Maintenance work on the ammeter of a 4160 V bus is believed to have cut power to two pumps at the Monticello nuclear plant Tuesday, leading to an automatic reactor shutdown from full power.According to a Nuclear Regulatory Commission event report, the shutdown happened at 10:44 a.m. when the bus locked out, removing power from a reactor feed pump and a reactor recirculation pump. The water level in the reactor fell, then began to rise to +48 inches, the reactor-water-level-high setpoint. The main turbine and another feed pump tripped, as designed, and the reactor shut down automatically.According to the report, the "reactor water level began to drop and C.4.A Abnormal Procedure for SCRAM was used to restart 11 reactor feed pump and recover water level. (The) minimum water level reached was -26 inches." A low-level signal triggered group 2 primary containment isolation. The emergency core cooling system was not activated, and safety relief valves were not used.In a release, plant owner Xcel Energy said: "All plant safety systems functioned as designed, and the situation posed no danger to the public or plant workers. Plant operators are investigating the cause of the shutdown, which is not expected to be lengthy."