A power failure contributed to a 4,500-barrel oil spill in the US in May.

The operator of the trans-Alaska pipeline system, an 800 mile long oil carrying system, has said a report has shown the power failure occurred during a planned shutdown, which caused valves to open.

With the valves open, an oil overflow occurred from a storage tank to a containment area.

According to the report the spill was made worse because workers became so focused on restoring power that they were unable to anticipate the tank overflow.