Residual-current device

A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a device that quickly breaks an electrical circuit to prevent serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human falls after receiving a shock, or if the person touches both conductors at the same time.A two-pole, or double-pole, residual-current device. The test button and connect/disconnect switch are colored blue. A fault will trigger the switch to its down (off) position, which in this device would disconnect both conductors.Log-log graph of the effect of alternating current I of duration T passing from left hand to feet as defined in IEC publication 60479-1.1: Electromagnet with help electronics 2: Current transformer secondary winding 3: Transformer core 4: Test push-button L: Line conductor N: Neutral conductor A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a device that quickly breaks an electrical circuit to prevent serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a human falls after receiving a shock, or if the person touches both conductors at the same time. RCD is the name used in the United Kingdom. In the United States and Canada, the terms ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or appliance leakage current interrupter (ALCI) (also known as a Leakage Current Detection Interrupter (LCDI)) are used.If the RCD device has additionally overcurrent protection integrated in the same device, it is referred to as RCBO (see below). An earth leakage circuit breaker (ELCB) may be a residual-current device, although an older type of voltage-operated earth leakage circuit breaker also exists. These electrical wiring devices are designed to quickly and automatically disconnect a circuit when it detects that the electric current is not balanced between the supply and return conductors of a circuit. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. Current of around 30 mA (0.030 amperes) through the human body is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. RCDs are designed to disconnect the conducting wires ('trip') quickly enough to prevent serious injury. RCDs are testable and resettable devices. A test button safely creates a small leakage condition, and a reset button reconnects the conductors after a fault condition has been cleared. Some RCDs disconnect both the energized and return conductors upon a fault (double pole), while a single pole RCD only disconnects the energized conductor. If the fault has left the return wire 'floating' or not at its expected ground potential for any reason, then a single-pole RCD will leave this conductor still connected to the circuit when it detects the fault. RCDs are designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 5–30 mA) and disconnecting quickly enough (<30 ms), they may prevent electrocution. They are an essential part of the automatic disconnection of supply (ADS), i.e. to switch off when a fault develops, rather than rely on human intervention, one of the essential tenets of modern electrical practice. To prevent electrocution, RCDs should operate within 25–40 milliseconds with any leakage currents (through a person) of greater than 30 mA, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would probably not increase the total current enough for a fuse or circuit breaker to break the circuit, and certainly not do so fast enough to save a life. RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between current flowing through the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground or to another circuit), and the device will open its contacts. Operation does not require a fault current to return through the earth wire in the installation; the trip will operate just as well if the return path is through plumbing, contact with the ground or any other current path. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete.