Editing Residual-current device (section)

==Typical design==
[[File:Schneider Electric A9D31620.JPG|thumb|right|An example of a rail-mounted RCBO]]
[[Image:ResidualCurrentCircuitBreak.jpg|frame|upright=1.4|right|Internal mechanism of an RCD]]
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{{nbsp}}A and is designed to trip on a leakage current of 30{{nbsp}}mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that [[Dangerous restart|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 (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 button (3) is pressed, the [[electrical connector|contacts]] ((4) and another, hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.

The sense coil (6) is a [[Test probe#Current probes|differential current transformer]]<!--better link?--> which surrounds (but is not electrically connected to) the {{Not a typo|line}} and neutral conductors. In normal operation, all the current flows in and out of the {{Not a typo|line}} and neutral conductors. The amount of current in the two conductors is equal and opposite and cancel each other out.

Any fault to earth (for example caused by a person touching a live component in the attached appliance) 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 {{Not a typo|line}} and neutral conductors (single-phase), or, more generally a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor), within the RCD.

This difference causes a magnetic flux in the toroidal sense coil (6), which, if sufficiently large, activates the relay (5), causing the switch to activate forcing the contacts (4) 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.{{Citation needed|date=January 2025|reason=Another americanism?}}

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). 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.<ref>[https://www.voltimum.se/articles/jordfelsbrytare Jordfelsbrytare (Swedish)]</ref>
=== RCD with integral overcurrent protection (RCBO) {{anchor|RCBO2}} ===
<!-- Courtesy note per [[WP:RSECT]]: [[RCBO]] and others link here -->
[[Image:FI-offen.jpg|thumb|upright=1.4|right|Opened three-phase residual-current device]]

Residual-current and [[overcurrent_protection|over-current]] protection may be combined in one device. Such a device is termed an RCBO (residual-current circuit breaker with over-current protection). In the US and Canada such devices (in the form of circuit breakers) are known by the term ''GFCI circuit breaker'' (lacking an American-specific acronym for this combination and seemingly taking for granted the presence of over-current protection in any circuit breaker type device). They are effectively a combination of a RCD and a [[Circuit_breaker#Arc interruption|MCB]].<ref>{{cite web |title=RCBOs {{!}} RS Components |url=https://sg.rs-online.com/web/c/automation-control-gear/circuit-protection-circuit-breakers/rcbos/ |website=sg.rs-online.com |accessdate=15 June 2020}}</ref>

As well as requiring both {{Not a typo|line}} and neutral inputs and outputs (or, full three-phase), some RCDs/GFCIs require a functional earth (FE) connection. This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but {{Not a typo|line}} 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 (Eaton/MEM) 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 [[Earthing system#TT_system|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 100{{nbsp}}mA (or greater) 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.

===RCD with additional arc fault protection circuitry===

In addition to ground fault circuit interrupters (GFCIs), [[Arc-fault circuit interrupter|arc-fault circuit interrupters]] (AFCI) are important as they offer added protection from potentially hazardous [[arc fault]]s resulting from damage in branch circuit wiring as well as extensions to branches such as appliances and cord sets. By detecting arc faults and responding by interrupting power, AFCIs help reduce the likelihood of the home's electrical system being an ignition source of a fire. Dual function AFCI/GFCI devices offer both electrical fire prevention and shock prevention in one device making them a solution for many rooms in the home.