to switch off when a fault develops, rather than rely on human intervention, one of the essential tenets of modern electrical practice. They are an essential part of the automatic disconnection of supply (ADS), i.e. 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 electrocution. 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. RCDs are designed to disconnect the circuit if there is a leakage current. 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. Some RCDs disconnect both the energized and return conductors upon a fault (double pole), while a single pole RCD only disconnects the energized conductor. RCDs are testable and resettable devices-a test button safely creates a small leakage condition, and another button resets the conductors after a fault condition has been cleared. 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. Alternating 60 Hz current above 20 mA (0.020 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. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. These electrical wiring devices are designed to quickly and automatically isolate a circuit when it detects that the electric current is unbalanced between the supply and return conductors of a circuit. An earth leakage circuit breaker may be an RCD, although an older type of voltage-operated earth leakage circuit breaker (ELCB) also exists. If the RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO. Injury may still occur in some cases, for example if a human receives a brief shock before the electrical circuit is isolated, falls after receiving a shock, or if the person touches both conductors at the same time. It is to protect equipment and to reduce the risk of serious harm from an ongoing electric shock. A residual-current device ( RCD), residual-current circuit breaker ( RCCB) or ground fault circuit interrupter ( GFCI) is an electrical safety device that quickly breaks an electrical circuit with leakage current to ground.
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