Wire Loop on the 18/F Landing Interlock
At a 30-storey commercial site in Kwun Tong, the operator is taking a materials cage down from 22/F when he notices that the landing gate at 18/F is held closed by a loop of tie wire around the interlock switch. The cage can be stopped about 500 mm below the 18/F floor and the gate can still be pushed open. The foreman says the wire was fitted 'only for unloading tiles' and will be removed on Friday. The cage door's mechanical lock also feels loose, and the interlock indicator lamp on the cage control panel stays green even when the gate is open. The hoist is needed for a concrete pour that starts in 30 minutes.
1. What immediate action should the operator take, and why is simply removing the wire and continuing not acceptable?
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The operator should stop the hoist at a safe landing, keep the cage gate closed, and take the hoist out of service. He should not open the 18/F gate or use that landing. He should isolate the control and apply a lock and tag, then report to the site agent and safety officer. Removing the wire and continuing is not acceptable because the wire is evidence of a defeated safety device, and the underlying switch may be damaged, misaligned or deliberately bypassed. The loose cage door lock and the false green indicator lamp show that the interlock circuit is not trustworthy. If the operator simply pulls the wire off, the gate may appear normal while the switch remains faulty. The correct response is to treat the whole interlock circuit as defective until a competent person repairs and tests it. The concrete pour is not a justification: a pour can be rescheduled or served by another hoist, but a fall into the shaft or movement with a gate open can kill.
2. Explain the safety functions that this interlock defeat has removed, and how the interlock should be tested after repair.
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A landing gate interlock normally does two things. It prevents the gate from being opened unless the cage is at that landing, and it prevents the hoist from moving unless every landing gate and the cage gate are closed and locked. The wire loop holds the switch in the 'closed' position, so the control system can believe the gate is secure even when it is open. That creates two hazards: a person can open the gate at the 18/F landing when the cage is 500 mm below floor level and fall into the shaft, and the cage can be started with a gate open. The false green lamp makes the defect harder to see. After repair, the interlock should be tested with the cage empty. With the cage stopped about 300 to 500 mm off the landing, an attempt to open the gate should fail. With the gate open, an attempt to start the hoist should fail. The same checks should be repeated at every landing, and the cage door lock should be checked. The test should be recorded, and the hoist should not carry persons until all gates prove correct.
3. What reporting and return-to-service steps should follow, and what should the operator refuse to do?
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The operator should report the defeated interlock, the loose cage door lock and the false indicator to his employer and the site safety officer in writing. He should record the location, the time, the foreman's explanation and the fact that the hoist was isolated. Photographs should be taken before the wire is disturbed. The landing should be barricaded so that no one uses the 18/F gate. Return to service should require repair by a competent person, replacement of any damaged switch or lock, and a full interlock function test at every landing. The repair should be entered in the log book. The operator should refuse to reset the alarm or run the hoist on the foreman's verbal assurance. He should also refuse to allow a helper to hold the gate closed while he moves the cage; that is another form of bypass. If the hoist is put back into use without repair, he should escalate to the principal contractor and consider reporting the matter to the Labour Department.