CLP said on Friday that a fluorescent light catching fire was the most probable reason behind a power cable bridge fire in Yuen Long in June that caused a major blackout in northwestern New Territories.

Some 160,000 households in Yuen Long, Tin Shui Wai and Tuen Mun were plunged into darkness for hours, as the entire cable bridge burned down and collapsed.

During a press conference to announce details of its final investigation report, CLP managing director Chiang Tung-keung said a light near the Kwong Yip Street end of the bridge was the most possible source of ignition.

"The most likely cause of the fire is the fluorescent light inside the cable bridge on the ceiling, which caught fire first. It affected the communication cable underneath, and the fire then spread to the power transmission cable and across the bridge," he said.

Chiang noted that the bridge had been in use for 30 years and had always functioned well, adding that the blaze was extremely rare.

CLP said during a regular inspection in November last year, technicians found everything in the facility – including the lights – to be functioning normally.

Asked whether the fire was caused by someone forgetting to switch off the lights after the check-up, Chiang said that’s hard to determine.

"Unfortunately, because the fire was so fierce that night, most of the equipment inside the cable bridge was already damaged. That's why it's very very difficult to determine the status of the fluorescent light before the incident," he said.

The report said the fire's damage to the communication cables meant technicians couldn't resume power supply using remote control, and some 150 technical staff had to be deployed to the scene for manual assessment and switching.

Chiang also pointed out that while there were two backup power systems for the area, one of them malfunctioned the day before the fire so the firm could only rely on one system to get power back up.

He said after the blaze, CLP had inspected and upgraded four similar bridges to avoid similar incidents, such as changing the light tubes there, painting fire-retardant coating and installing heat detection systems.

The company again apologised for the incident, saying it will start sending out HK$100 consumption coupons to affected households from the middle of next month.