How does a ground-fault interrupter notice missing return current?

A ground-fault circuit interrupter, or GFCI, looks for current taking an unintended return path. Related devices are called residual-current devices, or RCDs. They do not simply measure how much current an appliance draws. They compare the outgoing current with the current returning through the monitored conductors.

In a common single-phase arrangement, the live and neutral conductors pass through a sensing core. Their currents normally have equal magnitudes and opposite directions, so their magnetic effects cancel in the ideal sensor. If some current returns outside that monitored pair, the cancellation is incomplete. The residual magnetic signal lets the device detect an imbalance and command its contacts to open.

In the invented snapshot, 2.000 amperes leave through live and 1.992 amperes return through neutral. The difference is 0.008 ampere, or 8 milliamperes. Our illustration uses a supplied 5-milliampere trip criterion, so that imbalance commands disconnection. Equal 2.000-ampere currents give zero residual. A larger balanced load still gives zero residual in this model.

The numbers describe sensing before the contacts open, not sustained current after disconnection. Real products have specified thresholds, timing and responses to different current waveforms. The sensor detects the imbalance; opening the contacts then interrupts the circuit.

A fuse or ordinary circuit breaker instead responds to excessive current, although some products combine both functions. Comparing outgoing and returning current can detect a small diversion even when the appliance's overall current is not unusually large. Differential protection reduces certain shock risks, but current passing out through live and back through neutral can remain balanced and dangerous.