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.

[citation needed] For an RCD used with three-phase power, all three line conductors and the neutral (if fitted) must pass through the current transformer.

Electrical plugs 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 sink.

This is an active RCD; that is, it latches electrically and therefore trips on power failure, a useful feature for equipment that 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.

The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the line and neutral conductors.

This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but 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.

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.

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.

Major differences exist regarding the manner in which an RCD unit will act to disconnect the power to a circuit or appliance.

RCDs used on single-phase AC supplies (two current paths), such as domestic power, are usually one- or two-pole designs, also known as single- and double-pole.

This is not generally a problem for an overcurrent circuit breaker because the force from the amount of current involved with those when they trip can be sufficient to break them free if stuck, however an RCD is designed to trip on a very small amount of current which can excerpt far too weak a force to break a stuck switch free, thus failing to operate the safety device.

An RCD helps to protect against electric shock when current flows through a person from a phase (line / hot) to earth.

Fuchs Electrical Industries of Alberton Johannesburg, initially developed a cold-cathode system in 1955 which operated at 525 V and had a tripping sensitivity of 250 mA.

Very rapid tripping times were achieved through a novel design, and this combined with the high sensitivity was well within the safe current–time envelope for ventricular fibrillation determined by Charles Dalziel of the University of California, Berkeley, United States, who had estimated electrical shock hazards in humans.

In addition, the original prototype was able to trip at a lower sensitivity in the presence of an interrupted neutral, thus protecting against an important cause of electrical fire.

The circuit breaker type, installed into a distribution panel, suffered from accidental trips mainly caused by poor or inconsistent insulation on the wiring.

Belgian domestic installations are required to be equipped with a 300 mA residual current device that protects all circuits.

The Italian law (n. 46 March 1990) prescribes RCDs with no more than 30 mA residual current (informally called "salvavita"—life saver, after early BTicino models, or differential circuit breaker for the mode of operation) for all domestic installations to protect all the lines.

Additionally, all power sockets need to be protected by a residual current device of sensitivity not exceeding 30 mA and all equipment in wet places (water heater, water pump) need to be protected by a residual current device of sensitivity not exceeding 10 mA.

From January 2003, all new circuits originating at the switchboard supplying lighting or socket outlets (power points) in domestic buildings must have RCD protection.

Residential facilities (such as boarding houses, hospitals, hotels and motels) will also require RCD protection for all new circuits originating at the switchboard supplying socket outlets.

[25] In North America socket-outlets located in places where an easy path to ground exists—such as wet areas and rooms with uncovered concrete floors—must be protected by a GFCI.

South Africa mandated the use of Earth Leakage Protection devices in residential environments (e.g. houses, flats, hotels, etc.)

[31] The current (18th) edition of the IET Electrical Wiring Regulations requires that all socket outlets in most installations have RCD protection, though there are exemptions.

Provision of RCD protection for circuits present in bathrooms and shower rooms reduces the requirement for supplementary bonding in those locations.

RCDs used for shock protection must be of the 'immediate' operation type (not time-delayed) and must have a residual current sensitivity of no greater than 30 mA.

If spurious tripping would cause a greater problem than the risk of the electrical accident the RCD is supposed to prevent (examples might be a supply to a critical factory process, or to life support equipment), RCDs may be omitted, providing affected circuits are clearly labelled and the balance of risks considered; this may include the provision of alternative safety measures.