US Patent 7876448 Fiber-optic current sensor with sum detection

To measure a sum of electrical currents in different conductors, light is led in an optical sensing fiber around all the conductors. The sensing fiber can be wound around the conductors in a single loop or in several individual loops. Interspersed polarization maintaining fibers and retarders can be used to transfer the light between individual loops for preventing undesired signals from stray magnetic fields. The method has high accuracy is particularly suited for measuring large currents that sum up to zero.