TAIPEI (Reuters) - Taiwan said on Wednesday it was investigating a massive power blackout that hit businesses and residential homes, affecting close to seven million households on the heavily industrialized island amid sweltering heat.

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President Tsai Ing-wen apologized for the crisis which left millions of homes without power and hit offices and factories on the island of nearly 24 million people late on Tuesday.

The worst appeared to be over by Wednesday afternoon, with power fully restored and little impact on Taiwan’s leading technology manufacturers.

Taiwan Semiconductor Manufacturing Co (TSMC), the world’s largest contract chipmaker and a major supplier to Apple Inc, said its operations were not affected, as did electronics manufacturer Pegatron Corp.

ChipMOS Technologies Inc said the outage did not have a big impact on its operations. Its shares fell 2 percent in early trade but pared losses later to 1 percent, lagging a flat broader market.

Premier Lin Chuan promised to seek an external review of the island’s power supply network, and take questions in person at the legislature.

Officials said a task force would be set up to investigate the cause of the outage.

“The strength and stability of Taiwan’s power grid requires a large-scale inspection, and also examination by outside investigators,” government spokesman Hsu Kuo-yung said at a press conference.

Residents complained as temperatures hovered around 32 degrees Celsius (89.6°F), while the blackout caused havoc as restaurants and small businesses were left without power, traffic lights stopped working and elevators stalled.

The blackout was caused by “structural problems” and human error involving the replacement of equipment, which ultimately affected the operations of a state-owned Taiwan Power Co power plant, state-owned gas supplier CPC Corp said at a press conference.

It said it would fully compensate Taiwan Power Co and take responsibility, although it did not provide an estimate of the costs.

Taiwan Power Co said the outage at its plant in the northwestern city of Taoyuan caused six generators to stop working.

President Tsai, in a statement on Wednesday, again apologized for the blackout, describing electricity supply as a national security issue and stressing the importance of ensuring the safety of the island’s infrastructure facilities.

Government spokesman Hsu said the question of whether the chairmen of Taiwan Power Co and CPC Corp should step down should be answered after they tended to their responsibilities.

The administrative deputy minister for economics, Shen Jong-chin, temporarily assumed the duties of minister of economics affairs Chih-Kung Lee, who resigned over the incident.

The blackout was the country’s most severe since the 1999 Jiji earthquake, Taiwan Power Co said.