The incident sparked questions from many in the city, including lawmakers, on why the emergency power supply did not work, despite the firm having often emphasised its world-class service, with a reliability rate of more than 99.999 per cent.

About 140,000 affected households had their power gradually restored within hours of the outage on Tuesday night, while the remaining 20,000 had only begun to get their electricity supply back on Wednesday morning, even though CLP Power conceded that the power supply might remain unstable for the coming few days.

Chan, who worked for CLP Holdings for decades, said: “In addition to the reasons for the fire, the direction of the investigation would also include why the backup support provided from other areas took so many hours to fully restore power.”

Undersecretary for the Environment Tse Chin-wan said the bureau had told the firm to submit a preliminary report within three days and a detailed report within 14 days, which must also include proposals to prevent any similar incidents from happening again.

“From CLP Power’s explanation, they have three power cables installed in case of malfunctions. If one has problems, the other two can be backups, so this is very rare,” said Tse, who is set to be sworn in as the city’s environment minister on July 1.

A fire on a bridge, which carried the three high-voltage power cables along Kwong Yip Street in Yuen Long, broke out at around 7pm on Tuesday, causing the structure to collapse as firefighters attempted to contain the blaze.

Some 20,000 households in Tin Ching Estate, Tin Yuet Estate, Tin Chak Estate and Tin Fu Court were still experiencing outages on Wednesday morning. A number of people were seen charging their phones at Tin Chak Shopping Centre in Tin Shui Wai.

The Education Bureau said 14 schools, covering kindergarten, primary and secondary pupils, were forced to suspend classes due to the power outage.

Quince Chong Wai-yan, CLP Power’s chief corporate development officer, apologised for the inconvenience caused by the incident, saying the power supply might remain unstable for the coming few days.

“We have been repairing the power cables. Our goal is to complete the restoration as soon as possible,” she said.

While Chong added that the cause of the blaze was still under investigation, Secretary for Security Chris Tang Ping-keung said there was no evidence to suggest the fire was deliberate.

“There was no damage to the place where the cable bridge was entered,” he said.

Eric Cheung Po-chung, a CLP Power senior manager, said the electricity supply in Yuen Long relied on three 132KV power cables which were all connected to the same bridge.

“It is extremely rare that these 132KV cables would malfunction at the same time, which caused difficulties in resuming the electricity,” he added.

However, New Territories North lawmaker Edward Lau Kwok-fan, a member of the Democratic Alliance for the Betterment and Progress of Hong Kong, said if the firm had bundled the three cables together, then it was no surprise the backup system had not worked.

“CLP Power previously said that it had a centre controlling the electricity system, the backup plan is that if a cable did not work and an area is hit by outage, the centre would quickly arrange for electricity to be diverted from a different location,” he said.

“But they told me the system did not work last night, and electricians had to travel to different locations in the New Territories to rearrange electricity supply manually.”

Lau said he believed the contingency measure could only work if one or two cables in the area had broken down, but not all three.

He added that while strong typhoons had caused sporadic power cuts in recent years, the outage on Tuesday could be the worst incident since the Castle Peak power station was disconnected from the supply grid in 1984, causing chaos across the city.

Lawmaker Tony Tse Wai-chuen, who is a surveyor by profession, said the three cables could have been bundled together under the bridge because it was difficult and costly to install new underground ones with the growth of Tuen Mun and Yuen Long’s populations.

Senior adviser Chan said that based on his past experience, cable facilities were installed with fire service equipment and a protection system, with additional systems to detect any electric overload.

Dr Edward Lo Wai-chau of the Hong Kong Institution of Engineers’ electrical division said a possible explanation for the fire was an overload of electric current.

“The weather is hot now and the peak of electricity usage is around 6pm to 7pm in the evening. People will turn on the air conditioner after arriving home but air conditioners in office buildings are still on,” he said.

Tin Yat Estate resident Lau, 67, said he still had no power in his home as of Wednesday morning.

“I had not yet eaten anything for the whole night. I’m afraid the food in my fridge would be rotten and I dare not eat anything,” he said.

“CLP is always increasing electricity fees, but why would such an incident happen? I hope they can give us compensation regarding our loss in this incident.”

A 73-year-old housewife surnamed Chong, who lives in Tin Shui Estate, described Tuesday night as “total chaos” and “terrifying”.

“It was a total blackout. I was at home last night and everything went off. I could not cook, I had no lights, and I could not reach for any help, as my phone had lost its signal. Together with my husband, we looked for our neighbours to help,” she recalled.

Tin Yuet Estate resident Chan Suk-ying, 45, said her three children were affected and she had to bring them to work with her. “I cannot just leave them at home,” she said.

Wong, the owner of a frozen food shop at Tin Shing wet market, said the electricity had come back on at around 1am so their stock was fine as it had been stored in a big freezer. But another shop owner, Lee, 58, was not so lucky.

“We threw away some fish balls and marinated meat. We probably lost around HK$1,000,” she said.

Additional reporting by Charmaine Choi