Magnetic sensing techniques have already been developed jointly by EPRI and General Electric to detect intemal arcs in GIS [4] [5]. One of them uses a permanent magnetic sensor [4], and the other an battery-driven magnetometer [5]. It is reported that the latter can detect a fault current of 4 kA or larger when the two sensors are placed in the same axial location but separated by 180 degrees. Our sensing technique is based on the same principle of detecting the magnetic field change. However, it investigates the application of an opto-magnetic sensor on the detection of biased load-current distributions, which significantly differ from those of intemal arc current flowing from a conductor to an enclosure.