Safety Devices
Safety Devices
Safety devices are the exam's highest-stakes material because they prevent uncontrolled motion and door-related injuries. You must know what each device protects, how it is tested, and what makes it unacceptable during inspection.
Car safeties and governor
- The governor detects overspeed and trips the car safeties, which grip the guide rails. Governor tripping speed is set above rated speed; for passenger elevators the minimum is typically 115% of rated speed.
- Safeties must be tested with the car at rated load or as the code requires, and the governor must be sealed after adjustment.
Doors, limits, and pit devices
- Door interlocks prevent car motion unless the car door and hoistway door are closed and locked, except within the leveling zone.
- Final limit switches stop the car if normal limits fail; they are not operating stops.
- The pit stop switch must stop the car and prevent motion while a mechanic is in the pit.
Buffers and hydraulic safety
- Oil buffers absorb kinetic energy; spring buffers are allowed only within code speed limits. Buffer oil level and stroke must be checked.
- Hydraulic elevators need a rupture valve or restrictor, a relief valve, and a check valve to control descent if piping fails.
Worked example
A passenger elevator has a rated speed of 200 ft/min. The minimum governor trip speed is 200 × 1.15 = 230 ft/min. If the governor trips at 210 ft/min, it is below the minimum and fails. The mechanic must correct the setting and retest; simply resetting the governor does not prove the safeties will stop the car.
Traps
Treating the final limit as a normal stop; testing safeties without the required load; overlooking governor seal status; and assuming a door interlock is working because the door closes.