A current detection apparatus includes a magnetic core having a ring shape with a gap and a center opening portion through which a current path is disposed, and a magnetic sensor including a magnetic sensing element such as a Hall effect element arranged in the gap. If necessary, after heating, the memory may be refreshed. The rate of fiber digestion may be controlled based on a determination of the ruminal undigested NDF fraction of the analyzed feed components. A very big operating area for internal faults makes the method very sensitive for internal faults even for a heavy load and high resistance fault. The speed of the turbine may be varied to control the output frequency of the high frequency generation unit, thus controlling the output of the compression system by controlling the speed of the turbine driving the generation unit.