A voltage dip that disrupted the electricity supply in Kowloon and the New Territories and led to more than 50 reports of people being stuck in lifts on Saturday might have been caused by a safety feature that protects the power cables, according to an electrical and building services engineer.

CLP Power said a hill fire that broke out in Yuen Long might have been the cause of the voltage dip in its overhead cables connecting Yuen Long and Lai Chi Kok.

Ho Wing-ip told a radio programme on Monday that when hot air rises and carries ash onto power lines, it may trigger a short circuit in the cables to protect them and subsequently interrupt the current.

“Imagine that the entire power system has many different cables. If the overall electricity demand in Hong Kong remains the same, then the current remains the same. Suddenly, one group of cables stops operating, with no current flowing through. In other words, the current will change for the other groups of cables. So it causes a so-called ripple,” he said.

“It’s like a very calm lake, and all of a sudden a stone falls down and then the ripples appear.”

Ho said it is not rare for such a short circuit to happen, for instance when animals climb up power lines, and when plants grow too dense and branches hang on the cables which could also lead to similar incidents.

He added the entire Hong Kong power grid is inter-connected, which means it is difficult to accurately predict in advance which location a voltage dip will occur in, and what range would be affected.