Elevator Overspeed Governor Tripping Speed Band

THE ORDER MATTERS. The electrical overspeed switch trips first, cutting power and setting the brake, which stops most overspeed events without the safety ever engaging. Only if the car continues to accelerate does the governor mechanically grip its rope and pull the safety, which wedges the car against the guide rails -- a violent event that takes the car out of service and requires inspection afterward. A governor found with its electrical trip set at or ABOVE its mechanical trip has lost the gentle stop entirely: every overspeed goes straight to a safety application. The margin band is bounded at both ends for good reasons. The minimum, commonly 115 percent of rated speed -- 575 fpm on a 500 fpm car -- keeps the governor from tripping on normal operation, including the modest overspeed of a heavily loaded down run. The maximum exists because a safety must engage before the car reaches a speed at which the buffers below it cannot absorb the impact, and it is tighter as a percentage at higher rated speeds, so a 500 fpm car has a wider permitted band than a 1,200 fpm car does. GOVERNOR TRIP, SAFETY TYPE, AND BUFFER STROKE ARE A SET rather than independent choices: the mechanical trip speed is the speed at which the car can strike the buffers, and raising it to stop nuisance trips invalidates the buffer selection below it. That is why governor settings are not a field adjustment. One more thing worth stating: the governor rope runs at CAR speed regardless of the suspension roping ratio, so a mechanic used to thinking in 2 to 1 terms can misread what the governor is seeing. And verification is by test at the intervals the code requires -- a governor is a mechanical device with springs and pivots that age, and a setting recorded on a tag is not evidence of a setting that still holds. A limit comparison against code values the user supplies. The minimum and maximum tripping speeds, the relationship between the electrical and mechanical settings, and the testing intervals are set by the adopted code edition and by the jurisdiction. Governor setting and testing are performed by licensed elevator personnel using prescribed procedures, and a governor is typically required to be removed and calibrated by an authorized facility. It does not evaluate the safety itself, its type, condition, or capacity, size buffers, or address ascending car overspeed and unintended movement protection, which are separate required devices. ASME A17.1 and A17.2, the equipment manufacturer, the elevator authority having jurisdiction, and a licensed elevator mechanic govern.

minimum mechanical trip = rated speed x the code minimum percentage, commonly 115; margin = (trip - rated) / rated; the electrical overspeed switch must trip below the mechanical trip; buffer impact speed = the mechanical trip.

The ASME A17.1 governor tripping-speed limits by name -- a mechanical trip at least 115% of rated speed under a ceiling that tightens as rated speed rises, with the electrical switch set below it. Both bounds are entered from the adopted code table, not reproduced.

Percentage comparison against code values the user supplies; no A17.1 table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rated_speed_fpm, electrical_trip_fpm, mechanical_trip_fpm, code_minimum_pct, code_maximum_fpm, minimum_trip_fpm, mechanical_margin_pct, electrical_margin_pct, headroom_fpm, impact_speed_fps, implied_buffer_stroke_in

• The electrical trip must come first inverted settings send every overspeed straight to a safety applicationASME A17.1
• Trip, safety and buffer are one set raising the trip invalidates the buffer selection beneath itASME A17.1
• A tag is not evidence tripping speed is verified by test at the intervals the code requiresASME A17.2