Glossary term
interlock
A lock and switch combined, so a door cannot be opened unless the car is there and the lift cannot move unless the door is locked.
What it means
An interlock does two jobs at once. Mechanically it holds a door shut. Electrically it tells the controller that the door is shut and locked. Both conditions have to be true before the lift will move, and the mechanical lock will only release when a car is present at that floor. It is the single most important safety device on a lift with doors.
Where you see it
On every landing door, on the car door, and on gates and seat swivels of home equipment. On a stairlift the equivalent is the swivel interlock that stops the chair travelling unless the seat is locked in position.
How it works
A hook or latch drops into a keeper to hold the door. A contact carried on the same assembly closes only when the hook is fully home. All those contacts are wired in series, so one open contact anywhere breaks the chain and the lift stands still.
The release is deliberately mechanical: a cam or vane on the arriving car physically lifts the hook. No electronics are involved in deciding whether it is safe to open, which is why the arrangement stays safe even when the control system has failed.
Why it matters
Interlock faults present as a lift that is powered but immobile. Because the contacts are in series, the fault can be at a floor nobody is using. That is why an engineer's first question about a dead lift is often whether any door in the building looks out of line.
Commonly confused with
An interlock is described as a lock, but it is two things at once: a mechanical lock and an electrical contact. It is also confused with the door operator. The operator moves the door; the interlock decides whether the door may be opened and whether the lift may move.
Wear, service and lifespan
Interlocks are safety-critical and are examined at every service and at thorough examination. Their rollers, springs and contacts wear, and contacts can burn or oxidise. An interlock that has to be forced, that rattles, or that lets a door be pushed open with the car away is a stop-use condition, not a snag list item.
Signs to watch for
- The lift has lights and a display but will not move.
- The car arrives but the doors do not release.
- You can hear a repeated clicking at the door.
- The lift works from some floors and not others.
- A door feels loose or rattles when pushed.
Troubleshooting interlock
The lift arrives but the door will not open
What it usually means: The interlock is not releasing, often because the car is not quite level
Safe check: Note how far off level the car sits and at which floor.
The lift will not move even with the doors visibly shut
What it usually means: A door is closed but not locked, so the safety circuit stays open
Safe check: Look for a small gap or a door that has stopped just short of the frame, and report the floor.
A landing door can be pushed or rattled open by hand
What it usually means: A failed interlock, which is a serious hazard
Safe check: Stop use immediately, keep people away from the doorway and report it as urgent.
How to report it
Name the floor and say whether the fault is opening or moving. If a landing door moves at all when the car is not there, say so first, because that is treated as a dangerous occurrence rather than a routine breakdown.
Ask us about interlock on your lift
Tick anything you can see or hear from where you normally stand, add a note in your own words, and we will put it into the message below for you. Never open, force or enter any part of the lift to check.
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Related terms
Definitions that usually come up alongside interlock when you are describing a fault.
- landing door
The door in the wall at each floor, locked shut until a lift car is present and level behind it.
Doors and entrances
- car door
The powered door on the lift car itself, driven by the door operator and responsible for opening the landing door as well.
Doors and entrances
- controller
The electronic brain that reads calls and safety signals, drives the motor and doors, and records faults.
Controls and electrics
- entrapment
A passenger stuck in a lift car between floors or behind closed doors, released only by trained responders.
Safety devices
- landing gate
A manually opened gate at the landing of a platform or goods lift, interlocked so it cannot be opened unless the platform is there.
Doors and entrances