Intermittent Door Faults That Only Happen at Floors 5 to 8
A 12-stop geared traction elevator (1998) with a solid-state controller installed in 2014 logs intermittent faults two to four times a week. The log always shows the same thing - 'door open during travel' or 'gate switch open' - and the car stops at the next landing with the doors closed. Every event has the car at floors 5, 6, 7 or 8 when it happens; nothing has ever been recorded at the terminal landings. Events cluster in the afternoon. Twice the car stopped between floors and passengers were released after about 10 minutes.
Over three weeks a previous mechanic replaced the car door operator board, both car door interlock switches and one light curtain, then re-terminated the traveler cable connector at the car top. The failure rate did not change at all. The maintenance record shows the traveler cable is original and there is no record of the car-top junction box ever being opened.
The building has one elevator and a law firm as tenant; complaints are escalating and your supervisor wants the car running today.
1. What does the fault pattern tell you, and why did replacing the door components change nothing?
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The only variable that correlates with the fault is car position. Not door operation, not time of day, not temperature. Four adjacent floors span roughly one region of the hoistway, and at any car position there is one fixed point on the traveler cable that sits at maximum bend - typically where the cable leaves the car-top attachment or passes its first support. A conductor that is broken except for a few strands will open only when that specific section of cable is forced around that bend radius, which happens when the car sits at floors 5 to 8.
Nothing in the door operator, the interlocks or the light curtain is position-dependent in that way, which is why three weeks of parts replacement did not move the failure rate. The afternoon clustering is probably traffic, not physics: more runs means more passes through the fault zone. And the message itself is the giveaway - 'gate switch open' while the doors are closed means the controller is losing the circuit, not observing a real door condition. When the fault message contradicts the physical state of the door, suspect the wiring, not the door.
2. Describe the isolation test and what you would actually measure.
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Identify from the wiring diagram exactly which traveler conductors carry the door interlock and gate-switch circuit, then prove which ones by continuity from the car-top junction box to the controller terminals with both ends disconnected. Do not rely on a plain ohmmeter: a circuit hanging by two strands reads a few ohms and looks perfect.
Load the circuit instead. Put a battery and lamp across it, or read voltage across it while it carries its normal control current, and have an assistant jog the car through floors 5 to 8, stopping at each, while you watch the meter. A reading that jumps from zero to full supply and back as the car moves is your conductor. Confirm by opening the car-top junction box and flexing each conductor individually at the attachment point. If the fault will not reproduce on demand, leave a logging meter or chart recorder on the circuit overnight and read the timestamps against the controller's event log, so the next visit is diagnostic rather than exploratory.
3. What is the interim operating decision, and how do you handle the customer and your supervisor?
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A door interlock or gate-switch circuit is a safety circuit. A broken conductor in it is functionally a broken interlock, and the car can start or stop with the door circuit in an unknown state. There is no 'run it carefully' option and no jumping out the circuit, because that removes the protection that keeps the car from moving with a door open. The car stays out of service until the traveler is replaced.
Plan the work as one block: park the car to give slack at the car top, replace the cable, re-terminate at both ends, then prove every affected circuit by test before returning the car. Tell the tenant in writing what is wrong, why the car cannot run, and a realistic window. Report to your supervisor in terms of risk, not inconvenience, and put the diagnosis and the tests in the maintenance record so the next mechanic does not repeat the parts replacement. If the supervisor insists on running it, get the instruction documented; the safety circuit is not a place to absorb schedule pressure.