The weather doesn't look too bad. So why is the power out?

As many Nova Scotia Power customers around metro Halifax and elsewhere gazed out their windows at the relatively calm weather conditions late this morning, they may have wondered why they were in the dark.

Most of the Halifax peninsula was without power, along with much of Dartmouth and suburban Halifax, accounting for nearly half of the 249,000 customerswithout electricity.

Nova Scotia Power spokesperson Tiffany Chase said the problem affecting much of the mainland originated in Cape Breton, where heavy snow and tree branches on power lines caused an interruption in the transmission system near Port Hawkesbury.

There are two different kinds of lines in the system, Chase said.

Transmission lines are high-voltage connectors that run along highways and carry electricity from the power plants, while distribution lines, which have a lower voltage, branch off from those transmission lines and bring power directly to customers.

Most of the power used by the province originates in the power plants and wind farms in Cape Breton. That means a problem with the transmission line can affect distribution lines, including customers who are far away from the original problem, Chase said.

"So when we experienced an interruption on our own system near Port Hawkesbury this morning, it meant that we couldn't send electricity to some customers further down the line, specifically in Halifax metro and the South Shore as far as Yarmouth."

Protective measures

She said another factor in the widespread outages is the protective measures that come into effect in the grid when there's a dip in supply.

"If you have a large number of customers trying to draw power but you don't actually have the supply on the lines, the protection comes into place to actually shut off the draw of power, which creates an outage," Chase said.

In addition to the transmission interruption in Port Hawkesbury, a second interruption just across the New Brunswick border affected about 100,000 customers in Nova Scotia.

Thetwo province's grids are connected. So when New Brunswick Power experienced a transmission interruption in Memramcook, the system there tried to draw power from Nova Scotia's. But since Nova Scotia didn't have enough power, those protective measures came into effect and created outages on both sides.

Restoration times

As of about noon, electricity was expected to be restored to most of the metro Halifax area, the South Shore by 4 p.m., and most of northern Nova Scotia by 11:30 p.m.

Chase said restoration times listed on the utility's website are developed using the best information the company has at any given time.

She said the company needs to identify the exact location of where the interruption is, then make sure it's safe to ramp up power generation again.