UPDATE 1-Alberta regulator issues orders against Plains Midstream Canada

* Regulators issues orders following 2011 spill

* Says employees restarted line despite leak alarms

CALGARY, Alberta, Feb 26 (Reuters) - Alberta's Energy Resources Conservation Board, which regulates pipelines in the province, issued four "high-risk enforcement actions" on Tuesday against Plains All American Pipeline LP's Canadian unit as it wrapped up an investigation into one of the province's largest-ever oil spills.

The regulator said Plains Midstream Canada must implement new risk assessment procedures; conduct an emergency response exercise; and confirm that it has improved its backfill technique. The orders follow the April 2011 Rainbow pipeline breach that spilled 28,000 barrels of oil into a wilderness area near a native community.

The spill from the 187,000 barrel per day pipeline was one of the largest in the province's history and came on the heels of a number of other ruptures, including Enbridge Inc's high-profile 2010 spill near Marshall, Michigan, that caused U.S. regulators to characterize Enbridge employees as "Keystone Kops" because they failed to shut the pipeline after alarms sounded.

The ERCB said Plains Midstream also failed to shut down the Rainbow pipeline despite leak alarms sounding at its control center. The line was down and restarted three times before being finally closed more than eight hours after the initial alarm.

"Overall, the actions of Plains appeared to demonstrate a practice of placing higher priority on continued operation of the pipeline over any potential impacts related to a pipeline leak," the board said in its report.

The board also ordered Plains to improve its crisis communications, saying its efforts to keep the public informed after the spill were substandard.

Plains Midstream Canada could not be immediately reached for comment.