Residual-current device

A residual-current device, residual-current circuit breaker or ground fault circuit interrupter is an electrical safety device, more specifically a form of Earth-leakage circuit breaker, that interrupts an electrical circuit when the current passing through and neutral conductors of a circuit is not equal, therefore indicating current leaking to ground, or to an unintended path that bypasses the protective device. The device's purpose is to reduce the severity of injury caused by an electric shock. This type of circuit interrupter cannot protect a person who touches both circuit conductors at the same time, since it then cannot distinguish normal current from that passing through a person.
A [|residual-current circuit breaker with integrated overcurrent protection] combines RCD protection with additional overcurrent protection into the same device.
These devices are designed to quickly interrupt the protected circuit when it detects that the electric current is unbalanced between the supply and return conductors of the circuit. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. Alternating 60 Hz current above 20 mA 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 quickly enough to potentially prevent serious injury to humans, and to prevent damage to electrical devices.

Purpose and operation

RCDs are designed to disconnect the circuit if there is a leakage current. In their first implementation in the 1950s, power companies used them to prevent electricity theft where consumers grounded returning circuits rather than connecting them to neutral to inhibit electrical meters from registering their power consumption.
The most common modern application is as a safety device to detect small leakage currents and disconnecting quickly enough to prevent device damage or electrocution. They are an essential part of the automatic disconnection of supply, 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 reduce the risk of electrocution, RCDs should operate within 25–40 milliseconds with any leakage currents of greater than 30mA, 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. A small leakage current, such as through a person, can be a very serious fault, but does not increase the total current enough for a fuse or overload circuit breaker to isolate the circuit.
RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between current flowing through and neutral. If these do not sum to zero, there is a leakage of current to somewhere else, 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 or contact with the ground or anything else. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete.
RCDs are testable and resettable devices—a test button safely creates a small leakage condition, and another button, or switch, resets the conductors after a fault condition has been cleared. Some RCDs disconnect both the and neutral conductors upon a fault, while a single pole RCD only disconnects the conductor. If the fault has left the neutral 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.
For an RCD used with three-phase power, all three conductors and the neutral must pass through the current transformer.

Application

with incorporated RCD are sometimes installed on appliances that might be considered to pose a particular safety hazard, for example long extension leads, which might be used outdoors, or garden equipment or hair dryers, which may be used near a bath or basin. Occasionally an in-line RCD may be used to serve a similar function to one in a plug. By putting the RCD in the extension lead, protection is provided at whatever outlet is used even if the building has old wiring, such as knob and tube, or wiring that does not contain a grounding conductor. The in-line RCD can also have a lower tripping threshold than the building to further improve safety for a specific electrical device.
In North America, GFI receptacles can be used in cases where there is no grounding conductor, but they must be labeled as "no equipment ground". This is referenced in the National Electric Code section 406 2, however codes change and someone should always consult a licensed professional and their local building and safety departments. An ungrounded GFI receptacle will trip using the built-in "test" button, but will not trip using a GFI test plug, because the plug tests by passing a small current from to the non-existent ground. It is worth noting that despite this, only one GFCI receptacle at the beginning of each circuit is necessary to protect downstream receptacles. There does not appear to be a risk of using multiple GFI receptacles on the same circuit, though it is considered redundant.
In Europe, RCDs can fit on the same DIN rail as the miniature circuit breakers; much like in miniature circuit breakers, the busbar arrangements in consumer units and distribution boards provides protection for anything downstream.

RCBO

A pure RCD will detect imbalance in the currents of the supply and return conductors of a circuit. But it cannot protect against overload or short circuit like a fuse or a miniature circuit breaker does.
However, an RCD and an MCB often come integrated in the same device, thus being able to detect both supply imbalance and overload current. Such a device is called an RCBO, for residual-current circuit breaker with overcurrent protection, in Europe and Australia, and a GFCI breaker, for ground fault circuit interrupter, in the United States and Canada.

Typical design

The diagram depicts the internal mechanism of a residual-current device. The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13A and is designed to trip on a leakage current of 30mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation. Some early RCDs were entirely electromechanical and relied on finely balanced sprung over-centre mechanisms driven directly from the current transformer. As these are hard to manufacture to the required accuracy and prone to drift in sensitivity both from pivot wear and lubricant dry-out, the electronically amplified type with a more robust solenoid part as illustrated are now dominant.
In the internal mechanism of an RCD, the incoming supply and the neutral conductors are connected to the terminals at, and the outgoing load conductors are connected to the terminals at. The earth conductor is connected through from supply to load uninterrupted.
When the reset button is pressed, the contacts and another, hidden behind ) close, allowing current to pass. The solenoid keeps the contacts closed when the reset button is released.
The sense coil is a differential current transformer which surrounds the and neutral conductors. In normal operation, all the current flows in and out of the and neutral conductors. The amount of current in the two conductors is equal and opposite and cancel each other out.
Any fault to earth causes some of the current to take a different path, with some of the neutral current diverted, which means that there is then an imbalance in the current between the and neutral conductors, or, more generally a nonzero sum of currents from among various conductors, within the RCD.
This difference causes a magnetic flux in the toroidal sense coil, which, if sufficiently large, activates the relay, causing the switch to activate forcing the contacts apart and thus cutting off the electricity supply to the appliance. In some designs a power failure may also cause the switch contacts to open, causing the safe trip-on-power-failure behaviour mentioned above.
The test button allows the correct operation of the device to be verified by passing a small current through the orange test wire. This simulates a fault by creating a deliberate imbalance in the sense coil. If the RCD does not trip when this button is pressed, then the device must be replaced.

RCD with integral overcurrent protection (RCBO)

Residual-current and over-current protection may be combined in one device. Such a device is termed an RCBO. In the US and Canada such devices are known by the term GFCI circuit breaker. They are effectively a combination of a RCD and a MCB.
As well as requiring both and neutral inputs and outputs, some RCDs/GFCIs require a functional earth connection. This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but and earth remain.
For reasons of space, many devices, especially in DIN rail format, use flying leads rather than screw terminals, especially for the neutral input and FE connections. Additionally, because of the small form factor, the output cables of some models are used to form the primary winding of the RCD part, and the outgoing circuit cables must be led through a specially dimensioned terminal tunnel with the current transformer part around it. This can lead to incorrect failed trip results when testing with meter probes from the screw heads of the terminals, rather than from the final circuit wiring.
Having one RCD feeding another is generally unnecessary, provided they have been wired properly. One exception is the case of a TT earthing system, where the earth loop impedance may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100mA trip current time-delayed RCD is installed, covering the whole installation, and then more sensitive RCDs should be installed downstream of it for sockets and other circuits that are considered high-risk.