Many elevator installations now feature emergency power systems which allow elevator use in blackout situations and prevent people from becoming trapped in elevators.
Traction elevators
When power is lost in a traction elevator system, all elevators will initially come to a halt. One by one, each car in the group will return to the lobby floor, open its doors and shut down.
People in the remaining elevators may see an indicator light or hear a voice announcement informing them that the elevator will return to the lobby shortly.
Once all cars have successfully returned, the system will then automatically select one or more cars to be used for normal operations and these cars will return to service. The car(s) selected to run under emergency power can be manually overridden by a key or strip switch in the lobby.
In order to help prevent entrapment, when the system detects that it is running low on power, it will bring the running cars to the lobby or nearest floor, open the doors and shut down.
Some modern elevators are now featuring battery-powered emergency feature, such as Mitsubishi, Hitachi, Fujitec, Toshiba, etc.
Hydraulic elevators
In hydraulic elevator systems, emergency power will lower the elevators to the lowest landing and open the doors to allow passengers to exit.
The doors then close after an adjustable time period and the car remains unusable until reset, usually by cycling the elevator main power switch.
Typically, due to the high current draw when starting the pump motor, hydraulic elevators aren't run using standard emergency power systems.
Buildings like hospitals and nursing homes usually size their emergency generators to accommodate this draw. However, the increasing use of current limiting motor starters, commonly known as "Soft-Start" contactors, avoid much of this problem and the current draw of the pump motor is less of a limiting concern.
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