Energy utilities provider SP Group said yesterday that it was at fault for the blackout that occurred in the Central Business District last Friday.

"We have traced the cause of the incident to maintenance work by our crew at a substation that day," the group said.

"A test equipment component was not removed before the system was re-energised, resulting in supply disruption."

The power outage, which started at about 2.45pm and lasted half an hour, affected thousands of people working in the CBD and surrounding areas such as Chinatown.

Offices, banks, traffic lights and at least one shopping mall went dark during that time.

SP Group added that it takes "full responsibility" for the outage. "We are tightening our procedures to prevent a recurrence of this incident," it said. "We know that our priority must be to ensure a safe and reliable power supply to Singapore at all times."

In 2011, a contractor was fined $150,000 after it damaged a high-voltage electricity cable, leading to a 15-minute blackout in the Bishan and Ang Mo Kio area. Apart from affecting homes and businesses in the area, the power failure also stalled trains between Orchard and Marsiling, causing delays for more than 7,000 commuters.

One of the worst power outages in recent memory happened in 2004, when stoppages in the natural gas supply from Indonesia caused a two-hour blackout across parts of Singapore.

In 2006, disruptions in the gas supply from Malaysia also caused a 45-minute blackout in various parts of the island.