The authorities want to know why lifts in the city's tallest building plunged several floors last week when power supplies were briefly interrupted. Office workers in the 118-storey International Commerce Centre stepped white-faced from the lifts following the incident on Thursday. Some were still in shock yesterday. 'I was going down from the 22nd floor. Just as the door closed and the lift started to move, it suddenly rocked for a few seconds. Then I felt it plunge. It was very scary,' said Ron Shu. People going up the building felt something too. One woman said: 'I was going up from the ninth floor to get back to my office on the 21st floor. At first, there was nothing special, but when the lift reached the 20th floor the lift suddenly stopped for several seconds, like a car braking sharply. 'The lift rocked for several seconds before everything returned to normal and the lift went up again. I was so afraid of being trapped inside. I was so scared.' Passengers in lifts going down might have got the impression they were plunging at increasing speed when in fact they were slowing down because the lifts' emergency brakes had been activated, said Lo Kok-keung, a mechanical engineer at Polytechnic University. 'It is like when you are in a car going at high speed. When you suddenly hit the brakes, the car will move forward for some distance before it stops. And the force of the braking will make people move forward inside the car due to the force of gravity,' he said. Tse King-wa, chairman of the Hong Kong General Union of Lift and Escalator Employees, said if there had been any problem with the lifts' safety, they 'would not have resumed operation after the first so-called plunge'. The incident came barely a month after six labourers collecting construction waste were killed when the work platform they were using plunged 20 floors down a lift shaft at the ICC, possibly because of overloading or lack or maintenance. The building's manager, Kai Shing Company, said a citywide power dip had cut electricity supplies to the ICC on Thursday, triggering lift safety systems. That made them slow down, and passengers in its high-speed lifts might have felt uncomfortable. But the drop in voltage lasted less than a second and normal lift service resumed immediately. Electricity producer CLP Power said there was a voltage dip at about 12.41pm on Thursday. The Electrical and Mechanical Services Department said its preliminary investigations showed the lifts at the West Kowloon tower were in safe working order and could continue operating. 'The contractor was asked to submit a report giving full details of the incident [within seven working days],' a spokesman said.