Intermittent Call Cancellation in a Bank Elevator
A 12-stop traction elevator in a bank building has an intermittent complaint: during the morning rush, a registered car call sometimes cancels and the doors reopen just as the car starts from the lobby. It happens two to four times a week and has not been duplicated on a quiet afternoon. The controller log shows a door close limit fault and a safety-edge active event at the same timestamp; once it also logged a car door interlock fault. With the car parked, voltage at the door operator terminals is 120 VAC. During a run, the same measurement drops to about 92 VAC and fluctuates. Insulation resistance from one traveling-cable conductor to ground is 0.2 megohm with the car at floor 7, but greater than 50 megohm at the lobby. The door edge itself tests good when the car is stationary. Building staff want the door edge bypassed to stop nuisance reopenings.
1. What is the likely fault location, and what evidence supports it?
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The likely fault is in the traveling cable, not in the door edge. Three pieces of evidence point that way. First, the voltage at the door operator is normal when the car is parked but drops to about 92 VAC during travel, which indicates a high-resistance or intermittently open conductor in the moving cable. Second, insulation resistance to ground is 0.2 megohm at floor 7 but greater than 50 megohm at the lobby; a position-dependent insulation value is characteristic of a cable damaged at a flex point. Third, the controller logs several unrelated door-circuit faults at the same time. A single failed safety edge would not normally produce a door close limit fault and a car door interlock fault. The edge tests good stationary, and the complaint occurs when the car starts and the cable flexes.
2. What safe test sequence would you use to confirm and isolate the fault?
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Take the car out of service, isolate the mainline, lock and tag it, and verify zero energy at the door operator and controller terminals. Do not use an insulation tester on live circuits or on connected electronic boards. Disconnect the traveling cable at both ends as required, then test conductor-to-conductor and conductor-to-ground insulation with the correct instrument and test voltage. Flex the cable at its supports and at the points where it enters the car and the hoistway while watching the readings; a fault that changes with position will often reveal itself. Inspect the cable jacket, strain relief, and guide for abrasion, crushed areas, or broken strands. Also check the door operator terminals and the car station for loose connections. Record readings at the lobby, mid-hoistway, and floor 7.
3. Why is bypassing the door edge unacceptable, and what permanent repair is required?
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Bypassing the door edge is unacceptable because it defeats a safety device intended to stop or reopen the doors when a passenger or object is in the closing path. It is also likely a code violation and would leave the building with a concealed hazard. The intermittent faults are not caused by the edge, so a bypass would not even solve the complaint; it would only remove protection while the real traveling-cable fault continues to cause call cancellations and door faults.
The permanent repair is to replace the damaged traveling cable, or the affected conductors if the cable design and code allow a proper repair, and to restore the strain relief and supports. After replacement, repeat insulation and continuity tests, inspect the door edge and door operator, and run the car through the full travel with doors operating to confirm that the faults no longer occur. Document the before-and-after readings.