A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A shock, possibly lethal, is likely to result from these conditions; RCDs are designed to disconnect quickly enough to greatly reduce the harm caused by such shocks.

In the United States and Canada, a residual current device is also known as a ground fault circuit interrupter (GFCI), Ground Fault Interrupter (GFI) or an appliance leakage current interrupter (ALCI).

RCDs operate by measuring the current balance between two conductors using a differential current transformer, and opening the device's contacts if there is a balance fault (i.e. sufficient difference in current between the phase conductor and the neutral conductor). More generally (single phase, three phase, etc.) RCDs operate by detecting a nonzero sum of currents, i.e. the current in the "live" (phase) conductor plus that in the "neutral" cor must equal zero (within some small tolerance), otherwise there is a leakage of current to somewhere else (to earth/ground, or to another circuit, etc.). In the United States, the National Electrical Code, requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4–6 mA of current (the exact trip setting can be chosen by the manufacturer of the device and is typically 5 mA) within 25 milliseconds. GFCI devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCDs have trip currents of 10–300 mA.

RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30 mA milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. RCDs are intended to operate within 25–40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

Residual current detection is complementary to, rather than a replacement for, conventional over-current detection, as residual current detection cannot provide protection for faults which do not involve an external leakage current, for example faults that pass the current directly from one side of the circuit through the victim to the other. Notably, RCDs do not provide protection against overloads or short circuits between phase (live, hot, line) and neutral or phase to phase.

RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDs serve more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks.

In some countries, two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but it must be tagged as such (the GFCI manufacturers provide tags for the appropriate installation description).