A journey needs permission as well as a request.

A floor button asks the lift to make a journey. The safety arrangement establishes whether the protective conditions needed for that operation are present. A request cannot replace a missing permission.

In a conventional series safety circuit, several contacts form one electrical path. Each contact represents a specified condition. For normal travel, all the relevant contacts must be closed: one open contact breaks that path.

The contacts are part of the control arrangement; they do not carry the motor’s main power. The complete system uses the safety circuit to govern the equipment that permits or prevents movement.

Why a closed-looking door can prevent a start.

Follow a normal departure.

A passenger selects another floor. The doors close, but one landing door’s lock has not reached the condition that closes its safety contact. The stop-device and governor contacts in our example are closed.

That single open lock contact still interrupts the series path. The lift must not begin the normal journey, even though the call is registered and the doors look closed.

The missing lock indication explains which permission is absent. It does not, by itself, identify why: a mechanical locking problem, a contact problem or an interruption elsewhere in that part of the circuit can require different investigation.

One open contact breaks the path.

The same three-contact chain in two statesState A shows a closed door-lock contact, stop-device contact and governor contact in series, giving a continuous path to the permission stage. In state B the door-lock contact is open. The gap breaks the path even though the other two contacts remain closed. This is a conceptual fragment, not a complete wiring diagram.A · All three contacts closedB · Door-lock contact open12312344Continuous path · permission presentBroken path · normal start inhibited
  1. Door-lock contact: closed only when its specified locking condition is made.
  2. Stop-device contact: opens when that stopping device is operated.
  3. Governor contact: opens when its overspeed switching mechanism operates.
  4. Permission stage: receives the result of this chain. Permission from this chain is only one requirement for a journey.
A deliberately simplified series-contact example. Real lifts have additional devices, monitoring and installation-specific arrangements. The drawing shows the logical path only, not terminals, supply or return wiring.

Read series as “all these conditions together”. Closing two of the three contacts cannot compensate for the third being open. A closed chain also does not command the motor to run; the normal journey sequence and its other conditions still apply.

Safety electronics and ordinary signals have different roles.

Some lifts use suitable safety electronics for defined protective functions, rather than relying only on a long chain of mechanical contacts. These systems can evaluate safety-related inputs, monitor their own operation and control safety outputs.

The important distinction is the complete, engineered safety function. An ordinary position sensor connected to a standard software input does not become a safety device merely because the programme reads it. The sensor, signal path, processing and output arrangement must be appropriate for the function.

A diagnostic display can help identify the part of a circuit that changed. It does not replace that circuit’s protective action or prove every mechanical condition in the lift. If a condition changes during travel, the installation responds through its designed stopping function; the exact response depends on the condition and system.

A closer look

A contact chain is a useful first model, but a real protective system also has to deal with faults within its own components. Monitoring may check whether contactors, switches or safety outputs change state as expected. This is why a closed-path indication alone cannot establish the health of the whole safety function.

Safe Torque Off (STO), where used, prevents a drive from producing motor torque. It does not itself provide a holding force against gravity. Brake control and monitoring therefore remain separate parts of the lift’s overall protective design.

Check your understanding

Can ordinary movement logic simply ignore a missing safety permission to complete a call?

Reveal the explanation

No. Normal calls remain subject to the installation’s protective conditions. A journey request cannot turn an interrupted safety path into a valid permission.

Apply the idea

The call is registered. Can the car start?

The stop-device and governor contacts are closed, but the door-lock contact is open. What happens to the normal start permission, and why?

Show the reasoning

The normal start is inhibited. The contacts are in series, so the open door-lock contact breaks the path. The two closed contacts cannot complete it. This identifies an absent permission, not a definitive diagnosis of the cause.

Further reading.

Optional reading to explore this lesson in more detail.

  • NEW LIFT: FST controller manual (PDF)

    Legacy controller example (2003): section 3.1, printed page 10, distinguishes the safety-circuit, door and door-lock indications. These are examples of reported conditions, not a universal circuit layout.

  • BBH Products: SMX PESSRAL safety control

    Electronic safety example: compare the listed safety inputs, safety outputs and monitored functions with the conceptual contact chain. The page also distinguishes the connection to the standard lift controller.

  • Ziehl-Abegg: ZAdynpro manual (PDF)

    Technical detail: section 5.1 explains Safe Torque Off. Removing motor torque and holding a lift against gravity are different functions.