door locked when it should be free egress during a fire alarm). The decision tree below resolves which of those two scenarios is in play, what corrected, and how to verify before the building reopens. Getting this wrong on a commercial entry or a stairwell door creates legal exposure under NFPA 101 Life Safety Code and the local fire marshal's authority.

Symptom presentation

The complaint usually arrives within hours of the replacement when the facility manager realizes the door behaves differently. Get the door type, the building occupancy classification (school, office, healthcare, multifamily, etc.), and the door's function in the egress plan. Fire-rated doors, stairwell doors, and required exit doors have specific code requirements that override locksmith preference.

Test the door at the point of failure. Cut power to the lock with the building manager's supervision. If the door becomes unlocked (free pull from either side), the lock is fail-safe. If the door remains locked from the outside but free egress from the inside, the lock is fail-secure. If the door becomes locked from both sides (no egress), the wiring is wrong and the door is creating an immediate life-safety hazard.

Read the lock body. Most mortise locks are stamped with the function code (Schlage L9000 series, Best 35H, Yale 8800) and the function code defines the lock's behavior. Common functions include 24 (storeroom, fail secure), 80 (storeroom, fail safe), and other lettered or numbered variants per manufacturer.

Quick checks at the door

Verify the door's required behavior in the egress plan. Stairwell doors must allow free egress from inside even when locked from outside (NFPA 101 stair re-entry requirements). Stairwell re-entry doors must be unlocked from the stair side on every fourth floor minimum. Required exit doors must allow free egress at all times under normal operation.

Test free egress on the interior side. Pull the lever or push the bar. The door should open without a key, without a credential, and without electrical power applied. If free egress requires power, the install does not meet NFPA 101 and must be corrected before the building reopens.

Check the power supply polarity. Many electric strikes and locks are polarity-sensitive; reversing polarity reverses the fail mode on some manufacturer products. Read the manufacturer's wiring diagram and confirm polarity at the strike terminals.

Isolation tree

Branch 1 - door is fail-secure but the egress plan requires fail-safe. Most commonly happens when a stairwell exit lock or a smoke-compartment door is replaced with a storeroom (fail-secure) function instead of the original storeroom-by-power (fail-safe) function. The remedy is to swap the lock body or the function module to the fail-safe variant. Some major brands (Schlage, Best) allow function changes on the same chassis via a kit; others require a different chassis.

Branch 2 - door is fail-safe but the security plan requires fail-secure. Most commonly happens on a commercial entry replaced as a "stairwell" function. The door now stands wide open on every power loss, which is unacceptable. Swap to the fail-secure variant.

Branch 3 - door has lost free egress. This is the highest-severity scenario. A door that locks the occupants in is illegal under NFPA 101 and creates immediate civil and criminal exposure. Open the door, restore free egress (usually by removing a wiring jumper that has reversed the function, or by removing a deadbolt that was added on top of the electrified lock), and document the correction. The fire marshal's office will want a record if this was reported.

Branch 4 - door behaves correctly under steady power but fails the wrong way during a brownout or low-voltage condition. The power supply is undersized or marginal. Read the inrush current spec on the lock; mortise locks and electric strikes pull substantial current during the unlock pulse (typically 0.5 to 1.5 amps at 24 VDC for a brief pulse). A supply rated below the inrush will sag, which can either fail-open or fail-locked depending on the lock model. Upsize the supply or add a local boost capacitor per the manufacturer's spec.

Branch 5 - door behaves correctly mechanically but is wired to the wrong access control output. The access controller is dropping power when it should be applying it, or vice versa. Trace the wiring from the controller to the lock; confirm the controller output mode (energized-to-lock vs energized-to-unlock) matches the lock's fail mode. Reconfigure the controller output or swap the wiring at the relay.

Confirming diagnosis

The power-off test is the most definitive check. Cut power to the lock, test the door from both sides, and observe the behavior. The behavior under power-loss is the lock's fail mode. Test free egress under power-off and under power-on; both should allow egress on doors required to be free.

For wiring confirmation, use a multimeter at the lock terminals. Verify polarity, voltage, and the access control output state matches the manufacturer's wiring diagram. A reversed polarity on a polarity-sensitive lock will appear to work in some scenarios and fail in others; the meter reading at the terminals is the source of truth.

For function-code confirmation, read the manufacturer's catalog. Each function code has a specific intended use; using a storeroom function on a stairwell door is a configuration error even if the wiring is correct.

Remediation

For function-code mismatches, swap the lock body or the function module per the manufacturer's kit. For polarity errors, swap the wiring at the terminals after verifying with the manufacturer's diagram. For power supply problems, upsize the supply or add a boost capacitor.

References

• NFPA 101 Life Safety Code - the standard governing free egress, stairwell re-entry, and door behavior under fire alarm and power loss.
• IBC International Building Code Chapter 10 Means of Egress - referenced for egress door requirements in commercial buildings.
• ANSI/BHMA A156.13 American National Standard for Mortise Locks and Latches Series 1000 - the standard governing mortise lock function codes.
• ANSI/BHMA A156.25 American National Standard for Electrified Locking Devices - the standard for electric strikes and electrified mortise locks.
• UL 1034 Standard for Burglary-Resistant Electric Locking Mechanisms - the security listing referenced for commercial electric locks.