The diagram depicts the internal mechanism of a residual-current device (RCD). The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13 A and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that. Some early RCDs were entirely. The diagram depicts the internal mechanism of a residual-current device (RCD). The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13 A and is designed to trip on a leakage current of 30 mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that. 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 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 (1), and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted. When the reset but. A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device, more specifically a form of, that interrupts an when the current passing through line and neutral conductors of a circuit is not equal (the term residual relating to the ), therefore indicating to, or to an unintended path that bypasses the protective device. The device's purpose is to reduce the severity of injury caused by an. 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 (RCBO) combines RCD protection with additional into the same device. These devices are designed to quickly interrupt the protected circuit when it detects that the is unbalanced between the supply and return conductors of the circuit. Any difference between the currents in these conductors indicates, which presents a shock hazard. Alternating 60 Hz current above 20 (0.020 amperes) through the human body is potentially sufficient to cause 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 potentially prevent serious injury to humans, and to prevent damage to electrical devices. • 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 off (down) 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 60479-1 • Typical GFCI receptacle found in North America 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 (typically 5–30 mA) and disconnecting quickly enough (<30 milliseconds) to prevent device damage or. 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 reduce the risk of 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, the most common cause of death through electric shock. By contrast, conventional circuit breakers or 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 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. This measures the difference between current flowing through line and neutral. 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 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 line and neutral conductors upon a fault (double pole), while a single pole RCD only disconnects the line conductor. If the fault has left the neutral wire "" or not at its expected 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, all three line conductors and the neutral (if fitted) must pass through the current transformer. 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, 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 (D) 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 line 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 as the ; much like in miniature circuit breakers, the busbar arrangements in and provides protection for anything downstream. 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 like a fuse or a miniature circuit breaker (MCB) does (except for the special case of a short circuit from line to ground, not line to neutral). 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.
