In September 2009, a contractor was working on the line in Mississauga, Ont., and damaged part of it, causing gas to escape and ignite. A flame shot up 50 metres into the air.
"Examination of the evidence, documents gathered, and interviews conducted indicate that the incident was preceded by a number of contributing conditions and unsafe acts," the NEB said in a report released this week.
No one was hurt in the fire and the environmental impact was limited, but a building, vehicles and construction equipment were damaged. Nearby businesses, including a strip mall and daycare, were evacuated as a precaution.
In the report, the federal energy watchdog says the incident was "preventable."
"Though no people were injured and the incident resulted in minimal impacts to public safety, the environment or the economy, the potential impact of an event of this magnitude cannot be ignored," it said.
Enbridge spokeswoman Chris Meyer said the company is reviewing the NEB's findings.
"Safety is our priority and after the incident we reviewed what happened and we've implemented changes to help prevent a similar situation from occurring again," she said.
The NEB says Enbridge's gas distribution unit did not have a copy of its safety manual on site and didn't adequately communicate hazards to contracted workers or make sure they were following proper procedures.
The fire took place as Enbridge's contractor, Robert B. Somerville Co. Ltd., was installing new pipe at Enbridge's Lisgar Gate Station.
In order to facilitate the welders' work, the pipe they were working on was rotated. An Enbridge representative oversaw the first movement, but had left the site to buy supplies for a second rotation.
During the second turn, a piece of equipment capping off the end of the existing pipeline was knocked free, leading gas to be released, the NEB said.
"Interviews with (Enbridge Gas Distribution) staff, management and the contracted workers provided the evidence that there was inadequate communication between EGD field staff and the contractor," the report said.
Both Enbridge (TSX:ENB) and the contractor have pleaded guilty to violations under the Ontario Occupational Health and Safety Act and the Ontario Court of Justice has sentenced each to a $50,000 fine.
The NEB report comes as Enbridge tries to win regulatory approval to build a controversial $6-billion pipeline connecting oilsands crude to the West Coast for export. Opponents say a spill from the line itself or from tankers along the coast could cause dire environmental harm.