Editing Residual-current device (section)

==Purpose and operation==

RCDs are designed to disconnect the circuit if there is a leakage current.<ref name="SmithMadden2008">{{cite book|author1=Ken Oldham Smith|author2=John M. Madden|title=Electrical Safety and the Law|url=https://books.google.com/books?id=Pw93_N_LMPoC&pg=PA186|date=15 April 2008|publisher=John Wiley & Sons|isbn=978-0-470-77746-6|pages=186–}}</ref>  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{{nbsp}}mA) and disconnecting quickly enough (<30 milliseconds) to prevent device damage or [[electrocution]].<ref name="NagelSmith1991">{{cite book|author1=Joachim H. Nagel|author2=William M. Smith|title=Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society|url=https://books.google.com/books?id=6vdVAAAAMAAJ|year=1991|publisher=IEEE|isbn=978-0-7803-0216-7}}</ref> 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.<ref name="AtkinsonLovegrove2012">{{cite book|author1=Bill Atkinson|author2=Roger Lovegrove|author3=Gary Gundry|title=Electrical Installation Designs|url=https://books.google.com/books?id=I5ORKmGBPZ8C&pg=PT114|date=26 November 2012|publisher=John Wiley & Sons|isbn=978-1-118-47776-2|pages=114–}}</ref>

To reduce the risk of electrocution, RCDs should operate within 25–40 milliseconds with any leakage currents {{clarify|At what voltage? Smooth current or pulsing? AC or DC? |date=July 2021}} (through a person) of greater than 30{{nbsp}}mA, 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 [[Fuse (electrical)|fuses]] 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 [[current transformer]].  This measures the difference between current flowing through {{Not a typo|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 [[Earth Wire|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 {{Not a typo|line}} and neutral conductors upon a fault (double pole), while a single pole RCD only disconnects the {{Not a typo|line}} conductor. If the fault has left the neutral wire "[[floating voltage|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.{{Citation needed|date=January 2025|reason=Huh? Is this an American thing?}}

For an RCD used with [[three-phase power]], all three {{Not a typo|line}} conductors and the neutral (if fitted) must pass through the current transformer.