The power company behind the biggest electricity blackout in history said yesterday that it still had no idea what caused the catastrophic failure that led to chaos for 50 million people.
Niagara Mohawk, which distributes electricity across America's East Coast, said it did not believe the blackout had been caused either by a lightning strike or a fire at a generating plant or by a deliberate act of terrorism.
A spokesman for the company, which is based in Syracuse, New York, said that there was no obvious physical damage to its equipment that could account for the power cuts that affected huge swaths of Ottawa in Canada, New York, New Jersey and Ohio.
However, what is clear is that the system was running at nearly full capacity in the sweltering heat of a summer afternoon when something caused a power surge. Then, within seconds, this triggered an automatic cascade of generators shutting down.
"It will take time to find the root cause of the event but the result was an imbalance between electricity generation and demand," the Niagara Mohawk spokesman said.
The company said there was a small fire in a transformer but it thinks that this might have been caused by the power surge rather than being the cause of it. It has no evidence of any lightning strikes or terrorist activity that could have caused the blackout.
Niagara Mohawk said that the successive shutdowns would have occurred within seconds of just one of the generators coming off-line because the system was working close to full capacity.
The shutdowns brought condemnation from the former US energy secretary Bill Richardson, who said: "In my view, we're the world's greatest superpower but we have a Third World electricity grid."
Abbas Akhill, an energy specialist at the US government's Sandia National Laboratories in California, said that American power companies operated with only about 3 or 4 per cent spare capacity - the amount of extra power produced above the maximum expected demand.
This compares with about 15 per cent spare capacity produced before the American electricity grids were deregulated. Producing spare capacity is expensive because it is in effect electricity that in normal periods goes to waste.
Although it is not possible to store electricity, there are ways of storing energy that can be quickly converted to electricity in emergencies. Again, however, American utilities have very little storage capacity.
"In the US we also have the smallest amount of storage capacity, just 3 per cent, which is far less than in Europe," Dr Akhill said.
With such small margins, the grid becomes highly vulnerable to the surges in peak demand or a minor power failure - even one caused by a relatively trivial incident such as the branches of a tree touching live power cables.
"It could even have been something as mundane as a squirrel getting into some switching gear. When you're stretched to the upper limit, the smallest event can trip you over the edge," Dr Akhill said.
Tim Green, an electrical engineer at Imperial College London, said that a number of events occurring at the same time could have caused the disaster. "The voltage dropped to 80 per cent of its normal level and in some areas it fell to just 50 per cent of its value. At this point the automatic protection comes in and some power stations must have disconnected from the grid," Dr Green said.
This would have exacerbated the problem, causing extra load to be placed on other stations, which could not cope and so tripped into safety mode, causing the blackouts that affected the entire region.
"It looks like the area that lost power lost it within two minutes and in fact most of the loss occurred within the first 20 seconds," Dr Green said.