WOBURN (CBS) – The president of National Grid said “mistakes do happen” after human error led to an over-pressurized gas line that has around 300 homes still without utilities.

National Grid shut off gas service to more than 300 homes after a crew mistakenly over-pressurized the system on Monday. It will likely be until Thursday before service is turned back on to all residents.

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The state has now ordered National Grid to stop all non-emergency work as they investigate exactly what went wrong. A fill-in technician is believed to be responsible for this mistake.

About 1,200 union employees with National Grid have been locked out since the summer.

“Mistakes do happen. This could happen regardless of the workforce we have on the ground,” said National Grid president Marcy Reed. “It’s not a rare event to have this happen. I stand by the qualifications and expertise that our workers have. Sadly we’re all human, and mistakes were made.”

National Grid workers in Woburn. (Image Credit: Anna Meiler/WBZ-TV)

Woburn Mayor Scott Galvin said the city has been happy with National Grid’s efforts to restore service to residents. Galvin added that once work is completed, the focus will turn to figuring out how it happened and what can be done to prevent a similar situation in the future.

Reed said restoration will be “slow and deliberate” in an effort to make sure it is done properly.

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“We’re going to get this right. It’s not a race,” she said.

Joe Kirylo of Boston Gas Workers Union, Local 12003 was critical of the National Grid replacement worker’s mistake.

“It’s incompetence and that’s the only way you can describe it,” he said.

National Grid says no residents had to evacuate and no one was hurt. They anticipate beginning to turn gas back on for some residents by Tuesday night and early Wednesday morning.

Rocco Leo, a locked out National Grid employee said it was only a matter of time before an incident like this happened.

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“The gas company I believe is playing Russian roulette,” he said. “We’ve told the media, the (Department of Public Utilities), everyone that would listen to us it’s not a matter of if, it’s a matter of when. We believed something like this would happen”