Safety first.
I have never heard of elevators plunging, but I have heard about an elevator which cable snapped.
All elevators have at least one safety brake, most of them have more than two, which consist of both cables, electromagnetic brakes and mechanical brakes. So did this one. The elevator only fell for one meter until it was stopped. The people inside did not realize what had happened, they did not even feel much of the fall.
I do not think one should worry about elevators plunging at all. If one should worry one should probably worry about REAL OLD elevators, because they can have just one safety brake.
The elevator I studied in a digital course on an university in Sweden had these safety brakes.
- An extra cable which were not supporting the elevator in general. The cable was loosely fastened in the elevator and if the elevator moved to fast downwards a mechanism on the cable noticed and started to break the fall. This cable was fastened in another structure than the original cable which elevated the lift (the original cable also had some safety brakes).
- One set of mechanical brakes which was activated in the same manner as the safety cable (by speed downwards).
- One set of magnetic brakes a top the mechanical ones, same activation.
I have also heard about safety issues in the bottom of shafts but never seen one. Supposedly they use compressed air or foam in the bottom of the shaft slowing a falling elevator.