A fault indicator is a device which provides visual or remote indication of a fault on the electric power system. Also called a faulted circuit indicator (FCI), the device is used in electric power distribution networks as a means of automatically detecting and identifying faults to reduce outage time. Overhead indicators are used to visualize the occurrence of an electrical fault on an overhead electrical system. Underground indicators locate faults on an underground system. Often these devices are located in an underground vault. Some fault indicators communicate back to a central location using radio or cellular signals. Basic principles Typically fault indicators sense magnetic field caused by current flows through a conductor or cable. Some of them also use measurement of electric field caused by voltage in conductor. During an electrical fault on a grounded system, additional current flows through a conductor, inducing a magnetic field, which is detected up by the fault indicator causing a state change on the mechanical target flag, LED, or remote indication device. Ground fault indicators for isolated ground systems sense the vector sum of the current and look for an imbalance indicating a fault on one or more of the three phases. Systems with earthing through high resistance have low phase-to-ground fault currents so require high sensitivity of FI. When there are high load currents and low earth fault levels the line mounted fault indicators should look at the differential increase of the current (di/dt) instead of the absolute value. In insulated neutral systems and systems with earthing through a Petersen Coil, ground faults cannot be located with classical FI's at all. Capacitive current appears in overall faulted system so directional fault location devices are required. Upon energization of the lines there could be high inrush current which may lead the fault indicators to false operation. Some fault indicators can block inrush current. Advanced fault indicators should monitor the live line to distinguish fault currents from usual current surges. They operate only after making sure of the feeding circuit breaker/fuse has cut the power flow and the line is de-energized. To monitor the live line, some fault indicators look only at the magnetic field caused by the load current. to be independent from the load current, some FI’s look at the E field to check the voltage directly. This feature is used to reset the indicator after the line is re-energized too. Such indicators