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A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a device to quickly disconnect current 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.These electrical wiring devices disconnect a circuit when it detects that the electric current is not balanced between the energized (line) conductor(s) and the return (neutral) conductor. Under normal circumstances, these two wires are expected to carry matching currents, and any difference usually indicates that a short circuit or other electrical anomaly is present.
A RCD does not provide protection against unexpected or dangerously high current when current is flowing in the usual wires in the circuit, therefore it cannot replace a fuse or protect against overheating or fire risk due to overcurrent (overload) or short circuits if the fault does not lead to current leakage.
RCDs are usually testable and resettable devices. Commonly they include a button that when pressed, safely creates a small leakage condition, and a switch that reconnects the conductors when a fault condition has been cleared. Depending upon their design, some RCDs disconnect both the energized and return conductors upon a fault, while others only disconnect the energized conductor and rely upon the return conductor being at ground (earth) potential. The former are commonly known as "double-pole" designs; the latter as "single-pole" designs. 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.
Maximal rated current
The rated current of an RCD is chosen according to the maximal sustained load current it is expected to carry, so generally if the RCD is connected in series with a single fuse or circuit-breaker, the rated current of the RCD shall be at least the same as the fuse or circuit breaker, though in the case of an RCD feeding many circuit breakers, an allowance for load diversity may or may not be permissible depending on the type of load expected.
RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value:
high sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection),
medium sensitivity (MS): 100 – 300 – 500 – 1000 mA (for fire protection),
low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine).
Note that the nominal value of residual current indicated above is not an absolute value. More information is presented below.
Break time (response speed):
There are two groups of devices:
G (general use) for instantaneous RCDs (i.e., without a time delay);
minimal break time: immediate,
maximal break time: 200 ms for 1× IΔn, 150 ms for 2× IΔn, and 40 ms for 5× IΔn.
S (selective) or T (time-delayed) for RCDs with a short time delay (typically used in circuits containing surge suppressors);
minimal break time: 130 ms for 1× IΔn, 60 ms for 2× IΔn, and 50 ms for 5× IΔn,
maximal break time: 500 ms for 1× IΔn, 200 ms for 2× IΔn, and 150 ms for 5× IΔn.
A residual-current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detect overload conditions, phase-to-neutral short circuits or phase-to-phase short circuits (see three-phase electric power). Over-current protection (fuses or circuit breakers) must be provided. Circuit breakers that combine the functions of an RCD with overcurrent protection respond to both types of fault. These are known as RCBOs and are available in 2-, 3- and 4-pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but use a common interrupting mechanism.