TAIPEI (REUTERS) - Taiwan's state-owned electricity monopoly Taipower on Friday (May 14) blamed human error during a test for a power cut that affected some four million homes the day before, and said it would offer discounts worth almost US$18 million (S$24 million) to affected customers.

The rolling blackouts went on for around five hours before normal supplies were resumed early evening, briefly affecting the world's biggest contract chipmaker, Taiwan Semiconductor Manufacturing Co.

Taipower said engineers had been carrying out tests at a substation as part of a future expansion project for a power plant in the southern port city of Kaohsiung.

However, they pressed the wrong switch, causing a sudden drop in voltage, and the problem then cascaded, leading to the blackout, it added.

Taipower will offer discounts to customers affected by the power cut, which it said it expects will amount to some TWD$500 million (S$23.9 million). More than six million homes have been affected on the island of 24 million people, with Taiwan Semiconductor Manufacturing Co saying some of its facilities suffered a "brief power dip", but power had been restored.

In a text message just before the power failed early in the afternoon, the government said its grid did not have sufficient electricity capacity after the outage at the power plant in the southern port city of Kaohsiung.

However, the main international airport near Taipei, the capital, and the high speed rail line were operating normally, the transport ministry said. Power went on and off intermittently in central Taipei, though it never totally went out.