Ontario and large areas of the United States were about 20 seconds away from being plunged into darkness on May 27, according to American union officials. But hydro officials and the Ontario government are disputing the claims.

Union officials said the power outage would have been similar to the proportion of the cascading blackout that left 50 million people in eastern North America without power in August 2003.

The leading cause of the potential blackout was the failure of two computerized protection systems installed by Ontario's power transmission utility Hydro One to prevent a repeat of the 2003 outage, said Andy Banks, of the Maryland-based International Federation of Professional and Technical Engineers.

"People are very, very nervous because (the 2003 blackout) caused $10 billion in damage on the U.S. side of the border. That was a catastrophe and took a long time to recover. They don't want it to happen again," he told the Toronto Star.

The union represents 80,000 workers, including more than 1,000 striking Ontario engineers and others at Hydro One who have been on strike since earlier in the month.

Ontario's Energy Minister Dwight Duncan rejected the claims, saying the union should mind its own business and work to improve the American power grid.

While Hydro One officials acknowledged the May 27 incident occurred during a routine maintenance job at the central grid control centre in Barrie, Ont., they dismissed claims the outage would have been as widespread as the 2003 blackout.

"This was nothing even similar to what happened in 2003 at all. We did have an operator error that resulted in a small blip on the system," Peter Gregg, the utility's vice-president of corporate communications, said, appearing on CTV Newsnet's Countdown.

Gregg played down the alleged magnitude of the potential blackout, saying most people wouldn't have noticed when lights flickered and computers turned off during the May incident.

He charged that striking workers were responsible for fear-mongering.

"To equate what happened on May 27 to what happened in August of 2003 is entirely irresponsible and I think it's motivated by a group of employees, who are on strike, who were trying to raise attention to their cause," he said.

While Banks rejected the accusation, he said the problem likely won't be resolved until the workers go back on the job.

"The real issue here isn't about the operator area, it's that two pieces of equipment...that were put in after the 2003 blackout, didn't work and nobody knows why," Banks told CTV Newsnet's Countdown.

"And they won't know until the engineers who are on strike, who installed the system and designed the system, and who watched the system, are back at work."

A third-party investigation, commissioned by Hydro One, is expected to be completed in the next two weeks.