* Some 1,200 barrels spill from pipeline in Wisconsin
* Spill is two years after another major Enbridge spill
* Canadian company already battling safety concerns on its line
WASHINGTON, July 28 (Reuters) - The U.S. pipeline safety agency launched an investigation on Saturday into an oil spill in Wisconsin on Enbridge Inc's network that forced the partial shutdown of a main artery carrying light sweet Canadian crude to Chicago-area refineries.
Enbridge's 318,000 barrel per day Line 14 pipeline, part of the Lakehead system, was shut after an estimated 1,200 barrels of oil were leaked. This happened almost two years to the day after another major spill in a different section of the line, in Michigan.
Enbridge Energy Partners said on Friday there was not yet a time frame for when flows would resume, and the cause of the spill had not yet been determined.
"(The U.S. Transportation Department's Pipeline and Hazardous Materials Safety Administration) is investigating the cause of the Enbridge crude oil pipeline failure in Wisconsin," spokesman Damon Hill said in an email on Saturday, adding that an inspector had been sent to the location of the pipeline failure.
Line 14 is one of four lines that ship mainly Canadian crude via Lakehead, a 2.5 million bpd network that is the principal route for Canadian exports.
The news will not help Enbridge build public trust in its network, which has come under scrutiny following several high-profile incidents, including a spill in Alberta last month and the massive leak in Michigan two years ago.
Just weeks ago, the U.S. National Transportation Safety Board delivered a scathing report of Enbridge's handling of the July 2010 rupture of its Line 6B near Marshall, Michigan, which led to more than 20,000 barrels of crude leaking into the Kalamazoo River.
The NTSB said it found a complete breakdown of company safety measures, and that Enbridge employees performed like "Keystone Kops" trying to contain it. The rupture went undetected for 17 hours.
U.S. pipeline regulators fined it $3.7 million for the spill, their largest ever penalty.
The incidents have caused furor just as the company seeks approval for its C$6 billion Northern Gateway pipeline to Canada's West Coast from Alberta amid staunch opposition from environmental groups and native communities that warn against oil spills on land and in coastal waters.
Enbridge said Line 14 was a 24-inch diameter pipe that was installed in 1998, making it a relatively new line.
In most cases, smaller pipeline leaks can be repaired quickly allowing operations to resume pumping, although regulators may require significant work if they find any cause for alarm. Following the leak two years ago, the line was shut for over two months.
No injury was reported on Friday at the line, which is near Grand Marsh, Wisconsin, Enbridge said.