All they've been able to determine is that it's not an obvious problem, said BC Hydro spokesman Ted Olynyk.
"It's not like a tree fell on the line, or it was wind or snow or ice or there was physical damage to the property," explained Olynyk.
"What we are doing now is BC Transmission Corp. technical folks are reviewing computer information, data, to see if they can pinpoint where and when the fault happened."
The outage began with disruption of at least one south Island power line around 5:45 p.m. local time October 12. A cascade effect amplified the damage by subsequently tripping all the power lines south of Ladysmith, including the Gulf Islands, and leaving more than 200,000 customers in the dark during Thanksgiving dinner.
That's not supposed to happen, said Olynyk. The Island's power system, which includes 31 substations, was built to protect against cascade outages when a few lines go down, he said.
"It's designed so that doesn't happen," said Olynyk. Nonetheless, it has happened in the past, most often when lightening strikes, he said.
What caused the cascade is now a primary focus of the investigation, said Olynyk.
"It's very important to find the reasons for this, so we can take preventive measures and ensure this doesn't happen again, not only at the location it will have happened but throughout the power system."
When the power failed, BC Hydro crews were quick to try and visually inspect as many of the transmission lines as they could, said Olynyk.
Finding no obvious problems Â— cut cables, fallen trees or blown transformers, for example Â— they began activating lines one by one to see if they would hold power, said Olynyk. They did, and all were re-activated by 8 p.m.
It's similar, he said, to tripping a circuit breaker in a house. Sometimes the combination of appliances that trigger the outage is not clear, but power can be restored by resetting the breaker at the source.
"Sometimes we've gone out there and there's an outage and there's no indication of what caused the outage," he said. "We reset the system and it's back up, and sometimes we just don't know."