Electrical and Mechanical Controls and Safety Devices
What this domain tests
This domain tests the electrical control system and the mechanical safety devices that stop, hold, or protect the elevator. Expect questions on safety circuits, door interlocks, inspection controls, governors, safeties, buffers, brakes, and the electrical installation rules that support them.
Electrical controls and protection
- NEC Article 620 covers elevator feeders, branch circuits, disconnecting means, wiring methods, machine-room lighting, and the pit receptacle. Grounding and bonding must be effective; the safety circuit is not a substitute for equipment grounding.
- The safety chain is a series circuit. Opening any required device-door interlock, pit switch, governor switch, buffer switch, final limit, or emergency stop-must remove power from the driving machine and set the brake.
- Car-top inspection operation must be selected at the car-top station and must limit movement to the car-top controls at reduced speed. Normal operation must not be available while inspection is selected.
- Door interlocks must prevent motion with a landing door unlocked, and door reopening devices must protect passengers during closing.
Mechanical safety devices
- The governor senses overspeed and trips the car safeties, which grip the guide rails.
- Oil buffers absorb kinetic energy at the pit; their oil level and switch function must be checked.
- The brake must hold the car at rated load and stop it reliably; brake torque and lining condition are safety items.
- Rope traction, counterweight clearances, and overload detection affect safe operation and must be tested as required.
Worked example
A car-top mechanic selects inspection, moves the car with the car-top buttons, then returns the selector to normal while standing on the car top with the hoistway door open. The car must not begin normal operation. The inspection switch must restore normal service only after the car-top controls are secured and the car is in a safe condition; the door interlock and safety chain must still prevent motion. If the car moves, the inspection/normal circuit is defective and must be corrected and retested.
Traps that cost marks
- Assuming the safety chain is a grounding conductor.
- Bypassing an interlock or safety switch to “test” operation.
- Confusing governor tripping with safety engagement; both must be verified.
- Using the emergency stop as a routine stopping device.
- Checking buffer switches visually without verifying electrical function.
Exam approach
Trace the circuit from the driving machine to the device that should open it. If a question describes unexpected motion, ask which interlock, switch, or mechanical device failed to open or hold.