(copied from another forum)
Just a few strands, or maybe just one strand, broken on a cable can cause a severe mishap. This could cause the cable to come off of the pulleys at the supporting towers, or cause a car to not disengage where needed and the car crashes.
A direct lightning strike would severely damage the cable although I am not sure that could melt the cable all the way through and the cable breaks.
Nowadays they have sensors mounted in strategic locations and that continuously check the passing cable for frayed strands.