Automatic Door Lock: Types, Modes and Fail-Safe Options

An automatic door lock is what turns a powered glass entrance from an open doorway into a secure one. During trading hours the door opens for everyone who approaches. After hours it has to stay shut, resist being forced, and still let the right people in and everyone out safely. The automatic door lock, the operating modes and the access control together decide how well an entrance does that.

This guide explains the types of automatic door lock used on commercial sliding and swing doors, how locking works with the operator’s modes, the difference between fail-safe and fail-secure, what happens during load shedding, why a lock stops working, and what to decide before you specify one. It uses our MC-125 sliding door operator and MCW-100 swing door operator, which both support an electric lock and access control.

What is an automatic door lock?

On a commercial entrance, an automatic door lock is a lock that the door’s control system can engage and release by itself, rather than one that somebody has to turn with a key every time. It is usually electrically operated and wired to the operator, so the door locks when it is switched to its locked mode and unlocks when it is switched back to automatic, when a valid card is presented, or when the access-control schedule says the building is open.

This is different from the automatic door lock on a car or a smart lock on a front door at home. We deal with pedestrian entrances on commercial buildings, where the lock has to work with a powered leaf, safety sensing, an escape route and, often, a fire strategy.

Types of automatic door lock

The right automatic door lock depends on whether the door slides or swings, how secure the entrance needs to be, and what must happen when the power fails.

• Electromechanical lock on a sliding door. Fitted at the operator, it holds the drive or the leaf hangers so the leaves cannot be pushed open. It is the usual automatic door lock on a sliding entrance and is controlled by the operator.
• Leaf locks on framed sliding doors. Hook or bolt locks in the meeting stiles or bottom rail, which hold the leaves to each other or to the floor. These add security low down, where a leaf is easiest to lever.
• Electric strike on a swing door. Replaces the keep in the frame. When it releases, the latch can pass and the operator can push the door open.
• Magnetic lock on a swing door. An electromagnet on the frame holds a steel plate on the leaf. Strong and simple, but it needs power to stay locked.
• Manual key locks. A key-operated lock on the leaf, used on their own on simple entrances or alongside an electric lock as a second layer at night.

How the lock works with the door’s modes

An automatic door lock is only half of the story. The other half is the operating mode the door is in. The MC-125 has four operating modes:

• Automatic: the door opens for anyone approaching and closes behind them. The lock is released.
• Hold open: the door stays open, for deliveries or a busy period.
• Half open: the door opens to part of its width, reducing heating and cooling losses on quiet days.
• Locked: the door closes and the automatic door lock engages. Only an authorised input, such as access control or a key switch, opens it.

The mode is usually chosen with a key switch or mode selector, often at reception, or set automatically by the access-control schedule. Our guide to the automatic door push button explains how key switches and buttons are wired to the operator.

Fail-safe or fail-secure

Every automatic door lock has to do something when the power fails, and there are only two choices.

• Fail-safe: the lock releases when it loses power. People can always get out, but the entrance is unlocked during an outage unless something else secures it.
• Fail-secure: the lock stays locked when it loses power. The entrance stays secure, but people must still be able to get out by another means.

Neither is right everywhere. On a door that is part of an escape route, the fire strategy decides how the lock must behave, and that decision is made by the fire consultant or the local authority rather than by the door supplier. On a door that is not on an escape route, the choice is about security and how the building is used after hours. We set the lock to whatever the building’s strategy requires and demonstrate it at handover.

Automatic door lock and access control

Most automatic door lock installations are tied into access control. The MC-125 supports an electric lock with multiple access-control inputs, and the MCW-100 has an access-control input and an electric-lock connection. That allows:

• card readers, keypads, remotes and intercoms to release the lock and open the door
• a schedule that unlocks the entrance in trading hours and locks it outside them
• different rules for entry and exit, so staff can leave freely while entry is controlled
• interlocking between two doors in an airlock or security lobby

Access control and locking should be decided before the header goes in, because they change the wiring. Our automatic door access control page covers the options, and our guide to the door interlocking system explains airlocks.

Automatic door lock on swing doors

Swing doors need the lock and the operator to work in the right order. When someone presents a card, the electric strike or magnetic lock has to release first, and only then can the operator push the leaf. If the operator pushes before the lock has released, the door strains against the lock, the operator reports a fault and the lock wears quickly. That sequence is set at commissioning and tested with every input.

Magnetic locks are fail-safe by nature, because they need power to hold. Electric strikes can be either. Which suits a particular door depends on the escape route, the security needed and the frame. See the MCW-100 swing door operator page for its inputs and limits.

Exit at night

A locked entrance still has to let people out. Cleaners, security staff and anyone working late must be able to leave without a key, and on an escape route free exit is a requirement set by the fire strategy rather than a convenience. The usual arrangement is an exit device on the inside, such as a push button, a sensor or a release, that unlocks and opens the door for people leaving while entry stays controlled. Test it at handover from the inside with the door in locked mode.

Locking during load shedding

Load shedding is where an automatic door lock is really tested. When the mains drops, the lock and the operator both lose power unless something keeps them running. There are three ways to handle it.

• Battery backup on the MC-125 keeps the door and its lock working for a period, so the entrance behaves normally.
• An inverter or generator circuit keeps the door running for longer, provided the lock and the operator are on the same supply.
• A defined fail state, fail-safe or fail-secure, chosen for the building, with a manual release that staff have been shown.

The common mistake is having the lock on one circuit and the operator on another, so one keeps power while the other loses it. Our guide to automatic doors and load shedding covers the choices.

How secure is an automatic door lock?

An automatic glass entrance is not a security barrier, and no automatic door lock changes that. What good locking does is make the entrance a credible deterrent and a meaningful delay. The weak points are rarely the lock itself. They are usually a leaf that can be levered at the bottom, glass that falls out when broken, or a lock that is not engaging because the door has dropped on its track.

Locking low down on framed leaves, laminated glass and a well-maintained track do more for security than a stronger lock at the top alone. Our guide to automatic door security covers the layers behind the entrance.

Why an automatic door lock stops working

When an automatic door lock misbehaves, the cause is usually one of a few things. Check these before calling, and tell us what you found.

• The mode. Is the door actually in locked mode? A key switch left in the wrong position is the most common cause.
• The schedule. Has the access-control schedule changed, or is the clock wrong after a power cut?
• Alignment. If the leaves no longer meet where they should, the lock cannot engage. Worn rollers or a dropped leaf are often the real fault. See our guide to the automatic door track.
• Power. Has the lock lost its supply, or has a battery reached the end of its life?
• Access control. Is the reader or panel faulty, rather than the lock?
• The lock itself. Locks wear like any moving part, and a lock that clicks but does not hold needs attention.

Do not force a locked door or bypass the lock to get through the night. Switch it off at its isolator if necessary and call us. See book a technician or our guide to an automatic door not closing.

Adding a lock to an existing automatic door

Many older automatic entrances were installed without an electric lock and are secured at night with a key lock or a chain. An automatic door lock can often be added to an existing operator, or fitted as part of replacing the operator. We look at the leaves, the header, the existing wiring and the access control at a site survey, then quote the lock, the wiring and any work on the leaves. On some older doors, replacing the operator and adding the lock together is the more sensible route. See automatic door modernisation.

Automatic door lock by building type

• Shops: locked mode at closing, often with a manual key lock as a second layer and laminated glass. See automatic doors for shops.
• Office receptions: unlocked in business hours, card access after hours, free exit for staff.
• Clinics and pharmacies: controlled entry, sometimes with a staff-released door during quiet hours. See pharmacy automatic doors.
• Hospitals: locking set per zone and agreed with the fire strategy.
• Security lobbies: two locked doors interlocked, with an emergency override at each.

Key switches and who holds the keys

The key switch or mode selector is the part of the system that decides whether the entrance is open or shut, so who holds its keys matters. On most sites the keys sit with the manager or security, and the switch is mounted inside, out of reach of the public. Lost keys should be treated like any other lost building key. Where the schedule is run by access control instead, the key switch becomes an override for holidays, stock-takes and emergencies, and it is worth writing down who is allowed to use it.

Bi-parting and telescopic doors

Bi-parting doors have two moving leaves meeting in the middle, so the lock has to hold both. The operator lock holds the drive, and leaf locks at the meeting stiles or into the floor hold the leaves to each other low down, where a pair is easiest to lever apart.

Telescopic doors have several leaves on each side moving at different speeds, and every one of them has to be held when the door is shut. On both, the locking arrangement is part of the leaf design, so it is decided with the glass and frame rather than added later. See telescopic sliding doors.

What to put in the specification

Write the automatic door lock into the specification rather than leaving it to the installer. State the lock type and where it engages, fail-safe or fail-secure, who supplies the access control and how it connects, what happens on power failure and on fire alarm, the after-hours schedule, and how staff release the door manually. Contradictions between the door, access-control and fire specifications are the most common gap we find. Our automatic door specification guide shows how to set this out.

Commissioning and servicing the lock

At commissioning we test the automatic door lock in every mode and with every input: each reader, the key switch, the schedule, the fire link where there is one, and a power cut with and without the battery. Results are written down at handover. On every service we check that the lock engages fully, that the leaves meet where they should, and that the battery still carries the lock through an outage. Our standard service interval is six months, more often on busy entrances. See automatic door servicing.

Automatic door lock: quick answers

• Automatic door lock systems engage and release under the door’s own control.
• Automatic door lock types include electromechanical, leaf, electric strike and magnetic locks.
• Automatic door lock behaviour follows the mode: automatic, hold open, half open or locked.
• Automatic door lock power-failure behaviour is fail-safe or fail-secure.
• Automatic door lock settings on escape routes are decided by the fire strategy.
• Automatic door lock faults often start with a dropped leaf or a wrong mode.
• Automatic door lock wiring should be planned before the header goes in.

Automatic door lock questions

Need an entrance that is open by day and secure by night? Request a quote with photographs of the door and tell us how the building is used after hours, and we will recommend the right automatic door lock and access control. Requirements for powered pedestrian doors are published by the South African Bureau of Standards. We are on 010 412 0062, Monday to Thursday 08:00 to 16:30 and Friday 08:00 to 14:00.