Figure 1 — Under normal conditions, current flowing to the load is equal to current returning from the load.
As illustrated in this sketch, electrical current flows in a closed path from the source (labeled "line"), through the load, and back to the source. The current flowing toward the load is exactly equal to the current returning from the load. Current in both directions is made to pass through the center of a detection coil (L1). Current passing through the detector coil produces a voltage at the output terminals of the coil, but because the outbound and inbound current is exactly equal and opposite, the two currents together exactly cancel and produce no output voltage from the detector coil.
Figure 2 — When a ground fault occurs, part of the current returns via the ground path.
When some fault in the insulation of the conductors or the appliance, or accidental contact with a person or animal causes leakage of current to ground, that part of the current does not flow in the intended path, so the current in one conductor does hot quite equal the current in the other. In this condition, the magnetic fields of the hot and neutral conductors do not exactly cancel, and the detection coil produces a voltage. The ground fault detector circuit senses the voltage from the detection coil and sends a pulse of current through the trip solenoid.
Figure 3— When the GFI is tripped, the load contacts open to stop all current.
The solenoid unlatches the interrupter contacts, which spring open to disconnect the protected wiring. The GFI will remain tripped until it is manually reset, so that the cause of the trip can be corrected safely
Figure 4— When a ground fault occurs in the neutral wire, neutral fault detection signal current flows.
Neutral-fault detection is an additional feature of the GFI. If no load is connected, a ground fault in the neutral conductor could escape detection. This situation is not nearly so dangerous as a fault in the hot conductor, but could become dangerous if further faults develop. To detect the condition, the GFI generates a signal, which is induced into both conductors (i.e. it is a common-mode signal). If no ground fault exists, there is no closed path for the signal to follow, hence it causes no current and is not seen by the detection coil. When a fault develops, the signal returns to its source via the grounding system, producing a current which is detected, causing the GFI to trip. The voltage and current levels at which this signal operates are very small, and have no effect on electrical equipment.
