A major electricity supplier has blamed a 48-minute power outage that affected 44,000 households on Hong Kong Island last month on the insufficient alertness of staff and poor cable labelling.

HK Electric on Monday presented the findings of its final report into the blackout, and confirmed it was caused by a short circuit triggered by an accidental connection to a spare cable during refurbishment at a switching station at Cyberport in Pok Fu Lam.

The company said about 44,000 households were affected during the incident on April 19.

The firm’s operations director, Francis Cheng Cho-ying, said that type of incident was rare and highlighted that it was HK Electric’s first major power interruption in almost three decades.

Francis Cheng (left) and Ip Sung-tai explain the firm’s findings.

“The incident is most regrettable and we apologise again for the inconvenience caused to our customers,” he said.

“We will learn from this lesson and enhance our operations to sustain our commitment to delivering a reliable electricity supply to our customers.”

The outage lasted from 12.49am to 1.37am, and caused traffic lights to malfunction on Hong Kong Island. Police said between 10 and 20 sets of traffic lights were still out of order at 6.25am in Central, Western, Eastern, Southern and Wan Chai districts.

HK Electric said the incident was triggered at 12.45am during the refurbishment and commissioning of switchgear at the Cyberport switching station, which houses a 275kV supply system.

The firm said the short circuit triggered by the accidental connection to a spare cable led to a severe power disturbance.

Voltage collapsed to near zero for about half a second, causing all three interconnector circuits between HK Electric and CLP Power’s systems to disconnect automatically.

One coal- and one gas-fired unit in Lamma Power Station were severely disrupted and had to be shut down manually, which meant insufficient electricity generation to meet the load demand.

HK Electric engineers at the system control centre began to restore electricity to customers after restoring the interconnector circuits with CLP Power. The entire power restoration was completed at 1.37am.

Businesses and households were left without power on April 19.

“The spare cable was inadvertently energised because one of the energy management system circuit diagrams did not clearly mark the connection of the spare cable circuit,” HK Electric general manager Ip Sung-tai said.

“And the ‘spare’ labels on site were not sufficient to indicate that the spare cable circuit was connected to the switchgear.

“The engineer in charge was unable to identify the presence of the spare cable circuit because it was fully concealed by a fire-rated steel enclosure.”

HK Electric said it had taken immediate improvement measures including suspending all refurbishment works until new commissioning guidelines were formulated and disconnecting all four spare cable circuits including the one in the incident.

The firm is also set to update all site labels showing details of the spare cable circuits and related circuit diagrams by the end of June. Counter-check requirements have been adopted for commissioning of all gas insulated switchgear.

Cheng stressed the engineering staff had followed guidelines to perform site checks.

“I believe the employee failed to see there was a problem because he was not alert enough, handling tasks based on past experience instead of verifying when questions arise,” he said.

“Had there been clear labelling, staff would not have to spend too much effort on verification, so I think it was not their sole responsibility.”

HK Electric says it has taken immediate measures to rectify the problems that caused the blackout.

Under a reward mechanism in its contract, HK Electric is expected to yield an additional return of HK$7.4 million (US$942,723) because there was an 0.015 per cent incentive adjustment for power restoration within 65 minutes.

Cheng emphasised the mechanism was based on the firm’s annual performance, not on a single incident, and did not say whether the company would be rewarded or give any return earned to affected customers.

The Electrical and Mechanical Services Department said it was concerned about the incident and would examine the report in detail with an independent third-party expert to evaluate whether the identified cause was well-founded and improvement measures were appropriate.

Lawmaker Edward Leung Hei said he found HK Electric’s explanation unacceptable.

“The Electricity Ordinance clearly states that electricity supply should not be connected to a fixed electrical installation unless the power supplier has done an inspection and is satisfied that it is safe,” he said.

“The authorities should look into the incident to see if any regulations have been breached.”