Damage to an overhead power line plunged thousands of East Kowloon residents into darkness for up to an hour on Monday.
Supplier CLP Power, which frequently boasts of a "99.999 per cent" reliability rate, apologised yesterday to the few thousand residents affected, describing the occurrence as "rare".
"We are working to investigate how this overhead cable may have been damaged," CLP chief operating officer Chow Tang-fai said.
At least 20 apartment blocks in Wong Tai Sin, Kwun Tong, Kowloon Bay, Ngau Tau Kok, Yau Tong and Tseung Kwan O lost power after 10pm, some for as long as an hour.
The 10-year-old cable, near an electricity pylon at Kowloon Peak, was last inspected in 2013.
Chow said investigators had found there had been a splitsecond voltage dip in the 132-kilovolt overhead line.
The transmission problem was isolated within a fraction of a second, said Chow, but when voltage was restored, the sudden power surge overwhelmed the electrical systems of some buildings and caused circuit breakers to trip.
The number of air conditioners turning off and restarting on a hot night could have played a role in the circuit tripping, he said.
"The situation was different in every building," Chow said.
"Some protection devices took longer to switch back on," he added.
Meanwhile, a recent flood risk assessment by the company revealed that at least 11 of its transmission substations could be at risk in the event of a storm surge or super typhoon.
CLP said new prevention measures such as floodgates and pumps had been installed to cope with adverse and extreme weather.
Flood calculators, which use real-time tidal data from the Observatory, will be used to predict which substations will be affected.
About 500 other flood-prone distribution stations will be given further upgrades.
CLP claims each of its customers experiences an average of just 2.3 minutes of unplanned power interruptions on average each year.