Alcoa Corp has closed one of two electrolyzers at its Portland aluminum smelter in Australia following a power outage on Thursday, a company spokesman said on Friday.

The spokesman said it was still unclear when the shut down cell could restart production.

A failure of a transmission line from Victoria to south Australia caused power outages of up to five and a half hours on two production lines at the aluminium plant.

“As a result of the power failure and uncertainty, one of the electrolyzers has been temporarily shut down to ensure the safety of the refinery and employees, while only the second one is currently in operation.”

“It is too early to tell how long it will take for the impact of this outage or normal production to resume.”

The Portland smelter, which is jointly owned by Alcoa, CITIC and a unit of marubeni, produces about 300,000 tonnes of aluminium a year.