Safety devices, interlocks and emergency systems
This domain tests the protective systems that stop a builder's lift from becoming a falling or crushing hazard, and the operator's duty not to defeat them. Expect questions on what a device does, what a fault means, and what action is permitted.
Primary protective systems
- The overspeed governor and safety device detect excessive speed and grip or brake the cage. They are not normal stopping brakes.
- The service brake holds the cage at a landing and during normal stops. A brake that slips or drifts is a defect.
- Normal limit switches slow or stop the cage at the top and bottom. The final limit switch is a back-up and must not be used as a routine stop.
- Cage gate and landing gate interlocks prevent movement unless gates are closed and latched, and prevent a landing gate opening unless the cage is at that landing where so designed.
- Emergency stop buttons, overload devices, phase-failure protection, buffers and communication systems support safe operation.
Interlock logic
Interlocks are safety functions, not convenience switches. A faulty interlock means the hoist is out of service until repaired and tested. Never tape, wedge, disconnect or jumper an interlock to keep production going.
Worked example
A landing gate interlock does not register "closed", so the hoist will not start. The operator is told to tape the switch closed for one delivery. Decision: refuse. Taping defeats the guard; the cage could move with the landing gate open and a person could fall or be crushed. Isolate and tag the hoist, report the fault, and use an alternative safe method after authorisation. The correct repair is by a competent maintenance person, followed by a function test.
Traps that cost marks
- Using the emergency stop as a normal landing stop. It is for emergencies and may require a deliberate reset.
- Assuming the emergency stop isolates all electrical energy. It may not; isolation for maintenance is a separate procedure.
- Testing an interlock by moving the cage with the gate open. Test statically according to the site procedure.
- Resetting a safety device without finding the cause of its operation.