Elevator Safety Features
Today’s elevator systems incorporate a wide variety of features designed to help reduce the chances of accidents and give passengers a quick, dependable ride.
There are two basic types of elevator systems, “Traction” and “Hydraulic.” Because they differ in the way they operate, some of their safety systems differ as well.
Most elevators which travel six or more floors are “traction” design. Traction elevators are suspended by strong steel cables propelled by a hoisting machine. Counterweights help balance the load and make it easier for the machine to move the elevator. The steel cables are very strong, and can safely hold several times the weight of the elevator and its full load of passengers.
The safety brake, together with a speed-sensing governor, acts to stop an elevator if it should over speed in the down direction. If an elevator over speeds, the governor makes the safety grasp the rails on which the car travels, bringing the elevator to a safe emergency stop.
The door system on a modern elevator also includes several safety devices. Sensors detect passengers or objects in the door opening, preventing the continued closing of the doors. Older systems use mechanical “safety edges” which cause the doors to stop or retract when they make contact with a person or object. More modern systems use a large number of invisible light rays to detect people or objects in the doorway and reverse or stop the doors without having to make physical contact.
Door operators contain devices which limit the amount of closing force. Newer systems are better able to keep the closing force consistent even under unusual conditions such as the “stack effect” which can cause heavy air movement in elevator shafts.
Interlocks on the hoist way doors help assure that the elevator cannot leave a landing unless the doors are fully closed and secured. Should the doors be forced open, the interlock circuit will be broken, causing the elevator to immediately stop.
Various switches in the elevator shaft detect the presence of the car at certain stages of its journey. They initiate slowdowns and stops at the proper points, and help prevent over travel in the up or down direction.
To discourage the very dangerous practice of passengers trying to open the door of a stalled elevator, door restraints can allow normal operation of the door when the car is near the floor level, but will restrict forcible movement of the door when the car is away from the floor.
The emergency evacuation hatch on most elevators is designed to be opened only from the outside, by trained emergency personnel. This too is intended to help prevent any passenger from gaining access to the dangerous elevator shaft way.
Buffers, located in the “pit” below the car, serve to cushion any unplanned travel below the lowest landing.
In the elevator cab you’ll find several items to help increase safety. An emergency alarm switch will sound an alarm when activated by a passenger. In most elevators, an emergency telephone or intercom can serve as a link to assistance if the car should stall. And, in the event of a power failure, emergency lighting maintains illumination for hours. In some systems, emergency power is available to permit movement of the elevator and evacuation of the passengers.
In many elevator systems, a special fire emergency system has been installed. It may be manually activated, or may respond to smoke sensors in the building. Exact operation varies by local codes, but generally such systems return the elevator to the main floor, open the doors to allow passengers to exit, and make the elevators available to emergency personnel.
Hydraulic Elevators are propelled by a jack mounted below the elevator. A pump moves hydraulic oil into the jack, causing it to raise the elevator. For the down trip, valves control the oil’s to return to the system’s storage tank, safely lowering the elevator car.
Because of their bottom-supported design, hydraulic elevators do not utilize the type of safety brake found on traction elevators.
Other systems on hydraulic elevators are essentially identical to those on traction systems, and have similar safety features.