Yukoners were out of power for a time last night, after Yukon Energy's most efficient hydro unit stopped working just after 9:30 p.m.

Most of Whitehorse, Faro, and Ross River were without power for about an hour, while Deep Creek and Mendenhall residents waited two hours for power to return. Yukon Energy crews worked on restoring power last night, said spokesperson Janet Patterson. Today, many are wondering what caused the power outage in the first place.

'There was a problem with the fourth wheel that caused the power outage,' said Patterson. 'We don't know what the problem is, so we can't determine costs from here.'

The fourth wheel is the newest and strongest in the Yukon. It began operating in 1985, and handles 20 megawatts of power. The original three wheels, which are powering the city today, handle 20 megawatts combined, said Patterson.

In the case of a power outage, Yukon Energy usually relies on back up power from Aishihik's hydro generators. Last night, work was being done on them at the time of the power outage, forcing Yukon Energy to go to an expensive Plan B.

'We did have to turn on the diesel generator for a brief period last night,' she said. 'It's expensive, so we don't like to run it because it costs a lot of money.'

Diesel ran until word of the Whitehorse power outage reached the Aishihik hydroelectric dam, and maintenance work at the facility could be stopped temporarily to return power to the system. The two Aishihik generators handle 30 megawatts each.

Yukoners use more energy in the winter months than the summer, so the loss of the fourth wheel does not mean residents are in danger using up the power supply of the remaining three hydro generators.

'In the summer we use 20 to 35 megawatts, and in the winter we use 50 to 60, so we have lots of power to work with,' said Patterson.

If demand does exceed the 20 megawatts that the three Whitehorse generators can handle, the Aishihik generators will pick up the slack.

The Yukon Energy crew began looking at the fourth wheel this morning, trying to determine what caused it to break down.

'We still do not know what the problem is,' said Patterson. Until a cause is found, Patterson said she could not speculate on how long it would take to fix.