Failures are rarely isolated events, sometimes happening for days before they are reported. It is often the case that the elevator behaves normally while the technician is on site. This was the case shown at the left. (The blue bars show the normal acceleration and deceleration of the elevator. The red bars indicate abnormal accelerations. The yellow areas indicate that the door is open.) The elevator made two normal down trips (at left). Then it was making another down trip when it stopped abruptly with a deceleration of 20 ft/sec2. The elevator then restarted by itself, and stopped abruptly again. After this, it began to start and stop abruptly every 7 seconds, and continued for 8 minutes.
The problem was found to be due to a switch that detected stretching of the ropes. Every time the elevator started to move, it stretched the ropes slightly and shut down the elevator. Examining the data showed that the behavior occurred several times in the preceding week, but only when the elevator was empty and parking. If the limit on the acceleration had been set, the monitor would have alerted the maintenance company when the problem started.