Lesson 4 of 8How lifts work
Lift doors and interlocks.
Lift doors open and close at every stop, and the lift cannot move until every one of them is proved shut and locked. Here is how the car door and landing doors work together, and how they keep people safe.
Two sets of doors.
Every lift has two kinds of door. The car door travels with the car. The landing doors, one at each floor, close off the shaft. When the car stops at a floor, the two line up and open together, so it looks like a single door.
Only the car door has a motor. The landing doors are moved by the car door, and they lock themselves whenever the car is somewhere else. That arrangement means one door operator does the work at every floor, and every landing door stays locked unless the car is there to open it.
Explore the door system.
The diagram shows a centre-opening door twice: the car door seen from the landing, and a landing door seen from inside the shaft. Select a number or a part to see what it does.
Door operator
The door operator is a motor unit on top of the car door. It drives a belt or linkage that slides the door panels open and closed. Modern operators control the speed throughout the movement, slowing the panels at each end, and can be adjusted for the weight of the doors and how busy the lift is.
Car door hangers and track
Each door panel hangs from a hanger with rollers that run along a track above the doorway. Worn rollers or a dirty track make the doors noisy and uneven, and can make them hesitate or reopen.
Car door contact
A contact proves the car door is fully closed. It is part of the safety circuit, so the lift cannot run with the car door open. Where the gap between the car and the shaft wall is wider than the standard allows, usually 0.15 m, the car door must also be locked whenever the car is away from a floor.
Coupler
The coupler is a pair of vanes on the car door. When the car stops at a floor and the car door starts to open, the vanes close on two rollers on the landing door lock. That unlocks the landing door and carries it along, so the two doors move together.
Light curtain
A light curtain has two strips on the leading edges of the doorway, one sending infrared beams across the opening and one receiving them. If anything breaks a beam while the doors are closing, the doors reopen. Older lifts may have a mechanical safety edge instead, which reopens the doors when it is pressed.
Sills
Each door panel has guide shoes that run in a grooved sill at the bottom of the doorway: one sill on the car and one at each landing. Grit, coins and litter in the sill groove are among the most common causes of door faults.
Landing door lock
The interlock holds the landing door shut whenever the car is not at that floor. Its hook must be engaged by at least 7 mm before its electrical contact can be made, and that contact is part of the safety circuit. So the lift cannot move unless every landing door is closed and locked.
Pick-up rollers
Two rollers on the lock lever face into the shaft. When the car is at the floor, the car door coupler closes on them, lifting the lock and taking hold of the landing door. When the car is elsewhere, nothing touches them and the door stays locked.
Emergency unlocking
A triangular keyhole on the landing side of the door is linked to the lock. It lets a trained person unlock the door from the landing with a special key, for rescue or maintenance. Opening a landing door when the car is not there exposes an open shaft, which is why the key must only ever be used by trained, authorised people.
Landing door hangers and track
Like the car door, each landing door panel hangs from rollers on a track. Landing doors are often damaged by trolleys, bins and furniture, and a bent panel or track can stop the door closing or locking properly.
Door closer
Every landing door moved by the car door has a closer: a weight, often hanging in a tube, or a spring. If the landing door is opened when the car is not there, the closer shuts it again and the lock re-engages.
Interlocks and the safety circuit.
The landing door locks are among the most important safety devices on a lift. Without them, a landing door could be opened onto an empty shaft, or the car could move while someone was stepping in.
Each lock does two jobs. Mechanically, it holds the door shut. Electrically, its contact is wired into the safety circuit together with the car door contact and every other safety switch. On a lift built to BS EN 81-20, the contact can only close once the locking hook has engaged by at least 7 mm, so a door that is closed but not locked still keeps the lift out of action.
The law
The Lifts Regulations 2016 require landing doors to be fitted with interlocks that prevent the lift moving unless the doors are closed and locked, and treat devices for locking landing doors as safety components.
The lock works the other way too. A landing door can only be opened by the car door when the car is within a short distance of that floor, called the unlocking zone. Anywhere else, the coupler cannot reach the pick-up rollers and the door stays locked.
BS EN 81-20 limits the unlocking zone to 0.20 m above and below the floor, or 0.35 m where the car door and landing door are moved together mechanically, as with the coupler described above. The car door is restricted too: while the car is moving, it takes more than 50 N to open it, and away from the unlocking zone it must not open more than 50 mm even when pulled with 1000 N. That is why nobody should ever try to force lift doors open between floors.
Protecting people in the doorway.
Lift doors close automatically, so they are designed to avoid hurting anyone in the way. On a lift built to BS EN 81-20, four things work together:
- Limited energy: the moving doors may carry no more than 10 joules of kinetic energy at their average closing speed.
- Limited force: it must take no more than 150 N to stop the closing doors, apart from the first third of their travel, and stopping them makes them reopen.
- Reopening: a light curtain covering the doorway from 25 mm to 1600 mm above the sill reopens the doors if it detects anything 50 mm or more across. Older lifts may have a safety edge instead.
- A door open button: inside the car, it reopens automatic doors while the car is at a floor.
The doors also stay open for an adjustable time, which can be set longer for buildings where people need more time.
If something keeps blocking the doorway, some lifts switch the light curtain off after a set time and close the doors slowly with a warning sound. The standard then limits the doors to 4 joules. This stops one blocked door holding up everyone waiting on other floors.
Types of lift door.
| Type | How it opens | Where you see it |
|---|---|---|
| Centre-opening | Two panels part in the middle | Most passenger lifts; quick to open and close |
| Side-opening | Two or more panels slide to one side, one behind the other | Narrower shafts, goods lifts and wide openings |
| Swing landing door | A hinged landing door, opened by hand, with a sliding or folding car door | Some small and older lifts, and many platform lifts |
Some very old lifts still have collapsible lattice gates on the car or at the landings. If a lift you look after has them, ask your maintenance provider about the risks and the options for replacing them.
Why doors cause so many faults.
Doors move more than any other part of a lift: they open and close at every stop, many times a day. They are also the part passengers touch, push, kick and hit with trolleys. Our engineers treat them as the busiest and most fault-prone part of any lift.
Common door problems include:
- debris in the sill groove stopping the doors from closing fully;
- a dirty, damaged or misaligned light curtain making the doors reopen;
- worn hanger rollers or guide shoes making the doors noisy, slow or uneven;
- a landing door knocked out of line, so its lock does not engage and the lift stops;
- a door operator out of adjustment, so the doors slam or hesitate.
A proper maintenance visit cleans, checks and adjusts all of this. Our guide to lift faults covers the symptoms of door problems and what engineers check.
Engineer’s note
Building managers can prevent a lot of door faults: keep sills clear, stop people wedging doors open, protect landing doors during moves and deliveries, and report doors that bounce, stick or close hard as soon as they start.
Safety first
Never open lift doors with a release key, enter a lift shaft or pit, or try to free trapped passengers unless you are trained and authorised to do so. If someone is trapped, use the alarm or call your maintenance provider. If anyone is in danger, call 999.
Check your understanding.
Three quick questions on this lesson. Choose an answer to check it, or open the answer below each question.
-
What opens a landing door when the car arrives at a floor?
Show the answer
B. The coupler on the car door, which unlocks the landing door and moves it. Landing doors have no motor of their own. The coupler on the car door engages the rollers on the landing door lock, unlocks it and carries the landing door with the car door.
-
Why can one landing door stop the whole lift?
Show the answer
A. Its lock contact is part of the safety circuit, which must be complete for the lift to move. Every landing door lock has an electrical contact wired into the safety circuit. If any one is not made, the circuit is open and the lift cannot run.
-
What makes a landing door close when the car leaves?
Show the answer
B. A closer, such as a weight or spring, on the landing door. Each landing door has its own closer, so it shuts and locks whenever the car is not at that floor.
Sources.
The primary sources for this lesson. We checked them when the lesson was last updated on 18 October 2026. How we research and check our lessons.
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Related reading
Doors giving trouble?
Talk to an engineer about doors that bounce, stick, close hard or keep taking the lift out of service.
Common questions.
Quick answers to common questions about this topic.
How do lift doors know when to open?
The controller tells the door operator on the car to open the doors once the car has stopped level at a floor. The coupler on the car door then unlocks the landing door and carries it open with the car door.
Why do lift doors keep reopening?
Usually because something is breaking the light curtain beams or the door is meeting resistance as it closes. Common causes include debris in the sill track, a dirty or misaligned light curtain, worn door rollers and people holding the doors. If it keeps happening, report it to your maintenance provider.
What is a lift door interlock?
An interlock is the lock on a landing door. It combines a mechanical lock with an electrical contact in the lift’s safety circuit, so the lift cannot move unless the door is closed and locked, and the door cannot be opened from the landing unless the car is there.
Can a lift door close on someone?
Lift doors are designed to limit the force and energy with which they close, and a light curtain or safety edge makes them reopen if something is in the way. Faults can still happen, so doors that close hard or do not reopen should be reported straight away.